Early Pioneers of Photosynthesis Research

Summary

Before the early researchers on plant nutrition worked out the fundamentals of photosynthesis, plants were believed to obtain their nutrients from humus in the soil. Then, over a period of about 200 years, a succession of investigators supplied pieces of the puzzle that eventually allowed formulation of the overall equation for photosynthesis. Jan van Helmont, in the early seventeenth century, concluded that plants are composed not of humus but of water that has been transmuted into plant substance. In the early eighteenth century, Stephen Hales found evidence that “air” was a component of the plant body. The involvement of water and air remained conjectural, however, until the late eighteenth century, when the new chemical theories of Antoine Lavoisier allowed the understanding of plant nutrition to advance. Joseph Priestley found that plants and animals are interdependent through their complementary effects on the atmosphere, and his discovery of oxygen was important for Lavoisier’s formulation of the new chemistry. Jan Ingen-Housz discovered that plants need light in order to release oxygen and that the green color of plants is important. Jean Senebier showed that carbon dioxide is also essential. Nicholas de Saussure, with his embrace of the new chemical concepts and his well-targeted experiments, was able to assemble a more comprehensive picture of plant nutrition, encompassing carbon dioxide from the atmosphere, water and mineral nutrients from the soil, and light from the sun. In the mid-nineteenth century, Robert Mayer completed the outline of the overall scheme with his insight that photosynthesizing plants convert light energy into chemical energy. Some of the important eighteenth-century photosynthesis pioneers were overshadowed by the drama of the profound changes shaking chemistry and are less well known than they should be. Some of their discoveries are misattributed or misstated in the literature, even down to the present day.

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I. Introduction

Over a period of about 200 years, from the early seventeenth to the early nineteenth centuries, a succession of researchers unraveled the basic scheme by which plants nourish themselves. The story of this gradual assembling of fundamental facts is closely interwoven with the story of progress in chemistry. The earliest of the photosynthesis pioneers struggled with the limited scope of chemical knowledge, while the later ones had to adapt to, but ultimately benefited from, the rapid changes in chemical perceptions that occurred during the “Chemical Revolution” of the late eighteenth century. Collectively, the discoveries of the photosynthesis pioneers revealed how plants take simple, inorganic, raw materials – water and mineral salts from the soil, and carbon dioxide from the atmosphere – and, in the presence of sunlight, convert them into organic substances.

Until these fundamental facts of photosynthesis were established, the prevailing belief about plant nutrition was that held by Empedocles (ca. 490–430 B.C.), and developed more fully by Aristotle (384–322 B.C.). According to this view, plants derive their nutrition from humus, or organic material, in the soil, in a manner analogous to the ingestion of food by animals. The root system was thought to function as a “kind of diffuse mouth sucking nutrition from the Earth’s breast” (Galston, 1994), or, as Hill (1970) describes it, “…a plant would have seemed to resemble a foetus drawing its nourishment from Mother-Earth.” This concept persisted even long after the true sources of plant nutrition had been discovered.

Another idea of Aristotle’s that is often regarded as having impeded progress, in both chemistry and plant physiology, was his theory that there are only four elements – earth, air, fire, and water. An aspect of that scheme that was particularly significant during the Middle Ages was the idea that elements could be “transmuted” into one another. In addition, there was the sixteenth-century, Paracelsian notion of three chemical principles – salt, sulfur, and mercury. Advances in chemistry during the eighteenth century swept all these ideas away. With these changes in concepts came a change in terminology. For a chart of older terms and their meanings, see Table 30.1.

Table 30.1. Terms, old and new

Of special importance for this advancement of chemistry was the dawning understanding of gases. The atmosphere came to be seen not as one of four fundamental “elements,” but as a mixture of gases. In order to study gases, however, better techniques had to be developed for preparing, collecting, transferring, separating, mixing, and identifying them. Progress also came from the realization that gases participated in chemical reactions: They reacted with one another, and also could be “fixed” in solid substances and then released from a solid back into a gaseous form.

Before chemistry could advance significantly, however, it also had to abandon the theory of phlogiston, developed by Johann Joachim Becher (1635–1682) and Georg Ernst Stahl (1660–1734). During much of the eighteenth century, this theory was the prevailing explanation for combustion and many other chemical phenomena. Phlogiston was believed to exist as a component of all combustible materials and of all metals that could be calcined (turned into calxes, or what we call oxides; see Table 30.1). When materials burned or were calcined, phlogiston was believed to be lost to the surrounding air. Stahl posited a cycle in nature, such that phlogiston was exchanged among the plant, animal, and mineral realms. Plants recombined the phlogiston that was given off in burning or calcination. Animals derived phlogiston from the plants or plant-eating animals that they consumed. Plants also provided phlogiston to the mineral realm, in the form of the charcoal (or other organic substance) that was commonly used to reduce calxes to metals. Thus the metal regained the phlogiston it had lost when it had originally formed a calx (Brock, 1993).

The nutritional interactions of a plant with its environment are not obvious, and could not be understood until gases had been discovered and studied. Rabinowitch (1945, p. 13) noted that the growing knowledge of the chemistry of gases in the mid- to late-eighteenth century “made the discovery of photosynthesis possible, yes, almost inevitable.” In the end, the overall scheme of photosynthesis proved to be relatively simple, but, of course, that simplicity masks a complexity of processes still being unraveled today.

The photosynthesis pioneers whose discoveries helped establish the basic biochemical scheme were: Jan van Helmont in the early seventeenth century; Stephen Hales in the early eighteenth century; Charles Bonnet, Joseph Priestley, Carl Wilhelm Scheele, Antoine-Laurent Lavoisier, Jan Ingen-Housz (who also demonstrated the importance of light), and Jean Senebier in the mid- to late-eighteenth century; and Nicholas de Saussure in the late eighteenth to early nineteenth centuries. Those pioneers who worked during the period of rapid progress in chemical theory both benefited from, and contributed to, that progress. After the rudimentary biochemistry of photosynthesis had been unraveled, Robert Mayer, in the mid-nineteenth century, contributed the important insight that energy plays a role in the process. (Ingen-Housz, with his insight that light is important, had anticipated Mayer, but had been unable to interpret his finding.) An examination of the birth and death dates of the pioneers and others important in laying the groundwork for discoveries in photosynthesis reveals how brief was the period in which the essentials of the process were uncovered (Fig. 30.1).

Fig. 30.1.

Lifespans of the pioneers and some of the other researchers whose work contributed to the early discoveries in photosynthesis. The chart is constructed after the manner of Joseph Priestley’s Chart of Biography (1765) and similar charts in his works on electricity (1767) and optics (1772a). Note the clustering of photosynthesis pioneers in the late eighteenth century.

Natural philosophers (as eighteenth-century investigators of nature were called; see Table 30.1) pursued their interests more as a passion than a profession. The early workers in photosynthesis came mainly from the ministry, law, and medicine. Their rivalry with their fellow researchers could be as fierce as that among today’s professional scientists, however, and was a strong motivating force in their accomplishments (Djerassi and Hoffman, 2001). In addition, many of the pioneers, like many of their contemporaries, viewed the study of nature as a means of apprehending the creator (Kottler, 1973).

Previous writing about the early pioneers has covered them both individually and collectively. Early issues of the journal Plant Physiology paid tribute to a number of them, with articles on Van Helmont, Hales, Ingen-Housz, Senebier, and de Saussure. (The portraits included in these articles are reproduced here as Fig. 30.2; the figure caption provides article citations.)

Fig. 30.2.

Portraits of photosynthesis pioneers, taken from papers published in early issues of Plant Physiology. Artists mostly unknown. (a) Jan van Helmont (From Harvey, 1929); (b) Stephen Hales (From White, 1942); (c) Jan Ingen-Housz (From Harvey and Harvey, 1930); (d) Jean Senebier (From Bay, 1931); (e) Nicholas de Saussure (From Hart, 1930).

Nineteenth-century German plant physiologist Julius von Sachs (1832–1897), who laid the foundations of modern plant physiology (Pringsheim, 1932; Gest, 1988), provided an overview of the early pioneers in his History of Botany (Sachs, 1875; 1890 translation). In addition, Eugene Rabinowitch (1945, Chapter 2) wrote a useful summary of the early workers’ contributions. Leonard K. Nash, in his 122-page booklet Plants and the Atmosphere (1952), gave a detailed, scholarly account, drawing solely on primary source material.

Briefer, more recent, overviews of the pioneers include those by Rabinowitch and Govindjee (1969, Chapter 1), Hill (1970), and Rabinowitch (1971). Recent issues of Photosynthesis Research have published accounts of the entire history of photosynthesis research: Huzisige and Ke (1993), the “Celebrating the Millennium” series (Govindjee and Gest, 2002; Govindjee et al., 2003; Govindjee et al., 2004), and Govindjee and Krogmann (2004). These histories include brief surveys of the work of the early pioneers. The book Discoveries in Photosynthesis (Govindjee et al., 2006) incorporates these articles. Höxtermann (1992) also covered the full history of photosynthesis research. Morton (2008) discusses the early pioneers as well as some more recent figures. The present discussion, the first in many years devoted solely to an overview of the work of the early pioneers, provides references to recent secondary literature. One of the goals is to address the persistent confusion surrounding the attribution of the discoveries of several of these workers.

II. Jan van Helmont (1579–1644): A Possible Role for Water

The first unmistakable quantitative experiment of the Renaissance was Flemish physician Jan van Helmont’s study of the growth of a willow tree planted in a pot of soil of known weight (Hoff, 1964). Van Helmont’s son, Francisco Mercurio van Helmont, posthumously published Jan van Helmont’s collected works as Ortus medicinae (Van Helmont, 1648). In the English translation of that work, titled Oriatrike (Van Helmont, 1662), the following account of the willow experiment is given (p. 109):

But I have learned by this handicraft-operation, that all Vegetables do immediately, and materially proceed out of the Element of water onely. For I took an Earthen Vessel, in which I put 200 pounds of Earth that had been dried in a Furnace, which I moystened with Rain-water, and I implanted therein the Trunk or Stem of a Willow Tree, weighing five pounds; and at length, five years being finished, the Tree sprung from thence, did weigh 169 pounds, and about three ounces: But I moystened the Earthen Vessel with Rain-water, or distilled water (alwayes when there was need) and it was large, and implanted into the Earth, and least the dust that flew about should be co-mingled with the Earth, I covered the lip or mouth of the Vessel, with an Iron-Plate covered with Tin, and easily passable with many holes. I computed not the weight of the leaves that fell off in the four Autumnes. At length, I again dried the Earth of the Vessel, and there were found the same 200 pounds, wanting about two ounces. Therefore 164 pounds of Wood, Barks, and Roots, arose out of water onely.

Van Helmont (Fig. 30.2a) combined genuine scientific research with nonscientific, somewhat mystical pursuits (Pagel, 1972), and represents the transition from alchemy to chemistry (Partington, 1957). He studied gases, coined the term “gas,” and isolated two of them (including carbon dioxide, which he called gas sylvestre). He thought that these were just varieties of common air, however.

Van Helmont was a quiet, scholarly man (Harvey, 1929), who rebelled against the traditional medical training of his time. That training, however, instilled in him an interest in the physiology of both animals and plants. He was financially secure and could work on experiments uninterruptedly in his own private laboratory (Harvey, 1929). For biographical details, see Redgrove and Redgrove (1922), Harvey (1929), de Waele (1947), and Pagel (1972).

Van Helmont was spurred to conduct the willow experiment by his conviction that water is the “first matter,” or material cause, of all things and thus the chief constituent of organisms. The willow experiment was his way to test the Aristotelian notion that water could be transmuted into earth – in this case, the “earth” of plant substance (Brock, 1993).

The idea for the willow experiment may not have originated with Van Helmont. About 200 years earlier, a German, Nicolaus of Cusa (1401–1464), proposed an experiment (but presented no experimental data) much like Van Helmont’s willow experiment (Hoff, 1964). On the basis of his highly detailed proposal, Krikorian and Steward (1968) speculated that Cusa either performed the experiment himself or drew on earlier sources that may subsequently have been lost (see Howe, 1965).

Van Helmont’s willow experiment demonstrated that the major part of the weight gained by a growing plant does not come from the soil, as had been thought. Van Helmont believed that he had excluded all sources of nutrition save water, but, of course, he was unaware of the large, important contribution made by atmospheric carbon dioxide–a gas that, ironically, he himself had discovered. His conclusion, however, was logical based on the information available at the time (Harvey and Harvey, 1930).

Some years after Van Helmont carried out his experiment, chemist and physicist Robert Boyle (1627–1691) had his gardener perform a similar experiment, using squash and cucumber. Boyle concluded (1661, p. 109), “it appears that…the Main Body of the Plant consisted of Transmuted Water.”

III. Stephen Hales (1677–1761): The Presence of Air in Plants

According to Guerlac (1972), Stephen Hales (Fig. 30.2b) was “the leading English scientist during the second third of the eighteenth century” and “the acknowledged founder of plant physiology.” Hales made important discoveries regarding the movement of the sap in plants. Of special significance to the understanding of photosynthesis, however, was his demonstration that plants interact with the atmosphere. His experiments indicating that some of a plant’s nutrition was derived from “air” suggested that Van Helmont’s conception of the plant body as a structure built entirely of transmuted water was incomplete. Hales also speculated that light is involved in plant nutrition.

Hales was born at Bekesbourne, Kent. He studied Divinity at Cambridge University, where he was influenced by the ideas of Isaac Newton (1642–1727). In 1709, Hales was ordained and appointed curate at Teddington, near London, where he remained the rest of his life (Guerlac, 1972). Further biographical details about Hales can be found in Hoskin (1961) and Guerlac (1972).

Hales subscribed to Newton’s concept that most changes in nature can be explained by the action of gravitational attraction and other attractive and repulsive forces (Hoskin, 1961). Among these phenomena Hales included the physiological activities of animals and plants.

Physician William Harvey (1578–1657), a century earlier, had demonstrated the circulation of the blood (Harvey, 1628), and Hales followed up by conducting experiments, many by horrific means, on horses and other animals to obtain the first measurements of blood pressure. These investigations of fluid movement sparked his interest in the movement of the sap in plants.

Hales began to experiment on plants about 1718 (Hoskin, 1961), and published the results as Vegetable Staticks (Hales, 1727, reprinted 1961). His early experiments were measurements of the quantities of moisture taken in by the roots and transpired by the leaves. Using rudimentary laboratory apparatus and hydrostatic principles, he made many attempts to calculate the force with which the sap moves. In his day, the sap was thought to circulate in a manner analogous to the circulation of the blood, moving upward in the inner part of the stem and downward in the outer part (Hoskin, 1961). Hales concluded that the sap probably does not circulate, “…it probably having only a progressive and not a circulating motion, as in animals” (1961 reprint of Hales (1727), p. 6). Hales also found that the rate of transpiration depends in part on the amount of leaf surface area available (1961 reprint of Hales (1727), p. 185). The importance that he attached to leaves is evident in his statement that leaves contain the “main excretory ducts in vegetables,” and that these ducts “separate and carry off the redundant watry fluid” (1961 reprint of Hales (1727), p. 185).

While pursuing his research on sap movement, Hales noticed bubbles of air in the sap. He then showed experimentally that stems draw up air (1961 reprint of Hales (1727), pp. 85–88). Having discovered a role for leaves in transpiration, he was prepared to believe that they might also function in bringing air into the plant. He wrote (1961 reprint of Hales (1727), p. 87) “…it is very probable, that the air freely enters plants, not only with the principal fund of nourishment by the roots, but also thro’ the surface of their trunks and leaves, especially at night, when they are changed from a perspiring to a strongly imbibing state.”

Hales then began to investigate air as a component of a wide variety of plant, animal, and mineral substances. He tested for it by heating materials and capturing the ejected air, using an apparatus of his own devising, called a pneumatic trough (Fig. 30.3). (Later workers modified this device and used it to great advantage in the study of gases. For a history of its development, see Parascandola and Ihde, 1969.) With this apparatus, Hales demonstrated that many substances contained, or held within their pores, large quantities of air. He wrote (1961 reprint of Hales (1727), p. 166), “…we have from these Experiments many manifest proofs of considerable quantities of true permanent air, which are by means of fire and fermentation raised from, and absorbed by animal, vegetable and mineral substances.”

Fig. 30.3.

Stephen Hales’s pneumatic trough (Hales, 1727, Plate 17, Fig. 38). Hales heated materials, by fire, in a bent gun barrel (rr) and collected the “air” that was given off in a large glass globe (a) filled with water and suspended by a rope in a tub of water (xx).

Hales had little idea how to explain his observation that air in its “elastick” (gaseous) state was interconvertible with air that was “fixt” in substances. He wrote (1961 reprint of Hales (1727), p.179), “…they [air particles] are, we see, easily changed from an elastick to a fixt state, by the strong attraction of the acid, sulphureous and saline particles which abound in the air.” By means of fire or fermentation, air that is fixed may, in turn, resume “its former elastick state.” Air to Hales was (1961 reprint of Hales (1727), p.180) “this now fixt, now volatile Proteus among the chymical principles…this much neglected volatile Hermes, who has so often escaped thro’…burst receivers.”

Hales considered air to be a source of nourishment for plants, writing (1961 reprint of Hales (1727), p. xxiv):

The admirable provision she [nature] has made for them [plants], not only vigorously to draw to great heights plenty of nourishment from the earth; but also more sublimed and exalted food from the air, that wonderful fluid, which is of such importance to the life of Vegetables and Animals: And which by infinite combinations with natural bodies, produces innumerable surprizing effects; many instances of which I have here shewn.

Hales says further (1961 reprint of Hales (1727), pp.185–186) “We may therefore reasonably conclude, that one great use of leaves is what has been long suspected by many, viz., to perform in some measure the same office for the support of the vegetable life, that the lungs of animals do, for the support of the animal life; Plants very probably drawing thro’ their leaves some part of their nourishment from the air.”

Sachs wrote (1890 translation of Sachs (1875), pp. 476–477): Hales “was the first who proved, that air co-operates in the building up the body of the plant, in the formation of its solid substance, and that gaseous constituents contribute largely to the nourishment of the plant.” Nevertheless, Hales’s views had little immediate impact on general conceptions of plant nutrition because of the lingering conviction that plants derived their nourishment only from the soil or, as Van Helmont’s willow experiment seemed to demonstrate, water.

In Hales’s day, air was still not understood to consist of distinct gases, and Hales did not look for or discover any. Further, like his contemporaries, Hales believed that air was without chemical properties (Hoskin, 1961). Nonetheless, the discovery that air could be “fixed” was important for pneumatic chemistry and for the development of chemistry in general in the eighteenth century (Brock, 1993).

In addition to his insights into air in plants, Hales speculated, in 1725, about the role of light. He seemed to have sensed that sunlight might have some kind of direct effect on plants, apart from its provision of heat (Gest, 1988). Hales asked (1961 reprint of Hales (1727), pp.186–187):

And may not light also, by freely entring the expanded surfaces of leaves and flowers, contribute much to the ennobling the principles of vegetables? for Sir Isaac Newton puts it as a very probable query, ‘Are not gross bodies and light convertible into one another? and may not bodies receive much of their activity from the particles of light, which enter their ­composition? The change of bodies into light, and of light into ­bodies, is very conformable to the course of nature, which seems delighted with transmutations.’

After publishing Vegetable Staticks (1727), Hales turned to medical investigations. The second edition of Vegetable Staticks appeared in 1731, and 2 years later his (fruitless) work on kidney stones, together with his experiments on blood pressure, were combined into Haemastaticks, which was united with Vegetable Staticks to form a volume titled Statical Essays (Hales, 1733). A second edition of Statical Essays appeared in 1738, and by the time the fourth edition was published, in 1769, Vegetable Staticks had been translated into Dutch, French, German, and Italian (Hoskin, 1961).

IV. Charles Bonnet (1720–1793): A Useful Observation: Bubbles on Submerged Leaves

Following the publication of Hales’s Vegetable Staticks (1727), there was a virtual eclipse – for half a century – in progress towards understanding the biochemistry of plant life (Gest, 1988). During this period, however, Charles Bonnet (Fig. 30.4), in 1747, made an observation that, although unimportant in itself, was to prove useful as an experimental tool for later investigations, notably by Ingen-Housz and Senebier.

Fig. 30.4.

A portrait of Charles Bonnet.

Bonnet was born into a wealthy family in Geneva. He was a mentor and close friend of photosynthesis pioneer Jean Senebier. A lawyer by profession, Bonnet was drawn to natural history (Pilet, 1970). He became interested in the structure and function of leaves and the movement and exchanges of moisture in plants (Kottler, 1973).

In his book Recherches sur l’usage des feuilles dans les plantes (1754), Bonnet reported finding that air bubbles form on submerged, illuminated leaves. Because he observed bubbles on submerged dead as well as living leaves, he dismissed the possibility that the bubbles arose from a process going on within the leaves, despite the larger size of the bubbles on the living leaves. His (incorrect) explanation for bubble formation was that the sun’s heat dilates initially small bubbles that adhere to the plant surfaces (Bonnet, 1754). He did not analyze the air in the bubbles.

V. Joseph Priestley (1733–1804): The Role of Plants in “Purifying” Air, and the Discovery of Oxygen

Each of three men – Joseph Priestley, Carl Wilhelm Scheele, and Antoine Lavoisier (see portraits, Fig. 30.5) – has some claim to the discovery of oxygen. Of the three, Englishman Joseph Priestley (Fig. 30.5a) is most often given the accolade. Priestley also discovered the reciprocal relationship between plant and animal life, mediated by gases, thereby providing support for Hales’s view that air is important for plants. Priestley’s discovery of oxygen, which he achieved independently of his finding of the mutual interdependence of plant and animal life, was pivotal in Lavoisier’s formulation of the “new chemistry.”

Fig. 30.5.

Three researchers having roles in the discovery of oxygen: (a) Joseph Priestley; (b) Carl Wilhelm Scheele; (c) Antoine-Laurent Lavoisier.

Much has been written about Priestley, but Robert Schofield’s two-volume biography, which takes a detailed look at the life and achievements of this versatile man, stands out. Volume I is The Enlightenment of Joseph Priestley: A Study of his Life and Work from 1733 to 1773 (Schofield, 1997); and Volume II is The Enlightened Joseph Priestley: A Study of his Life and Work from 1733 to 1804 (Schofield, 2004). The publication of the second volume, in 2004, marked the bicentennial of Priestley’s death, in 1804. Schofield (1975) provides a shorter biographical account of Priestley.

Priestley was born in Birstal Fieldhead, a small village near Leeds, England. He was trained as a minister, but his career and interests were wide ranging. In addition to being a nonconformist (Unitarian) theologian, he was a teacher, radical political theorist, and chemist (Schofield, 1975). He first ventured into natural philosophy by writing a book on electricity (Priestley, 1767), in which he included some experiments of his own.

By this time, gases had begun to be studied and identified: Van Helmont’s gas sylvestre (carbon dioxide) had been rediscovered by Joseph Black in 1754 and renamed “fixed air” by him (Partington, 1957); and Henry Cavendish (1766) had studied both fixed air and hydrogen, which he called “inflammable air.” Priestley’s interest in gases was kindled when, having taken a job as a minister in Leeds in 1767, he settled next to a brewery, where vats of fermenting beer were producing abundant fixed air. While experimenting on this air, he developed a technique for impregnating water with fixed air, thus artificially preparing soda water (Priestley, 1772b).

Priestley made full use of the pneumatic trough to isolate gases for qualitative study, in contrast to Hales, who had studied “air” only quantitatively (Parascandola and Ihde, 1969). Priestley’s pneumatic trough, like one devised by English physician William Brownrigg (1712–1800) in 1765, included a shelf for collecting gases over water (Badash, 1964; Parascandola and Ihde, 1969; see Fig. 30.6). (Cavendish (1766) had also improved the pneumatic trough and was the first to collect water-soluble gases over mercury; this allowed gases to be collected without loss due to solution in water.) By 1772, Priestley had identified a number of new gases, including nitric oxide (Priestley, 1772b).

Fig. 30.6.

Laboratory apparatus of Joseph Priestley (Priestley, 1775c, plate opposite title page). Pneumatic trough (a) with shelf; trough contains water in which stand jars (cc) containing gases; another jar (c), on the shelf, is receiving gas from a generating bottle (e). The inverted glass (d) contains a mouse; mice are kept for use as in 3. A plant is growing in a gas in 2. Other apparatus for the manipulation of gases are also depicted (Explanation based on Partington, 1957).

Priestley also experimented on the effects of different “airs” on the survival of small animals. Because animals could live only a limited time in an enclosed air space, he concluded that they converted “pure” into “impure” air, and that they could not live in impure air. Similarly, candles were extinguished after a time in an enclosed space. Because he believed strongly in the unity of living things, Priestley expected that a sprig of mint placed in an enclosed space would, like a mouse, render the air impure and that the plant would die. To his surprise, the plant flourished. To his even greater surprise, a growing plant restored air that had been “vitiated” by the burning of a candle. Priestley wrote of his momentous finding (Priestley, 1772b, pp. 166–168):

…I flatter myself that...I have accidentally hit upon a method of restoring air which has been injured by the burning of candles, and that I have discovered at least one of the restoratives which Nature employs for this purpose. It is vegetation.

…One might have imagined that, since common air is necessary to vegetable, as well as to animal life, both plants and animals had affected it in the same manner, and I own I had that expectation, when I first put a sprig of mint into a glass jar, standing inverted in a vessel of water; but when it had continued growing there for some months, I found that the air would neither extinguish a candle, nor was it at all inconvenient to a mouse, which I put into it.

…Finding that candles burn very well in air in which plants had grown a long time, and having had some reason to think, that there was something attending vegetation, which restored air that had been injured by respiration, I thought it was possible that the same process might also restore the air that had been injured by the burning of candles.

Accordingly, on the 17th of August, 1771, I put a sprig of mint into a quantity of air, in which a wax candle had burned out, and found that, on the 27th of the same month, another candle burned perfectly well in it. This experiment I repeated, without the least variation in the event, not less than eight or ten times in the remainder of the summer.

(Note that, in the eighteenth century, the term “vegetation” meant the active process of plant growth and development, rather than today’s meaning of plant life in general; see Table 30.1.)

Priestley obtained the same results using groundsel and spinach in place of mint (Priestley, 1772b, pp. 169–170). He also showed that plants thrive in air made noxious by the respiration of animals or by animals putrefying (Priestley, 1772b). In Volume I of his three-volume work, Experiments and Observations on Different Kinds of Air (Priestley, 1774, 1775a, 1777), Priestley concluded (1774, pp. 86–87), “plants, instead of affecting the air in the same manner with animal respiration, reverse the effects of breathing, and tend to keep the atmosphere sweet and wholesome, when it is become noxious, in consequence of animals either living and breathing, or dying and putrefying in it.”

Priestley was on the trail of something new (oxygen), but at that time he had no idea what he had. He thought that plants, rather than producing a new air, removed from “vitiated air” a “putrid effluvium,” which he equated with phlogiston. He wrote (1772b, pp. 231–232), “May not plants … restore air diminished by putrefaction, by absorbing part of the phlogiston with which it is loaded?… May not this phlogistic matter be even the most essential part of the food and support of both vegetable and animal bodies?”

Even before he isolated oxygen gas, Priestley had developed a chemical test for the “goodness” of air, that is, its concentration of “pure” air (oxygen) (Priestley, 1772b, pp. 210–216). This “nitrous air test,” as he called it, gave him a quantitative tool to use instead of having to rely on the responses of small animals confined to chambers containing the air in question. The nitrous air test was based on the reaction between pure air and nitric oxide (NO), a colorless, insoluble gas: The reaction product, nitrogen dioxide (NO₂), dissolved in water, causing a decrease in the volume of the gas phase. This decrease was a measure of the amount of pure air in the original sample. The nitrous air test was widely used in the 1770s and 1780s and led to the construction of an apparatus that came to be called the eudiometer. (See Golinski (1992) for discussion of this instrument.)

In 1773, the Royal Society of London awarded Priestley its highest honor, the Copley Medal, in recognition of his work in electricity and on the “different kinds of air” (Partington, 1962). Physician John Pringle, in his presidential address to the Society on the occasion of the award, noted both Priestley’s invention of soda water and his discovery of the balance of nature, mediated by gases. A printed form of his address was widely circulated, arousing great interest in Priestley’s work (Nash, 1952). One of those inspired by the finding of complementarity in nature was Jan Ingen-Housz.

The same year in which he received the award, Priestley left Leeds to become librarian and literary companion to William Petty, Second Earl of Shelburne (1737–1805). Priestley and his family settled near Shelburne’s country estate, Bowood House, in Calne, Wiltshire, where they remained for 7 years. Lord Shelburne provided Priestley with laboratory facilities at Bowood House. This post allowed Priestley the leisure to pursue his experiments, and he did his most productive work in the experimental chemistry of gases while there. He discovered many new gases, including oxygen, and published his findings prolifically.

One method Priestley used to study airs was to heat chemicals, such as calxes (metal oxides), by means of a large convex lens, or “burning glass” (Fig. 30.7). With the lens, which was a time-honored instrument for heating chemicals, he focused the sun’s rays onto substances he had placed in the receiver of a pneumatic trough, and heated the materials hot enough that “air” could be extracted from them. He then collected the air over water or mercury and analyzed it. The chemical that led Priestley to oxygen was mercuric oxide (HgO), also known as red precipitate or mercurius calcinatus per se. The behavior of this compound had long puzzled chemists. Most metal oxides can be reduced to the metal only by heating them with a reducing agent, such as charcoal. In the process, the oxygen in the oxide combines with the carbon of the charcoal, thus yielding carbon dioxide and the metal (Brock, 1993). Mercuric oxide, in contrast, can be converted into the metal mercury when heated quite hot without charcoal. Oxygen is released in the process (Conant, 1950).

Fig. 30.7.

A compound burning-glass used by Joseph Priestley. The lenses are 16 and 7 in. in diameter, 16 in. apart. The glass is housed in the Archives and Special Collections, Dickinson College, Carlisle, Pennsylvania (Photo by the author).

Priestley describes his experiment with red oxide of mercury and his surprise at the results (Priestley, 1776, pp. 33–34):

Having … procured a lens of twelve inches diameter, and twenty inches focal distance, I proceeded with great alacrity to examine, by the help of it, what kind of air a great variety of substances, natural and factitious, would yield, putting them into the vessels … which I filled with quick-silver, and kept inverted in a bason of the same….

With this apparatus, after a variety of other experiments, … on the 1st of August, 1774, I endeavoured to extract air from mercurius calcinatus per se; and I presently found that, by means of this lens, air was expelled from it very readily. Having got about three or four times as much as the bulk of my materials, I admitted water to it, and found that it was not imbibed by it. But what surprised me more than I can well express, was, that a candle burned in this air with a remarkably vigorous flame… I was utterly at a loss how to account for it.

Priestley at first believed that he had obtained “modified nitrous air” (N₂O, or nitrous oxide) (Priestley, 1776, p. 39), an air that he knew supported combustion, as did this gas. Then, on a trip to France with Lord Shelburne in October 1774, Priestley told Lavoisier and others of the air he had obtained with a burning glass, and how it supported combustion. Clearly this was not fixed air, which would have been formed by the usual method of reducing a calx. Although Priestley had not grasped the importance of this air, Lavoisier saw its likely significance for the new system of chemistry he was developing.

Early in 1775, after discovering that mice lived longer in the air derived from mercuric oxide than they did in an equal volume of ordinary air, Priestley finally concluded that he had discovered something new. He called it dephlogisticated air (Priestley, 1775b), because he thought that it was air from which phlogiston had been removed and that could therefore readily absorb more of that substance. Ironically, although his discovery of oxygen was a central event in the ultimate demise of the phlogiston theory, Priestley himself persisted in explaining his oxygen experiments in terms of phlogiston. He is generally credited with the discovery of oxygen but faulted for not recognizing its significance.

In 1777, Priestley resumed his experimentation on plants upon learning that several investigators, notably Carl Wilhelm Scheele, had been unable to duplicate Priestley’s earlier finding that plants “improve” the air. During this new round of plant studies, Priestley obtained mostly negative results, in marked contrast to his initial successes. He did not understand that his failures were due to poor experimental conditions, and, at this point, he more or less lost his way in plant research. Priestley and others were unable to demonstrate consistent oxygen production by plants because they were unaware of the requirement for light (Gest, 2000).

Priestley published his new results in the first volume of his second three-volume physicochemical work, Experiments and Observa­tions Relating to Various Branches of Natural Philosophy (Priestley, 1779, 1781, 1786). Sounding less confident than previously, he nonetheless maintained (1779, p. 302), “Upon the whole, I still think it probable that the vegetation of healthy plants, growing in situations natural to them, has a salutary effect on the air in which they grow.”

Priestley also had difficulty interpreting what he called the “green matter” (actually, microscopic green algae) that developed in the jars of pump water in which he grew his experimental plants. He at first did not identify this matter as living (Priestley 1779, p. 342). He also had difficulty recognizing that the green matter was a source of dephlogisticated air. At one point, he even thought that water under the influence of light produced this air (Priestley, 1779, p. 349). He did not satisfy himself until late in 1779 that the green matter is a plant, that it converts impure air contained in the water into pure, or “dephlogisticated,” air, and that light is necessary for this process. Before he reported these findings (Priestley, 1781, pp. 16–17, 21–25), however, Jan Ingen-Housz (1779) beat him to publication on the light requirement.

In 1780 Priestley left Calne for a ministerial post in Birmingham, England, where he resumed his experiments on gases. He also completed his second series of Experiments and Observations and published new editions of earlier works. In 1790, he published a revised, abridged edition of his two, three-volume works as Experiments and Observations on Different Kinds of Air and other Branches of Natural Philosophy.

Then, in 1791, Priestley’s life took a difficult turn. An ardent religious and political nonconformist, he was often in conflict with the authorities. Because of his support for the French Revolution, his house and laboratory in Birmingham were burned by an anti-radical, anti-republican, “church and king” mob. In 1794, he emigrated to the United States of America. He settled at Northumberland, Pennsylvania, where he lived until his death, in 1804 (Fig. 30.8). He resumed his chemical experimentation and wrote prolifically in his new land, especially in defense of the phlogiston theory, but he had little new to contribute, other than the discovery of carbon monoxide. His work was largely ignored (Schofield, 2004).

Fig. 30.8.

Joseph Priestley House, Northumberland, Pennsylvania, where Priestley lived from 1794 until his death in 1804; now a Pennsylvania State Museum (Photo by the author).

VI. Carl Wilhelm Scheele (1742–1786): Early Identifier of Oxygen

There is little doubt that the Swedish apothecary Carl Wilhelm Scheele (Fig. 30.5b) discovered oxygen about 1772, 2 years before Priestley did. He did not publish his discovery for several years, however.

Scheele was born in Stralsund, Pomerania, Germany, a city that was in Swedish hands at the time (Hoffman and Torrence, 1993). He was a modest man whose circumstances were often poor, and he used simple laboratory equipment (Partington, 1957), yet he was an outstanding experimental chemist (Hoffman and Torrence, 1993). Scheele discovered many acids and several elements. Biographical information is available in Boklund (1975).

Scheele achieved the isolation of oxygen through the decomposition of various compounds, including nitric acid, saltpeter (potassium nitrate), manganese dioxide–and mercuric oxide, the compound that later led Priestley to oxygen (Partington, 1957). Due to delays in publication, however, his book, Chemische Abhandlung von der Luft und dem Feuer (Scheele and Bergman, 1777), did not appear until at least 4 years after he had made the discovery, and 2 years after Priestley had published his own (Priestley, 1775b). Thus Scheele did not receive credit for the discovery.

Scheele states in his book that the atmosphere is composed of two gases: “fire air,” which supports combustion, and “foul air” (nitrogen), which prevents it. Unlike Priestley and Lavoisier, however, Scheele did not demonstrate the importance of the new gas in respiration and combustion. Further, his attempts to confirm its production by plants failed.

Nearly 3 years before his book was published, Scheele communicated his finding of the new gas to Lavoisier, in a letter written on September 30, 1774. Lavoisier received the letter in mid-October 1774. No reply from Lavoisier is known (Partington, 1962). Swedish historians of science have not forgiven Lavoisier for his failure to respond (Poirier, 1996). Lavoisier gave neither Scheele nor Priestley credit for the discovery.

VII. Antoine-Laurent Lavoisier (1743–1794): The “New Chemistry”

During the 1770s and 1780s, the new, more systematic and quantitative chemistry developed by Antoine-Laurent Lavoisier (Fig. 30.5c) and his colleagues replaced the older concepts, including the notions of the four Aristotelian “elements,” the Paracelsian chemical “principles,” and the phlogiston theory. Lavoisier’s quantitative approach did not represent a complete break with the past, however, but rather a difference of degree and of technical expertise (Newman and Principe, 2002).

Lavoisier was born in Paris, the son of a wealthy French magistrate. He attended the College Mazarin, in Paris, and then was trained as a lawyer. Like Priestley, he had wide-ranging careers: He was a geologist, tax collector, administrator, and chemist. His life and work are the subject of two in-depth biographies published during the 1990s, approximately coinciding with the bicentenary of Lavoisier’s death, in 1794. These works are: Antoine Lavoisier: Science, Administration and Revolution, by Arthur Donovan (1993); and Lavoisier: Chemist, Biologist, Economist, by Jean-Pierre Poirier (1996).

Lavoisier himself did no experimental work with plants. His chemical ideas, however, were gaining acceptance at the time that photosynthesis pioneers Jan Ingen-Housz and Jean Senebier were carrying out their experiments, and the new concepts influenced their work. Nicholas de Saussure, who came later, had the full benefit of the new ideas in chemistry and was able to use them skillfully.

Early in his career in chemistry, Lavoisier recognized the need to test the long-held notion that the four Aristotelian elements could be transmuted into one another through such processes as heating, fermentation, and plant growth. He knew that if they could, then these processes could not be used to determine the quantities of the elements present in various compounds. Lavoisier charged that previous investigators had “pretended” to prove that water could be transmuted into earth, by two kinds of experiments: growing plants in water as Van Helmont had done; and repetitive distillation of water in a glass vessel, which led to a slight earthy residue (Lavoisier, 1770). Pointing to Hales’s (1727) largely forgotten finding that the solid parts of plants contain a great deal of air, Lavoisier noted that Van Helmont and others had overlooked air as a possible source of vegetative weight gain. Lavoisier concluded, “The experiments on vegetative assimilation of water prove nothing regarding the possibility of changing water into earth” (Lavoisier, 1770, p. 8, as translated by Donovan, 1993, p 93). In addition, Lavoisier demonstrated that the small but significant residue left after repeated distillation of water came from the glass of the vessel, not from a transmutation of water (Lavoisier, 1770).

By the time Lavoisier began his work in chemistry, the atmosphere was coming to be seen as a mixture of different kinds of gases rather than as a single element. Further, these gases were being found to be able to enter into chemical reactions. This knowledge was pivotal in Lavoisier’s challenge to the phlogiston theory, even before the discovery of oxygen. He recognized that a major problem with the theory was that, during calcination, metals gained weight rather than losing it as one might expect if phlogiston were being released from the metal during the process. Based on the knowledge that most calxes (oxides) can be reduced to the metal only by heating them with a reducing agent, such as charcoal, thereby producing fixed air, Lavoisier supposed that the calxes might originally have been formed by a combination of the metal with fixed air, but he had been unable to prove it (Conant, 1950; Brock, 1993).

After receiving the tip from Priestley, and presumably having read Scheele’s letter claiming discovery of “fire-air,” Lavoisier performed experiments showing that heating mercuric oxide with charcoal yielded fixed air, which did not support combustion, whereas heating this oxide by means of a burning glass yielded an air that supported a vigorous flame (Partington, 1962). He concluded that, if other calxes could be reduced without addition of carbon, they, too, would give this kind of air.

At first Lavoisier reported that the weight gained by a metal in forming a calx is due to the reaction of the metal with air in itself, but he later modified this view, saying that the reaction was a result of the addition to the metal of “the purest part of the very air which surrounds us, which we breathe” (Lavoisier, 1775, p. 127, as translated by Conant, 1950, p. 27). In Lavoisier’s view, calcination (and combustion, as well) was the combination of substances with oxygen. The remainder of the atmosphere, which he called “azote” and we know as nitrogen, was nonreactive. His findings cast serious doubt on the phlogiston theory because he had shown that a metal could be regenerated from a calx without a source of phlogiston in the form of charcoal (Brock, 1993). Lavoisier has been criticized for not giving Priestley and Scheele due credit for the discovery of oxygen, but Lavoisier understood the meaning of the finding in a way that had eluded both of them (Conant, 1950).

In 1777, Lavoisier renamed the gas that Priestley had called dephlogisticated air “principe oxygène” (oxygène meaning “acid former” in Greek; thus, “acid-forming principle”) (Partington, 1957). Even though he was wrong that all acids contain oxygen, the misnomer, oxygen, stuck.

Lavoisier built his new chemical system on the law of the conservation of mass, first advanced in 1748 by Mikhail Lomonosov (1711–1765) (Kedrov, 1973). Lavoisier also based his system on the idea of persistent, well-defined elements (Nash, 1952), which were not decomposable by any known means of analysis. Lavoisier and several chemical colleagues published Méthode de nomenclature chimique (Morveau et al., 1787), in which they renamed chemical substances so as to bring them into line with the new chemical theory. This helped in the acceptance of the theory, as did Lavoisier’s magnum opus, Traité élémentaire de Chimie (1789) (see Klein and Lefèvre, 2007, on the interplay of theory with classification and nomenclature in the chemical revolution). In the Traité, which was highly influential and used as a chemistry textbook for decades, Lavoisier states (p. 149, as translated by Partington, 1957, p. 124):

…for nothing is created in the operations either of art or of nature, and it can be taken as an axiom that in every operation an equal quantity of matter exists both before and after the operation, that the quality and quantity of the principles remain the same and that only changes and modifications occur. The whole art of making experiments in chemistry is founded on this principle: we must always suppose an exact equality or equation between the principles of the body examined and those of the products of its analysis.

During the late 1770s and the 1780s, Lavoisier and his collaborators made a number of chemical discoveries that were of great importance for the understanding of plant and animal physiology. These included the discovery that fixed air is composed of carbon and oxygen, and that water, rather than being an element, is a compound of hydrogen and oxygen. These investigators also contributed to the understanding of respiration, through studies on humans and guinea pigs (Partington, 1957).

Unfortunately for Lavoisier (and for chemistry), his achievements could not save him from the wrath of the French Revolution’s Reign of Terror. In 1794, he was beheaded, on a trumped-up charge related to his activities as a tax collector and a director of the French Gunpowder Administration. Hoffman and Torrence (1993, p. 91) comment that Lavoisier “died at the hands of the perversion of the revolution that Priestley supported, and for which the latter was hounded out of his native country.”

Thomas Kuhn, in his influential book The Structure of Scientific Revolutions (1996), ranks the oxygen theory of combustion as a “paradigm shift.” He says (p. 56), “What the work on oxygen did was to give much additional form and structure to Lavoisier’s earlier sense that something was amiss. It told him a thing he was already prepared to discover – the nature of the substance that combustion removes from the atmosphere.”

Several recent works of popular literature have focused on subjects related to the new chemistry and to oxygen and its discovery. In Oxygen: A Play in Two Acts (2001), chemists Carl Djerassi and Roald Hoffmann address the question who deserves credit for the discovery of oxygen. In Oxygen: The Molecule that Made the World (2002), Nick Lane discusses the role of oxygen in geological and evolutionary history and in aging and disease. Other works include Madison Smartt Bell’s biography Lavoisier in the Year One: The Birth of a New Science in an Age of Revolution (2005), and Joe Jackson’s A World on Fire: \A Heretic, an Aristocrat, and the Race to Discover Oxygen (2005).

VIII. Jan Ingen-Housz (1730–1799): The Role of Light, and the Importance of Plants’ Green Color

The discovery of the reciprocal nature of plant and animal life, mediated by gases (Priestley, 1772b), seems to have inspired at least three independent researchers in plant nutrition–Jan Ingen-Housz, Jean Senebier, and Nicholas de Saussure (Hill, 1970). Less well known than Priestley and Lavoisier, these men have often had their discoveries overlooked or misstated, or misattributed, either to one another or to Priestley.

The first of the three to make a significant contribution was the Dutch physician Jan Ingen-Housz (Fig. 30.2c). His most important discovery was that plants require light in order to produce dephlogisticated air (oxygen). He also showed that leaves are the primary sites of the formation of dephlogisticated air, and he discovered plant respiration.

Ingen-Housz’s life and work are detailed in Julius von Wiesner’s biography, Jan Ingen-Housz. Sein Leben und Sein Wirken als Naturforscher und Arzt (1905) and Magiels (2010). Shorter accounts include those by Harvey and Harvey (1930), Reed (1949), Van der Pas (1973), Smit (1980), Gest (2000), and Magiels (2007).

Ingen-Housz was born in Breda, The Netherlands. His father was a leather merchant and, after 1755, may have been a pharmacist. British physician John Pringle mentored the young Ingen-Housz, who earned a medical degree from the Catholic University of Louvain, Belgium, in 1753. Ingen-Housz then pursued further studies at the University of Leiden and possibly at the universities of Paris and Edinburgh (Van der Pas, 1973). He practiced medicine in Breda until 1765, when, at the invitation of Pringle, he moved to London to learn the then-new art of smallpox inoculation (Beale and Beale, 2001). The technique involved the use of live smallpox virus. (Although viruses had not been discovered yet, it was known that serum taken from an infected person and injected subcutaneously in an uninfected person provided protection. This method was superseded by the introduction of vaccination with cowpox virus, by English physician Edward Jenner, in 1798.) In 1768, British King George III sent Ingen-Housz to Vienna to inoculate the Habsburg royal family. Ingen-Housz’s success at this endeavor so impressed Empress Maria Theresa that she appointed him court physician and endowed him with a life-long annual income (Van der Pas, 1973).

Upon reading Pringle’s address to the Royal Society on the occasion of the awarding of the Copley Medal to Joseph Priestley, in 1773, Ingen-Housz became interested in the ability of plants to purify the air. He later wrote (Ingen-Housz, 1779, p. xv) of being impressed at Priestley’s discovery that “plants wonderfully thrive in putrid air; and that the vegetation of a plant could correct air fouled by the burning of a candle.” In the summer of 1779, having obtained a leave of absence from the Austrian court, Ingen-Housz went to England. There, at a country house at Southall Green, near London, he performed more than 500 experiments in less than 3 months’ time (Ingen-Housz, 1779, p. xlii).

Ingen-Housz reported his discoveries almost immediately, in the book Experiments Upon Vegetables, Discovering Their Great Power of Purifying the Common Air in the Sun-shine, and of Injuring it in the Shade and at Night (1779). The book was an immediate success. He himself translated it into French (Ingen-Housz, 1780). He later published a second French edition, in two volumes (Vol. I in 1787, Vol. II in 1789). In 1949, Howard S. Reed republished the text of Ingen-Housz (1779) in combined issues of Chronica Botanica, with a few interpolations from the French edition of 1787. Reed omitted Ingen-Housz’s experimental protocols but added extensive commentaries of his own.

Ingen-Housz made an important innovation in experimental technique. Based on Charles Bonnet’s (1754) observation that bubbles form on submerged, illuminated, green leaves, Ingen-Housz suspected that detached leaves might perform the function that Priestley had regarded as an activity of the plant as a whole. Using eudiometry, Ingen-Housz determined that the “air” produced by submerged leaves was dephlogisticated air. He then proceeded to do most of his experimental work with such leaves. This substitution, because of its simplicity, enabled him to make observations that had eluded Priestley (Nash, 1952).

Ingen-Housz (1779, pp. 14–16), in explaining his techniques and the resulting formation of bubbles, wrote that leaves:

…are to be put in a very transparent glass vessel, or jar, filled with fresh pump water;… which, being inverted in a tub full of the same water, is to be immediately exposed to the open air, or rather to the sun-shine: thus the leaves continuing to live, continue also to perform the office they performed out of the water, as far as the water does not obstruct it. The water prevents only new atmospheric air being absorbed by the leaves, but does not prevent that air, which already existed in the leaves, from oozing out. This air, prepared in the leaves by the influence of the light of the sun, appears soon upon the surface of the leaves in different forms, most generally in the form of round bubbles, which … rise up and settle at the inverted bottom of the jar: they are succeeded by new bubbles, till the leaves, not being in the way of supplying themselves with new atmospheric air, become exhausted. This air, gathered in this manner, is really dephlogisticated air...  .

Ingen-Housz used Priestley’s pneumatic trough and nitrous air test. He controlled the illumination more carefully than had his predecessors, however. By using more intense illumination, he was able to carry out the experiments more rapidly, before green matter could develop and improve the air, or putrefaction could vitiate it (Nash, 1952). He said that Priestley’s and Scheele’s failures with plants (Ingen-Housz, 1779, p. 45) occurred because “These gentlemen expected the good effects from the vegetation of the plants, as such. By making a plant grow night and day in ordinary air kept in a phial with the plant, the effect will depend upon the greater or less exposure of the plant to the light.” Ingen-Housz stated his belief (1779, p. 44) that “the faculty which plants possess of yielding dephlogisticated air, of correcting foul air, and improving ordinary air” is attributable to the influence of sunlight on a chemical process within the leaf. He pointed out that if the process of vegetation were responsible, as Priestley claimed, then dephlogisticated air would continue to be produced even in the dark, since plants are known to grow in the absence of light (as in etiolation). Plants do not produce dephlogisticated air in the dark, however.

For many years, Ingen-Housz did not understand that fixed air was important in plant physiology. He nowhere stated in his 1779 book that fixed air gives rise to dephlogisticated air, and he quarreled over this point with Senebier, who strongly believed that it did. Further, even though sunlight was presumably operating both for plants immersed in water and for those growing naturally in the atmosphere, Ingen-Housz did not regard the two situations as comparable. He thought that plants not growing under water purify the atmosphere by removing its phlogiston, but for plants growing under water, the mechanism was a transmutation brought about by light, with no involvement of phlogiston. He seems to have believed that the dephlogisticated air produced by plants submerged in water came from a transmutation of the water itself rather than from a gas in the water (Kottler, 1973).

Ingen-Housz thought that the phlogiston that plants obtained from the atmosphere served them nutritionally. He wrote (1779, pp. 74–75):

Vegetables seem to draw the most part of their juices from the earth, by their spreading roots; and their phlogistic matter chiefly from the atmosphere, from which they absorb the air as it exists. They elaborate this air in the substance of their leaves, separating from it what is wanted for their own nourishment, viz., the phlogiston, and throwing out the remainder, thus deprived of its inflammable principle, as an excrementous fluid….

Ingen-Housz made another misinterpretation in assuming that dephlogisticated air is highly soluble in water. He did not test this assumption (though Senebier later did). Instead, Ingen-Housz thought that bubbles appeared on leaves if the water was saturated with air of any kind, but if the water was unsaturated, the dephlogisticated air simply dissolved in the water without forming bubbles (Nash, 1952).

Despite such interpretive errors, Ingen-Housz refined the knowledge of plant nutrition in several ways, in addition to his discovery of the light requirement. He wrote (1779, p. xxxiv–xxxv): “I found …that this office [the production of dephlogisticated air] is not performed by the whole plant, but only by the leaves and the green stalks that support them.” He also recognized the green matter as a plant before Priestley did, and he demonstrated that, as the intensity of the illumination increased, plants produced more dephlogisticated air. Further, he found that, contrary to Bonnet’s (1754) hypothesis, it is the sun’s light, not its heat, that leads to the liberation of gas bubbles from submerged leaves. He thereby confirmed Hales’s suspicion that sunlight has some effect on plants other than through warming them. Ingen-Housz wrote (1779, pp. 29–30):

I placed some leaves in pump water, inverted the jar, and kept it as near the fire as was required to receive a moderate warmth, near as much as a similar jar, filled with leaves of the same plant, and placed in the open air, at the same time received from the sun. The result was, that the air obtained by the fire was very bad, and that obtained in the sun was dephlogisticated air.

Ingen-Housz also discovered that plants perform respiration, as Lavoisier (1777) had demonstrated for animals. Ingen-Housz (1779) showed not only that all parts of plants vitiate the air in the absence of light, but also that the non-green parts, such as the stems and roots, vitiate it in the daylight as well. Senebier did not accept Ingen-Housz’s finding of plant respiration, and this caused further quarreling between them.

Ingen-Housz considered vitiated air to be highly toxic, and greatly exaggerated its dangers. Höxtermann (2007, p. 147, as translated from the German by K. Nickelsen) quotes Ingen-Housz (1786, p. Lvf.):

I observed … that in general fruit at any time keep this noxious property, especially in the dark, and that this poisonous property goes so far that even the most delicious fruit, such as peaches, are able to contaminate the air in a single night, so that one would be in mortal danger if one were locked up in a small room with a great many of these fruit.

He discovered, however, that the production of pure air normally more than compensates for the production of this poisonous air. He says (1779, p. 47), “The plants evaporate by night bad air, and foul the common air which surrounds them; yet this is far over-balanced by their beneficial operation during the day.” Had Ingen-Housz used whole plants in his experiments, as had Priestley, he would have had a much more difficult time discovering this opposition of activities (Nash, 1952).

In 1780, Ingen-Housz returned to the Viennese court, where he pursued plant research as time permitted. He incorporated some of his new findings in the two-volume, French edition of his book (Ingen-Housz, 1787, 1789). In this edition, he still did not acknowledge that fixed air is the ultimate source of the dephlogisticated air produced by plants.

In 1788 Ingen-Housz went back to England for good. According to Harvey and Harvey (1930), he wished throughout this period to return to Vienna. Conley and Brewer-Anderson (1997), however, point to letters he wrote to his friend Benjamin Franklin in which he confided that he had chafed at his confinement at the court, which he was permitted to leave only with permission of the emperor. His correspondence with Franklin reveals further that, although he received a good income from the court, he suffered financial difficulties due to unwise investments, and as a result was unable to fulfill his wish to emigrate to the United States in his later years to be near Franklin (Conley and Brewer-Anderson, 1997). During his final years in England, Ingen-Housz spent some of his time at Bowood House, the estate of Priestley’s former patron, the Earl of Shelburne (who in 1784 had become the First Marquess of Lansdowne). In a laboratory that Lord Lansdowne equipped for him there, which included some of Priestley’s apparatus (Gest, 2000), Ingen-Housz resumed his research on plant nutrition (Harvey and Harvey, 1930).

It was during these years in England that Ingen-Housz wrote his second, and last, important work in plant physiology: An Essay on the Food of Plants and the Renovation of Soils (1796). In this short work, which appeared as an appendix to an obscure publication by the English Board of Agriculture, Ingen-Housz re-interpreted his earlier results in conformity with the new chemistry developed by Lavoisier. Fixed air became carbon dioxide, and pure (vital or dephlogisticated) air became oxygen. This was the first publication to describe clearly the importance of the process of gas exchange for plant nutrition.

Oddly, Ingen-Housz (1796, p. 16) claimed to have originated the idea that carbon dioxide is a plant nutrient. Nash (1952, pp. 104–105) quotes that passage, adding his own sarcastic, parenthetical comment:

I inferred from these, and some other facts quoted before, that the plants in the common course of nature draw from the air, in a great measure, what is necessary for their subsistence; and that being thus incessantly occupied in decomposing the common air, they render a part of it miscible with the ground, or with substances inherent in the earth, such as moisture, salts, &c.; that the carbonic acid, which is now admitted (according to my original idea) [this is rather cool: Senebier’s priority is completely ignored] as a nourishing substance for plants, is prepared without intermission, day and night by the roots and flowers, and in the night by the leaves and the rest of the whole plant, must have been destined by nature to some important use for the plants themselves….

Further arguing that plants obtain their carbon mainly from carbon dioxide in the atmosphere rather than from the soil solution, Ingen-Housz wrote (1796, p. 9):

…there is no difficulty in... conceiving how the largest tree finds, during centuries, that immense quantity of food it requires for its maintenance, growth, and abundant production of fruit or seed, all which is certainly derived in part from the soil; but I still believe chiefly from the atmosphere, by means of the leaves absorbing and decomposing the air in contact with them.

Ingen-Housz mistakenly thought, however, that plants take up nitrogen from the atmosphere (1796, p. 10).

Like his 1779 book, which was reprinted by Reed (1949), Ingen-Housz (1796) saw its way into a twentieth-century reprinting: J. Christian Bay (1933) printed 100 copies for private distribution. Gest (1997) points out that the reprint omits the marginal notes and comments that Ingen-Housz had included in his original publication.

Ingen-Housz’s achievements in plant physiology have, according to some authors, been overlooked and neglected. Gest (1991) ventures that this was “in no small measure because Priestley’s prolific writings obfuscated the great advances made by the Dutch physician.” Magiels (2007) also maintains that Ingen-Housz did not receive due credit. Gest (2000), after analyzing letters and other writings of Priestley, concluded that Priestley could refer only to his letters, not to any “publication” in the generally accepted sense, indicating a discovery by him of the light requirement, predating Ingen-Housz (1779).

Ingen-Housz (1796, pp. 2–3) asserted his priority in this discovery:

I was fortunate enough to discover the true reason, why plants did sometimes correct bad air, and sometimes made it worse, which reason was never so much as even suspected either by Dr. Priestley or by Scheele; and indeed if either of them had had the least suspicion of it, their known eagerness for fame would not have allowed them to keep the discovery from the public eye, and Dr. Priestley would not have gone much farther than Mr. Scheele did; viz. to acknowledge openly, (even in his book printed 1779,) that he had been mistaken, and that he was entirely ignorant of the reason why vegetables are so inconstant in their effects on the air in contact with them.

With all his quarreling with colleagues, Ingen-Housz’s personality has been a subject of discussion. There is considerable disagreement. Van der Pas (1973) characterizes him as a “shy, kind man.” Schofield (1966, p. 360) comments that he was a conformist socially and religiously and had “the genial manners and social graces that Priestley lacked.” Smit (1980) claims that Ingen-Housz “did not have the character of a fighter” so did not defend his priority in his discoveries.

In contrast, Beale and Beale (2005) state that, although Ingen-Housz may have been mild-mannered in social affairs, he was also called pompous and dictatorial, including in his relations with Edward Jenner. There is evidence, Beale and Beale say (p. 97), that Ingen-Housz could be “stubborn and persistent in scientific disputes. For example, he doggedly fought his corner after discoveries of his own had been claimed by others such as Joseph Priestley and Jean Senebier.” Rabinowitch (1945, p. 18) claims that Ingen-Housz realized the importance of his discovery of the light requirement “and was decided not to let anybody deprive him of it or even as much as share in it.” In the view of Partington (1962, p. 282), Ingen-Housz’s defensiveness may have been justified: “Priestley and Senebier, both ministers of religion, seem to have been anxious to give Ingen-Housz as little credit for his work as they possibly could.”

Although Ingen-Housz published before Priestley on the light requirement, Schofield (2004, pp. 155–156) notes that Priestley could have discovered this requirement on his own during the interval between the completion of Ingen-Housz’s book in October 1779 and the appearance of the printed volume later that year. Schofield (2004) points out that Priestley may not have seen that book when he wrote, on December 12, 1779, to a friend, “I soon discovered that the ‘green matter’ …is a vegetable substance, and that all other water plants do the same, converting the impure air contained in water into pure air, and therefore I conclude that all plants do the same in the light.” Nash (1952) offers a similar opinion. Priestley never acknowledged Ingen-Housz’s priority in the discovery of the light requirement.

Because Ingen-Housz had priority in publication, some scholars credit him with the discovery of photosynthesis. Others give the credit to Priestley–for example, Hill (1972)–or to Senebier. In reality, however, a number of individuals contributed, supplying various pieces of the puzzle, and no single person is responsible for the “discovery” of photosynthesis as such.

Ingen-Housz died at Bowood House, Calne, England. In 1999, a ceremony there marked the bicentennial of his death. Gest (2000) reported on the event.

IX. Jean Senebier (1742–1809): The Role of Carbon Dioxide

Swiss clergyman and naturalist Jean Senebier (Fig. 30.2d) provided evidence that plants must have access to fixed air (carbon dioxide) in order to produce dephlogisticated air (oxygen). He demonstrated further that the amount of dephlogisticated air that a plant produces is related to the amount of fixed air available (Senebier, 1782). He thus corrected the interpretation of Ingen-Housz (1779), who did not concede the importance of carbon dioxide until 1796, 14 years after Senebier had published on it.

Biographical information about Senebier is sparse (Kottler, 1973). Bay (1931), Kottler (1973), and Pilet (1975a) provide some background. Senebier was born in Geneva, the son of a wealthy merchant. Although he was interested in natural philosophy, his family directed him into the ministry. He was ordained a minister in Geneva in 1765. In 1769 he became pastor of a church in Chancy, near Geneva. In 1773, he left that post to become librarian for the Republic of Geneva, a position he held for the rest of his life (Bay, 1931; Pilet, 1975a).

The young Senebier was attracted to the circle inspired and guided by Charles Bonnet (Bay, 1931). Bonnet enabled Senebier to perform his first experiments in plant physiology (Pilet, 1975a). Following Bonnet’s advice, in 1769 Senebier answered a question on the art of observing, posed by the Haarlem Academy of Sciences (Kottler, 1973). He published an expanded version of his essay as L’Art d’observer (Senebier, 1775), and subsequently a lengthier edition (Senebier, 1802). According to Bay (1931), Senebier’s book was the first systematic attempt at a philosophy of the art of experimentation.

In the late 1770s, Senebier wrote several memoirs on phlogiston in the economy of nature. Then he turned to the nature of light and its interactions with natural objects, including plants, and began experimenting on etiolation (Kottler, 1973). Upon reading the French translation of Experiments Upon Vegetables (Ingen-Housz, 1780), Senebier’s experimental focus veered to gas exchange in plants. He repeated nearly all of Ingen-Housz’s 1779 experiments (Nash, 1952). Senebier claimed to have written of his ideas to Bonnet before he saw Ingen-Housz’s book, although he admits he made use of the book. His publications mention Ingen-Housz only in passing (Partington, 1962).

To study air production by plants, Senebier used an experimental setup similar to Ingen-Housz’s: leaves submerged in water (Fig. 30.9). He repeated the same experiments hundreds and hundreds of times, “now with reference to Ingen-Housz, then testing the exactness of Priestley’s work, then again striking a new track” (Bay, 1931). Senebier published voluminously on his results. Most of these results were reported in four works (Senebier, 1782, 1783, 1788, 1800). His other works in plant physiology include Senebier (1791, 1792).

Fig. 30.9.

Jean Senebier’s laboratory apparatus for studying production of dephlogisticated air by submerged leaves (Senebier, 1782, Plate I, Fig. 2). AB, receptacle with glass tube, closed at A and filled with water to D; CDEFGH, divisions on tube; IK, saucer filled with water, on which the receptacle containing water and the leaf is placed.

Senebier’s important findings on the exchange of gases between plants and the atmosphere are contained mostly in Volume I of Mémoires physico-chimiques sur l’influence de la lumière solaire pour modifier les êtres des trois règnes de la nature, & sur-tout ceux du règne vegetal (1782). Volumes II and III of that work present his views on the influence of light on plants (Kottler, 1973). With characteristic prolixity, Senebier devoted more than 400 pages of his books of 1782 and 1783 to his finding of proportionality between the amount of air present in the water at the outset and the amount given off by the leaves (Nash, 1952). He states (1782, Vol. I, pp. 178–179):

It was presently a matter of knowing, if really the waters the most charged with fixed air were also those that made leaves, that were plunged into them and then exposed to the sun, render the largest quantity of pure air. To discover this, I filled several recipients with boiled water: I introduced in some one measure of fixed air…; I introduced into others two measures of this air, into others three, to others four, etc. When this air was absorbed, I passed leaves under each of them. I then exposed them to the action of the sun; and I found that … the quantity of air, produced by the leaves, had accrued in proportion to the numbers of measures of fixed air previously absorbed by the water…

Fixed air is present in significant concentrations in water that has been exposed to the atmosphere for some time because, even though fixed air composes only a tiny percentage of the atmosphere, it is highly soluble in water. Thus some dephlogisticated air can always be produced by the illumination of leaves immersed in water that has been in contact with the atmosphere (Nash, 1952). Fixed air is especially abundant in pump water, which Senebier and Ingen-Housz used in many of their experiments.

When his experimental plants ceased to produce dephlogisticated air, a change of the water led to renewed production but a change of the leaves did not. He wrote (Senebier, 1783, p. 326), “I am convinced that the quantity of fixed air contained in the water was strongly diminished, when the leaves that I exposed to the sun had furnished their air.” He also wrote (Senebier, 1783, pp. 15–16), “...fresh leaves, exposed anew to the sun in water where this has already been going on, while sunlight acted on them, furnished much less air than the first.”

Because Senebier repeated most of Ingen-Housz’s experiments, it is not surprising that many of the findings of these two researchers were similar. Both men found that abundant dephlogisticated air is produced by leaves submerged in pump water, but none is formed by leaves submerged in water that has been successively distilled and boiled (thus containing no fixed air) (Nash, 1952). The similarity of Senebier’s experiments to Ingen-Housz’s, and Senebier’s failure to acknowledge the similarities and differences (Magiels, 2007), irked Ingen-Housz. Rabinowitch (1945, p. 20) observed, “Ever after he [Senebier] found himself exposed to the merciless irony and clever insinuations of Ingen-Housz, whose wrath would not be assuaged by the long-winded explanations of the Swiss pastor. The subsequent publications of both adversaries… are filled with acid polemics, and make sad reading.”

In contrast to Ingen-Housz’s denial of the role of fixed air, Senebier concluded that both submerged leaves and plants growing in their natural environment require fixed air in order to produce dephlogisticated air (Kottler, 1973). This led to further quarreling. They also argued over plant respiration. Senebier (1782) maintained that the gas produced in the dark by submerged, experimental plants must be a result of a diseased condition arising from the unnatural conditions, and that plants growing in the atmosphere did not normally produce fixed air. In this heated dispute, Ingen-Housz successfully maintained his position that plants respire (Nash, 1952).

Their views differed in other ways, as well. Senebier showed that dephlogisticated air tends to dissolve in water only if it is agitated with it, thus disproving Ingen-Housz’s contention that dephlogisticated air is very soluble in water. Senebier wrote (1782, Vol. I, pp. 35–36), “This conclusion…assures us that, in all the products of air furnished by the leaves, one has almost entirely the air that they have really filtered, and that the water has absorbed little of it….”

Senebier also contributed the insight that dephlogisticated air originates in the green parenchyma, not in leaf ribs or epidermal structures. Further, in comparing the effects of different colors of light on plants’ production of dephlogisticated air, he anticipated an important approach to the study of photosynthesis in the nineteenth and early twentieth centuries (Kottler, 1973). He was wrong, however, in stating (Senebier, 1782, Vol. I, pp. 255–258) that plants growing normally in the atmosphere absorb fixed air in solution through their leaves and roots. In contrast, Ingen-Housz ultimately concluded (1796) that the source of the carbonic acid is chiefly the atmosphere.

In his several treatises on vegetation written during the 1780s, Senebier showed a gradual conversion from the phlogiston theory to oxygen chemistry (Kottler, 1973). (For a detailed discussion of the evolution of Senebier’s theoretical views during the period 1782–1792, see Nash, 1952, pp. 77–95.)

Senebier’s discovery of the importance of fixed air is another of those early, major advances in the understanding of photosynthesis that has at times been overlooked, misstated, or misattributed. Strangely, confusion has persisted regarding which man, Ingen-Housz or Senebier, deserves credit for this important finding, even though Senebier (1782) presented voluminous evidence supporting the idea long before Ingen-Housz accepted its validity. Although none of Senebier’s evidence was individually conclusive, the many complementary findings argued strongly in favor of his view (Nash, 1952).

Two relatively recent books in which Senebier’s contributions are overlooked, misattributed or misstated will be mentioned here. One is John King’s Reaching for the Sun: How Plants Work (1997), which, on pages 19–20, credits Ingen-Housz with finding the “first hint that light and carbon dioxide were linked in some way leading to the release of oxygen,” thus denying Senebier credit for the carbon dioxide finding. The second example is Maurice McDonald’s Photobiology of Higher Plants (2003). McDonald (2003, p. 34) states: “In 1782, … Jean Senebier claimed that ‘fixed air’ (CO₂) produced by animals and by plants in darkness stimulated production of ‘purified’ air (O₂) by plants in light.” Senebier, however, did not believe that healthy plants produced carbon dioxide. A further example is Huzisige and Ke (1993), in which a chart lists both Ingen-Housz and Senebier as discovering the involvement of fixed air in photosynthesis.

The confusion over priority may be attributable partly to the unsupported claim of Ingen-Housz (1796) that he originated the hypothesis regarding fixed air as the source of nourishment for plants. Partington (1962, p. 280, footnote 5) points a finger at Sachs (1875), who, Partington notes, “mistakenly says Ingen-Housz found in his earlier work [i.e., 1779] that ‘green plants, under the influence of light, take up carbonic acid, separate the oxygen, and so assimilate carbon.’”

Another reason for the confusion may be that, in contrast to Senebier’s voluminous writings, Ingen-Housz (1796) presented a concise, largely accurate summary of the fundamentals of plant nutrition in terms of the new chemistry, including Senebier’s findings regarding fixed air. The actual achievement of a milestone in research (the discovery of the importance of fixed air) became obscured by a subsequent, persuasive synthesis of a range of findings, including the discovery in question, into a coherent statement.

Senebier may have suffered a somewhat similar fate vis-à-vis Nicholas de Saussure, who was economical both in his experimentation and in his writing. Further, since de Saussure’s work included precise measurements of gas exchange, de Saussure sometimes gets the credit.

X. Nicholas-Théodore de Saussure (1767–1845): Confirming the Role of Water, and Developing a Unified Concept of Plant Nutrition

Nicholas-Théodore de Saussure (Fig. 30.2e) clarified and expanded upon the data and interpretations of plant aerial nutrition and the scheme of plant nutrition in general (Nash, 1952). He is particularly known for his conclusive demonstration that water is a reagent directly involved in photosynthesis. De Saussure also provided quantitative evidence that plants obtain all their carbon from carbon dioxide in the atmosphere rather than from humus in the soil, and that the soil provides plants with minerals. Details about his life can be found in Hart (1930) and Pilet (1975b), and in Freshfield’s (1920) biography of Nicholas’ father, Horace-Bénédict de Saussure (1740–1799), who was a distinguished geologist and alpine explorer, and a close friend of Senebier’s.

Nicholas de Saussure was born in Geneva and attended the Geneva Academy (Hart, 1930). He developed an interest in natural philosophy and assisted his father on a number of alpine expeditions (Pilet, 1975b). Then he became intensely interested in chemistry and plant physiology.

There was a gap of well over a decade between Senebier’s achievements in plant physiology and de Saussure’s. Younger than his immediate predecessors in plant physiology, de Saussure did not have to labor under the weight of the phlogiston theory. He was an outstanding experimenter. (Fig. 30.10 shows examples of his experimental arrangements.)

Fig. 30.10.

Some experimental arrangements used by Nicholas de Saussure (1804, Plate). Fig. III: The roots of a horse chestnut (FG) were inserted into a recipient and the neck was sealed at E; the vessel was then filled with distilled water, and a gas (such as nitrogen, hydrogen, or carbon dioxide) was introduced through the opening CD; the upper part of the root was in contact with the gas, and the lower part with the water, HI. The vessel rested in a basin of mercury. The survival times of plants in the various gases, and the uptake of gases, were compared. Fig. IV: Mercury bath (CD) with recipient (AB) containing atmospheric air; the mercury surface in the recipient (HI) is covered with a layer of water (FG); a root (EK) of Polygonum was inserted from below. Water and oxygen, but not nitrogen, were taken up by the root.

Following the precedent of Lavoisier, de Saussure emphasized measurements of weight. Thus, instead of just observing whether his experimental plants did well, he measured the weight they gained. He also measured the weight of carbon and mineral elements that the plants contained. He carried out his research with relatively few experiments (in contrast to Senebier’s extensive and repetitious work) and aimed his work at particular problems (Nash, 1952). From small, experimental plants grown with their shoots exposed to the open atmosphere and their roots in distilled water, de Saussure learned that, despite the unnatural conditions, the plants contained more total carbon than did plants that were at the same stage of development as his experimental plants had been when he had begun his experiments (Nash, 1952). He concluded that plants obtain all their carbon from the carbon dioxide of the atmosphere (Partington, 1962). His findings disproved Senebier’s hypothesis that carbon dioxide dissolved in water is the source of the carbon in plants. (Ingen-Housz (1796) was in essential agreement with de Saussure on this point, but had not shown it quantitatively.)

Further, in contrast to his immediate predecessors in plant nutrition research, de Saussure did not focus solely on aerial nutrition, but also studied water and soil, both of which had long histories as suspected plant nutrients. Van Helmont (1648) had concluded from his willow-tree experiment that transmuted water forms the dry weight of plants, but his experiment did not really prove a role for water. Similarly, others had conjectured, without experimental evidence, that water is important for plants to gain in dry weight, because, knowing that plants contain abundant hydrogen, they could point to no other logical source.

De Saussure provided the first real evidence for a role for water. He showed that growing plants gain more in weight than can be accounted for by the assimilation of the carbon in the carbon dioxide that they take up. He reasoned that, since the amount of oxygen lost to the atmosphere is approximately equivalent to the amount of oxygen in the carbon dioxide absorbed, the extra weight gain must come from the water (Rabinowitch, 1945). De Saussure wrote (1804, pp. 269–270) that the results of his quantitative experiments “prove equally that the extract from the earth, the gas, and all the soluble principles in the water… do not make up the major part of the plant’s dry weight if the water is excepted…. One will recognize that the water that the plant draws up and solidifies, be it from the soil or from the atmosphere, makes up, in weight, the greater part of the dry substance of the plant.” De Saussure thus confirmed the guess that Van Helmont had made 200 years earlier, as Priestley and Ingen-Housz had confirmed the guesses of Stephen Hales (Rabinowitch and Govindjee, 1969).

De Saussure also demonstrated that plants take up various minerals from the soil. He contended that minerals found in the ash of plants had not been accidentally absorbed with the soil water, but were essential to the plant as nutrients, though they were often present in very small quantities (Hart, 1930).

With his highly targeted experiments and broad research program, de Saussure, according to Nash (1952, p. 117), “developed a conceptual scheme that allowed him to assign the source and route of supply of every major element that analysis showed to be present in mature plants.” He presented his findings and interpretations in his single book, Recherches Chimiques sur la Végétation (1804), which laid the foundations of a new science, phytochemistry. The book was an immediate success. It was translated into German in 1805 and again in 1890. Despite an initial positive reception, however, there was a delay of 20–30 years before his ideas were fully understood and appreciated, because the old humus theory retained a strong hold on thinking about plant physiology (Hart, 1930).

In 1802, expecting to occupy a promised chair in plant physiology at the Geneva Academy, de Saussure was instead named honorary professor of mineralogy and geology. Disappointed at not being able to teach plant chemistry, he obtained a leave of absence. He never gave a course (Pilet, 1975b). Starting in 1808, he published a series of articles, mostly analyzing biochemical reactions in plant cells.

XI. Summary of the Early Contributions to the Modern Chemical Theory of Plant Nutrition

Sachs summed up the early work on photosynthesis (but said nothing about the role of light) (1890 translation of Sachs (1875), p. 491): “The discoveries of Priestley, Ingen-Housz and Senebier, and the quantitative determinations of de Saussure in the years between 1774 and 1804, supplied the proof that the green parts of plants, and the leaves therefore especially, take up and decompose a constituent of the air, while they at the same time assimilate the constituents of water and increase in weight in a corresponding degree; but that this process only goes on copiously and in the normal way, when small quantities of mineral matter are introduced at the same time into the plant through the roots.” Schofield (2004, p. 139) echoes Sachs: “It took the combined efforts of Jan Ingenhousz, Priestley, Jean Senebier, and Nicolas T. De Saussure to demonstrate, against much hostile criticism, that green vegetation, under the action of sunlight, absorbed carbon dioxide and released oxygen to the air.” Hart (1930) concludes, “With Ingen-Housz and Senebier, de Saussure is responsible for founding the modern theory of plant nutrition.”

The chemical equation of photosynthesis could now be written:

$$ {\text{CO}}_{2}+{\text{H}}_{2}\text{O}\underset{\text{green plant}}{\overset{\text{light}}{\to }}{\text{O}}_{2}+\text{organic matter}$$

Nash (1952, p. 106) considered de Saussure and Van Helmont transitional figures: Each represented the culmination of an extensive tradition, yet the work of each foreshadowed a new era. Van Helmont was still a partisan of alchemy, yet he dealt with subjects that became major fields of inquiry in the ‘new experimental philosophy.’ In similar fashion, de Saussure brought the studies of plant nutrition, begun by Priestley, Ingen-Housz, and Senebier, close to completion: he finished the fundamental experimental work and supplied a convincing theoretical interpretation of the whole. But de Saussure also opened up new vistas of experiment and thought in this field...

The new era that de Saussure inaugurated was long in coming, however. After de Saussure, there was “…a rapid decline, both in investigation and in the presentation of existing knowledge on the whole subject of plant metabolism…. The beautiful experiments [of Ingen-Housz, Senebier, and de Saussure]…were either forgotten or directly misinterpreted…”(Spoehr, 1919). No substantial advance was made in the overall inquiry on plant nutrition until roughly 60 years after the appearance of de Saussure’s 1804 book (Rabinowitch, 1971).

It should be pointed out that, starting with the early pioneers, an erroneous view about photosynthesis was perpetuated. Ingen-Housz (1796), Senebier (1788), and de Saussure (1804) all expressed the belief that the oxygen emitted by plants originates in the carbon dioxide rather than in the water. For example, De Saussure (1804, p. 237) wrote, “plants, in no case, directly decompose water, assimilating its hydrogen and eliminating its oxygen in a gaseous state; they do not exhale oxygen gas except by immediate decomposition of carbonic acid gas.”

Arnon (1991) comments, “...photodecomposition of CO₂, as proposed by Ingen-Housz, was the universally accepted concept; and with some elaboration it became a dogma that persisted well into the twentieth century.” Then Dutch-born microbiologist Cornelis B. van Niel (1897–1985) proposed that the oxygen produced in green-plant photosynthesis comes from water, not carbon dioxide (Van Niel, 1931). He was subsequently proven correct.

XII. Julius Robert Mayer (1814–1878): The Final Component, Energy

The early workers in photosynthesis, from Van Helmont through de Saussure, were “chemical” pioneers. When they were doing their experiments, the concept of energy had not yet been formulated, although Ingen-Housz had discovered that light was essential to plants. Then, in the mid-nineteenth century, German physician Julius Robert Mayer (Fig. 30.11), the “physical” pioneer in our story, applied the newly emerging concept of energy to biological systems. For a detailed treatment of the development of Mayer’s thought, see Caneva (1993). For a briefer account of his life and thought, see Turner (1975).

Fig. 30.11.

A portrait of Robert Mayer.

Mayer discounted the possibility that energy (a term that was not widely used at the time; his word was “kraft,” which is translated as “force” or “power”) was created in organic systems, since it was not created in inorganic ones (Nash, 1952). In his book Die Organische Bewegung in ihrem Zusammenhang mit dem Stoffwechsel (1845), Mayer contended that energy is conserved in biological as well as in physical systems, and that plants, in carrying out photosynthesis, store the energy of sunlight in the form of chemical energy. Animals acquire this chemical energy through food and convert it to body heat and the mechanical force of muscle movements (Turner, 1975). Before Mayer, plants could only be understood as chemical manufacturers of organic matter; after him, plants could also be seen as storers of energy.

Mayer (1845) wrote (translation taken from Rabinowitch and Govindjee, 1969, p. 9):

Nature has put itself the problem how to catch in flight light streaming to the earth and to store the most elusive of all powers in rigid form. To achieve this aim, it has covered the crust of earth with organisms which in their life processes absorb the light of the sun and use this power to produce a continuously accumulating chemical difference.

These organisms are the plants; the plant kingdom forms a reservoir in which the fleeting sun rays are fixed and skillfully stored for future use; an economic provision to which the physical existence of mankind is inexorably bound.

The plants take in one form of power, light; and produce another power: chemical difference.

Rabinowitch and Govindjee (1969) note that the term ‘chemical difference’ is what we would call chemical energy.

The equation of photosynthesis could now be written:

$$ \begin{array}{l}{\text{CO}}_{2}+{\text{H}}_{2}\text{O}+\text{light}\stackrel{\text{green plant}}{\to }\\ \text{}{\text{O}}_{2}+\text{organic matter}+\text{chemical energy}\end{array}$$

This equation sums up the overall scheme of photosynthesis, showing both the material and the energy balance (Rabinowitch and Govindjee, 1969). It completes the story of the early pioneers’ achievements in elucidating the process by which plants nourish themselves and ultimately much of the rest of life on Earth. The carbon dioxide component was due to Senebier; the water to de Saussure; the light to Ingen-Housz; the oxygen to Priestley; and the chemical energy to Mayer. The other pioneers provided important stepping-stones to the unraveling of this scheme.

I end my chapter by showing my photograph with Govindjee (Fig. 30.12), who provided inspiration and impetus for this chapter.

Fig. 30.12.

A photograph of the author with Govindjee, Aug 2006.