1 (page 1)p. 1What is a plant?

Plants, like love, are easier to recognize than to define. At the entrance to many areas of outstanding natural beauty in England can be seen a sign that asks visitors to avoid ‘damaging trees and plants’. It is fair to ask in what way is a tree not a plant. A plant is often defined simply as a green, immobile organism that is able to feed itself (autotrophic) using photosynthesis. This is a heuristic definition for plants that can be refined if some more characters are added. Sometimes plants are described as organisms with the following combination of features:

So trees are clearly plants, and it is not difficult to think of other organisms that are unequivocally plants even though they lack one or more of these characteristics. For example, the orchid Corallorhiza wisteriana has the flowers of an orchid, produces tiny seeds typical of the family Orchidaceae, and has the vascular tissue that you find in the majority of land plants. However, what (page 2)p. 2it does not have are green leaves, because this orchid is mycotrophic, meaning that it lives off fungi which themselves derive their energy from decaying material in the forest floor. It is able to do this because of a very intimate relationship with a fungus, a characteristic found to varying degrees throughout the orchid family. In a similar vein, Lathraea clandestina, which can be seen growing on the banks of the River Cherwell in Oxford, has flowers reminiscent of a foxglove, yet it too has neither shoots nor leaves. Its flowers emerge directly from the soil because this plant has roots that are able to infiltrate the roots of willow trees and divert the nutritious contents of their vascular tissue. Both of these plant species have lost the ability to photosynthesize, but they are still plants because they share many, many other features with those plants which do still photosynthesize.

The problem with the definitions above is that they are too limited, because they do not take into account some of the algae that live in water. In order to arrive at a sensible and unambiguous definition for plants, we need to consider how we classify biological organisms. Similar individuals are grouped together into a species. Similar species are then grouped into a genus. Similar genera are grouped together into a family; and similar families are grouped into an order; similar orders into a class; similar classes into phylum; and similar phyla into a kingdom. Each of the groups in this hierarchy can be referred to as a taxon, and the study of groups is known as taxonomy. Prior to the 19th century, taxonomists tried to create a natural classification that revealed the plan of the Creator. Since the 19th century, biologists have questioned whether species can change and evolve by retaining those changes and passing them on to their offspring.

A great deal of work is currently being carried out to build the ‘tree of life’ (or phylogeny) that shows how all living organisms are related to each other. This work received its kickstart in 1859 with the publication of Darwin's On the Origin of Species, and it is still ongoing. An evolutionary tree is the only illustration in (page 3)p. 3(page 4)p. 4The Origin, and Chapter 13 of the first edition is still an eloquent introduction to taxonomy. Darwin talks about the possibility of building a natural classification, but now natural means revealing the course of evolution and not the mind of God. Classifications now are based on what Darwin called commonality of descent. All the members of a taxon must share a common ancestor, and the group must contain all of the descendants of that ancestor. If these criteria are fulfilled, then the group is said to be monophyletic. Monophyletic groups occur at every rank in the classification from species to kingdom.

If we see the evolution of species over the past 3,800 million years as a branching tree, then plants are one set of the branches on the tree of life, and this set of branches is all connected back to one crutch. The arguments start when you try to decide which crutch marks the start of plants. It is worth saying at this point that fungi are definitely not plants. Fungi are in fact on the branch next to animals on the tree of life. Despite this, mycologists (who study fungi) do tend to be grouped with botanists rather than zoologists in university departments.

At the heart of any definition of plants is the ability to photosynthesize. Unfortunately, there are organisms that photosynthesize but which cannot be considered by anyone to be plants. In particular, there are the photosynthetic cyanobacteria.

It is currently believed that life has evolved just once and that this happened about 3,800 million years ago. At that time, the world as an environment for biology was very different. There was no protective ozone layer to absorb the harmful ultraviolet light from the Sun. Furthermore, the atmosphere contained a great deal of carbon dioxide but very little oxygen.

(page 5)p. 5The first living organisms were simple compared to the majority of plants that we see around us today. For a start, they were unicellular. They were prokaryotes. There are many prokaryotic organisms still extant in two big groups: the archaea and the bacteria. (The other major group of organisms are the eukaryotes, that is, plants, animals, fungi.) Fossilized prokaryotes have been found in rocks dated at nearly 3,500 million years old. The fossils of these early bacteria are grouped in structures that look the same as the stromatolites that can be seen in several places around the world today.

A stromatolite is a cushion-shaped rock that is found on the edges of warm shallow lakes, most commonly salt-water lakes, and they are (very simply) laminated accumulations of microbes. The colonies of the unicellular cyanobacteria live in a film of mucus. Calcium carbonate builds up on the mucus and the cyanobacteria migrate to the surface and a new layer of mucus is formed. These alternating layers are then fossilized and the bacteria enclosed in the rocks. So it was clear to see that prokaryotic life had evolved perhaps as early as 3,800 million years ago, but it was not so easy to determine how these early living entities found the energy to live. Some may have synthesized enzymes to break down minerals, but this was slow. There is now compelling evidence that the cyanobacteria in these fossil stromatolites were able to capture the energy of the Sun and use it to synthesize molecules containing carbon derived from the abundant carbon dioxide in the atmosphere. This evidence is based around the fact that the enzyme that drives the capture of carbon from carbon dioxide preferentially fixes one carbon isotope (¹²C) over the other that is also present in the atmosphere (¹³C). So if carbon compounds contain the two isotopes in different proportions from those in the atmosphere, then the compounds were the product of photosynthesis. Carbon compounds have been found in rocks in Greenland that have the carbon isotope ratio produced by photosynthesis.

(page 6)p. 6Photosynthetic organisms, with which we are familiar, use water as a source of electrons. The oxygen in the water is then released into the atmosphere as gas. It is thought that the first photosynthetic cyanobacteria may have used hydrogen sulphide (H₂S) rather than water (H₂O). It is currently believed that by 2,200 million years ago, cyanobacteria were generating large amounts of oxygen and that this was accumulating in the atmosphere. This may seem like a small point, but the fact that cyanobacteria began using water as a supply of electrons led eventually to the levels of oxygen in the atmosphere that made aerobic respiration possible and the majority of biology as we know it. The generation of oxygen had another effect, namely the formation of the layer of ozone in the upper atmosphere, whose absence has already been noted and whose protective function is so important for biology. Prior to this, the mucus in the stromatolites may have helped to protect the cyanobacteria. Living in water would also have afforded some protection.

So to recap, we see that by 2,000 million years ago, there was a large population of prokaryotic cyanobacteria that was generating oxygen by photosynthesis, but there was still nothing that we could describe as a plant. The evolution of plants required an event that must have happened but for which we do not have a complete cast list. This event was the formation of the first eukaryotic cell. Eukaryotes cells are more organized internally than prokaryotes. They have organelles enclosed by membranes such as the nucleus and mitochondria and, in the case of plants, chloroplasts. Organelles are just small ‘organs’ found inside cells which perform specific functions within the cell.

It is believed that 2,700 million years ago, an unidentified unicellular prokaryotic organism engulfed another but did not break it down. The engulfed cell retained its membrane and gave up some, but not all, of its genes to be included in the nucleus of the host cell. This engulfing followed by ‘cooperation’ is known as endosymbiosis. This early eukaryotic organism (known as the (page 7)p. 7proto-eukaryotic cell) lived by metabolizing photosynthetic products from free-living cyanobacteria. The evidence for this endosymbiosis is simple: the organelles have two membranes – their own and one from the host that engulfed them. The evidence for the timing of the first endosymbiosis is equally simple. One of the unique features of all eukaryotes is the production of sterols. When a eukaryote dies and breaks down, the sterols are converted into steranes, and these persist in rocks for a very long time. Rocks 2,700 million years old contain steranes, and so there is a trace of dead eukaryotes but no fossils of intact organisms.

Many years passed and the diversity of eukaryotic organisms increased, resulting in evolutionary lineages that produced many other species (both extant and extinct) but not plants. However, having recruited one type of prokaryotic organism, the proto-eukaryotic cell recruited another, and this time it was a photosynthetic cyanobacterium. As before, the incoming organism became an organelle, and some, but not all, of its genes were transferred to the nucleus of the host cell. Again, as before, the organelle, known now as a chloroplast, has a double membrane.

The oldest fossil evidence of the structure of a probable eukaryote is in rocks 2,100 million years old. The organism, named Grypania spiralis, has no extant descendants. It looks a bit like an algae, and so it is believed (or perhaps hoped!) that it was photosynthetic. At 2 millimetres in diameter, it is big enough to be an ancestor of some of today's algae, but it cannot be proved to be this ancestor. The first undisputed fossil of a photosynthetic eukaryote that can be placed in an extant taxon has been found in rocks 1,200 million years old. The organism, Bangiomorpha pubescens, is a red algae and is thus named because it resembles the extant red algae Bangia atropurpurea. In addition to looking similar, these two species also share a habitat, the margin of land and water.

(page 8)p. 8Bangiomorpha is significant for another reason: it is currently the earliest example of a multicellular eukaryotic species that not only has cells with specific functions, but also one of the functions of these specialist cells is to indulge in sexual reproduction. Multicellularity is one of those important biological events that has evolved more than once on different branches of the tree of life. The most recent common ancestor of plants and animals was unicellular and yet both are now dominated by multicellular organisms.

The fossils of Bangiomorpha are so well formed that it has been possible to reconstruct its life cycle and it is similar to some of those found in the red algae. The spores germinate and grow into the multicellular body of the plant. The spores contain only one set of chromosomes (that is, they are haploid), so the algae plant is haploid. At the base of the plant is a holdfast that fixes the plant tightly to a rock. At the top, the plant becomes flattened, and as it grows upwards it is able to capture more light. Some of the cells in this thallus differentiated into haploid gametes that are a prerequisite for sexual reproduction.

So at some time between 2,100 and 1,200 million years ago, the first photosynthetic eukaryotic organisms emerged. These were the first plants, and every subsequent organism on this branch of the tree of life is a plant. The two endosymbiotic events that define this branch happened once and are known together as the primary endosymbiosis. The evidence for this has been derived from the analysis of DNA sequences. This technique, available from 1993, has been very important in unravelling previously tangled problems of evolution.

So plants as described in this book are monophyletic. Intriguingly, it is becoming clear that there has also been a secondary endosymbiosis in which some of the true plants have been incorporated into non-plant organisms and the resulting organisms are also not plants. Perhaps the most familiar of these (page 9)p. 9(page 10)p. 10are the brown algae such as kelp. So if you visit a beach and start looking at the ‘seaweeds’ left behind by the tide, the green and red ones are plants and the brown ones are animals.

The lowest side branch on the plant branch of the tree of life consists of a very old (at least 1,200 million years), very small (13 species) group of tiny (microscopic) freshwater algae in three genera – Glaucocystis, Cyanophora, and Gloeochaete – known collectively as the glaucophytes. The lowest branch on a phylogenic tree contains the organisms that are thought to have changed the least of any others on the tree. This does not mean that they look exactly like the original ancestors. The evidence that supports their basal position is two-fold. Firstly, in addition to the two membranes around the chloroplasts, there is a peptidoglycan layer. This is similar to the envelope found around bacteria, and the chloroplasts are sometimes referred to as cyanelles to distinguish them from those found in the rest of plants. This peptidoglycan layer is not found in any other plants, the inference being that it has been lost early in the evolution of plants. Secondly, they have pigments called phycobilins in their plastids. Plastids are organelles found in plants where important chemicals are manufactured or stored. These pigments are only found in cyanobacteria, the glaucophytes, and the red algae, which are the next side branch on the plant branch of the tree of life. In all three groups, these pigments are bound into phycobilisomes. In addition to the phycobilins, they have chlorophyll a.

This small group is of interest to evolutionary botanists because it is thought that they are the closest extant species to the original endosymbionts. Some species can move and some cannot, while some have cellulose cell walls and some do not. Sexual reproduction is unknown in this group of plants. The evidence from DNA sequences confirms the morphological evidence, and so (page 11)p. 11clearly these little plants are strong contenders for the title of the earliest diverging branch on the plant tree of life. The next side branch up is the red algae. It is worth reiterating that the term ‘algae’ is a vernacular term that is applied to a group of organisms, some of which are plants (red algae and green algae) and some of which are not plants (blue-green and brown algae).

There are many species on this branch: somewhere between 5,000 and 6,000, and perhaps as many as 10,000, of which just a small percentage live in fresh water. The red algae share some characteristics with the glaucophytes that have subsequently been lost and so do not appear in the rest of the plants. These characters are phycobilin pigments and phycobilisomes and having just chlorophyll a. These pigments, other than the chlorophyll, give these plants their distinctive red colour. Red algae have other features in common such as storing energy as glycogen (or floridean starch). Glycogen is a large molecule with lots of glucose in a chain off which come side chains of molecules. Some species secrete calcium carbonate and these are important in the construction of coral reefs, hence why these are known as coralline algae. Red algae all have a double cell wall. The outer layer is economically important because it can be made into agar, which has many uses including in cooking. The internal wall is made in part of cellulose, like most plants.

As one would expect, in a taxon as species-rich as the red algae, there is a lot of diversity, but there are common patterns to their life histories. However, that pattern is complicated and very different from the life cycle of mammals with which we are most familiar. This familiarity colours our preconceptions and assumptions about plant reproduction, and it can create a barrier that makes understanding plant life histories much more difficult than it need be.

(page 12)p. 12The first false assumption is that every free living organism needs two complete sets of chromosomes just because we do. This is not true, and we have already seen a fossil species (Bangiomorpha) which spent much of its life in the haploid state with one of each chromosome, rather than in the diploid state in which it would have had two sets. It is reasonable to ask if there are any advantages or disadvantages attached to being haploid or diploid. A fact of life as a result of being haploid is that any deleterious mutation will be expressed and the organism may perish as a result. So it may appear that having two copies of each gene is better because the harm of a deleterious mutation can be overcome by the good copy, or perhaps the combination of two versions of a gene might be better than just one version. However, this is a twin-edged sword because it means that diploid cells can build up many potentially crippling mutations. Given that there appears to be an evolutionary trend towards diploidy in all the major taxa in the tree of life, it appears that this is, however, a more stable evolutionary strategy. Despite this, it cannot be denied that being haploid has not prevented many taxa from surviving successfully for hundreds of millions of years. Among these taxa are red algae, green algae, mosses, liverworts, and ferns.

The second false assumption that we make is that there will be a group of cells (the germ line) that from a very early stage in the life of an organism will be responsible for making the gametes – the sperm and eggs. This is true in mammals and many other animals but not in plants. In general, the development of plants and the differentiation of their cells into different structures is very flexible and certainly not determined early in the life of the embryo. This can be seen clearly when a gardener roots a cutting, or when a biennial plant like a foxglove changes from vegetative growth and starts to produce flowers. Plants do not have a group of germ cells. Instead, they have a distinct and finite stage in their life histories when a haploid plant produces sperm and/or eggs. Because the plant is already haploid, there is no need to halve the number of chromosomes before the gametes are formed (page 13)p. 13by differentiation and mitosis (cell division whereby the chromosome number remains the same and two identical cells are produced). The haploid stage in the life history produces gametes, and so is very sensibly known as the gametophyte, ‘-phyte’ being derived from the Ancient Greek word φυτόν, meaning plant.

A third false assumption (that is clearly wrong given the previous paragraph) is that diploid plants will produce haploid gametes by the process of meiosis (cell division whereby the number of chromosomes is halved). Instead, the diploid plants produce haploid spores by meiosis. Unsurprisingly, the diploid stage in the life history that produces spores is known as the sporophyte. These haploid spores grow into the haploid gametophyte, and the life history repeats itself. Plant life histories therefore consist of alternating stages or generations: between a haploid generation and a sporophyte generation. Plants are often described as demonstrating an alternation of generations.

Red algae (or their ancestors) were the first plants to display sexual reproduction, and so it is worth describing it here as it makes what comes later easier to understand. What follows is a description of the life history of Polysiphonia lanosa. This is a red algae that may be familiar to anyone who has spent time rock-pooling in the intertidal zone of the coast of Britain and Ireland. This species of red algae grows on the outside of Ascophyllum nodosum (a brown algae and so not a plant) which is very common all the way up the west coast of Europe and the north-east coast of North America. The Polysiphonia is probably an epiphyte on the Ascophyllum, but some people have recorded that the former actually penetrates the latter, and this would make it parasitic. The Polysiphonia plants look like cheerleaders’ pompoms on the surface of the Ascophyllum.

These plants of Polysiphonia may be male or female gametophytes, but they look very similar. The male plants (page 14)p. 14produce male gametes. These are known as spermatia rather than sperm because they do not have a flagellum, or tail, for propulsion. These are released into the water around the plants and the hope is that they will find a female gametophyte. The female gametophyte produces (but does not release) a female (page 15)p. 15gamete. This is a cell that is retained inside a structure known as the carpogonium which consists of the female gamete and a trichogyne. This trichogyne is a protuberance whose function is to catch a passing spermatium. Once caught, the spermatium will donate its nucleus to that of the female gamete, and a diploid cell is formed known as a zygote.

The zygote develops into a diploid structure that remains enclosed in, and therefore completely parasitic on, the haploid female gametophyte. The diploid structure is known as the carposporophyte, from which we can deduce that this pustule produces diploid carpospores. These carpospores are released into the water and drift around hoping to land on a suitable substrate, which in the case of Polysiphonia is an Ascophyllum frond. These diploid carpospores germinate and grow into diploid tetrasporophytes. Rather unhelpfully, these tetrasporophytes look very similar to the gametophytes despite the former being diploid and the latter haploid. When a species has haploid and diploid generations that look similar, it is said to be isomorphic. When the two generations look different, they are said to be heteromorphic. Some red algae are isomorphic and others are heteromorphic.

When mature, tetrasporangia form on the surface of the branches of the tetrasporophyte. The tetrasporangia are the sites of the production of haploid tetraspores, so called because they are formed in tetrads (or 4s). This means that they are joined together in a triangular pyramid with each spore attached to each of the other three. These haploid spores are produced by a diploid plant and therefore they are the result of meiotic cell division, whereby the number of chromosomes is halved. The tetrad of spores is released, and hopefully it will land on a frond of Ascophyllum. One life history of Polysiphonia lanosa is now complete.

The internal structures of red algae are as variable as their life cycles, but Polysiphonia lanosa can be used as an example. Each branch has a central axis of elongated can-shaped cells (page 16)p. 16that are joined end to end. These cells are joined by pit connections that form during the process of cell division. Associated with these connections are pit plugs, which are able to seal the connection should one of the cells die. Intercellular connections are an important component of multicellularity. Around this central axis of cells is a layer of periaxial, or pericentral, cells. These are the same length as each cell in the central core and aligned with the cells therein, thus making the branches of the plants look like a series of repeating units. There may be a further layer of cells, known as the cortical cells, around the periaxial layer.

Green algae belong to a much larger and very diverse group of plants called simply the green plants. It is currently believed that they have all evolved from one common ancestor. They share a number of features, including both forms of chlorophyll – a and b – and they have a cell wall made of cellulose. The majority of the green plants are the land plants; the rest are the green algae.

In the same way that ‘algae’ is a vernacular term that is applied to a group of organisms, some of which are plants (red algae and green algae) and some of which are not plants (brown algae), the term ‘green algae’ refers to two different groups that do not share one unique common ancestor. The green algae are two branches on the evolutionary tree: the chlorophyte algae and the charophyte algae. The evolution of these plants and their relationships is not yet fully understood; in fact, it is a bit of a mystery because the land plants have received much more attention than the algae in the past twenty years. However, it appears that one branch is very much less species-rich than the other: the charophytes, including the Charales. This smaller group is arguably more important in that it is the sister group of the land plants that now dominate the world.

(page 17)p. 17Rather than try to give a comprehensive survey of the green algae, a few species will be described in detail to illustrate the diversity found in this group. Green algae are found in fresh water and sea water. Many species are unicellular but some are filamentous, while others spend some of their time as single-celled organisms that then form a multicellular colony in which some cell differentiation occurs. Volvox carteri is a good example of this latter behaviour. Some algae form symbiotic relationships with fungi known as lichens. While the algae can live without the fungus, the reverse is not true, and so lichens are known by the fungus's name not the algae's. Furthermore, one species of algae can form a symbiosis with many different fungi. (Lichens are not plants.)

The first group of green algae many people encountered was Chlamydomonas because it grew in the school pond. This unicellular genus, along with the Volvox and Ulva described later, belong to the chlorophytes. Chlamydomonas has two flagella and has been used extensively as a ‘model organism’ in the movement of flagella. The adult plant is haploid, with just one set of chromosomes. This cell can reproduce asexually by simply dividing mitotically. Before these new adults are produced, the Chlamydomonas lose their flagella and group together. The cells then divide but in an uncoordinated fashion, producing new unicellular organisms.

However, the adult Chlamydomonas can differentiate into a gamete. In some species, the male and female gametes are the same size (isogamous), whereas in others the females are larger (oogamous). The two gametes fuse in the water and form a diploid zygote that is encased in a thicker wall that can protect the young zygote. This zygospore can withstand harsh conditions, but when it is feeling that conditions are correct, it will divide meiotically and four new adult plants are released from each zygospore and the life history is completed.

(page 18)p. 18Chlamydomonas is in the family Chlamydomonaceae which is in the same group of families (or order) as the next genus we shall look at – Volvox. Volvox, like Chlamydomonas, has been investigated in depth because it is capable of existing as a unicellular plant or in a spherical colony of several thousand cells. Volvox carteri is thus a useful subject if you want to study the evolution of multicellularity, which has evolved many times in plants, let alone independently in plants and animals.

So the unicellular Volvox plants come together to form a colony whereby several thousand individuals become embedded around the outside of a gelatinous ball of glycoprotein (known as a coenobium) that is up to 1 millimetre in diameter. This colony works as a collective, with coordinated flagella beatings that can move the ball towards the light. Sometimes connecting strands of cytoplasm can be seen connecting the cells. The cells perceive the light through eyespots which are more common on one side of the colony than on the other, thereby giving the ball a front and back, or anterior and posterior pole. This coordinated group of individuals then becomes a truly multicellular organism when some cells begin to divide asymmetrically to give one small and one larger cell. The two new types of cell are incapable of an independent existence. The small cells are somatic cells whose function is to propel the colony with their flagella. The larger cells, known as gonidia, accumulate at the posterior pole, where they divide and give rise to daughter colonies. These young colonies are initially retained inside the coenobium, with their flagella orientated inwards, but when the parent colony ruptures to release the offspring, the cells reorientate and the flagella are on the outside of the sphere. Vegetative reproduction is complete. Bizarrely, sometimes granddaughter colonies form inside the daughter colonies before they are released by the mother colony.

Sexual reproduction of Volvox is also different from that of Chlamydomonas. Some species have colonies that produce both (page 19)p. 19sperm and eggs (monoicous), while in other species the colony will only produce sperm or eggs (dioicous). (It should be noted that this is different from monoecious and dioecious which is used when diploid plants are male or female as opposed to bisexual.) At the commencement of sexual reproduction, some of the generative cells in the colony will either begin to produce sperm which are released or to develop into egg cells which are not. The sperm are produced in sperm packets, which are simply bags of sperm that are released from the parental colony. There is some evidence that these packets release a pheromone to make other colonies sexually active. When the sperm finds an egg and successfully fertilizes it, a thick-walled spore results that contains the zygote. This diploid spore, known as the meiospore, is capable of withstanding harsh conditions, but in the correct circumstances will undergo meiosis and release haploid offspring.

One of the more familiar green algae to anyone who has been rock-pooling along the shores of temperate regions of the world is sea lettuce, or Ulva lactuca. This often scruffy-looking plant is found attached to rocks by a round holdfast or drifting freely. The rest of the plant is up to the size of a dinner plate and is a thallus of just two layers of cells thick, making it very flimsy, and so it regularly gets torn by the action of the waves on the rocks. However, this is not a problem as it lives in water and the plant is buoyant and supported by the water. The cells in each layer are arranged randomly and any one of them can divide. This means that there is no equivalent of the meristems that we find in flowering plants. The individual cells are not interconnected in any way, making this little more than a colony in some people's minds. This is an over-simplification as the plants are in fact more organized than that, in that they are like herbaceous perennial land plants because they can regrow from the holdfast each spring or if the thallus breaks off. The pieces of thallus that break off have been found to form new plants in laboratory conditions, but it is not known if this happens in the wild.

(page 20)p. 20Ulva, like all sexually active plants, has a life history that includes an alternation between a haploid generation and a diploid generation. The twist in this part of the tree of life is that the haploid and diploid generations look the same and so are described as isomorphic. The haploid plants produce gametes. To do this, cells around the margin of the gametophyte thallus divide and differentiate into biflagellated sperm or eggs. These are similar morphologically except that the females are slightly larger. Both male and female gametes are able to photosynthesize and to swim towards a light source (positively phototaxic). This means that the gametes swim up to the surface of the water. It is believed that the flagella are more than just a means of propulsion; they are implicated not only in sexual identity but also as facilitators of adhesion once a gamete of the other sex has been located. The gametes of both sexes have eyespots at the base of their flagella. The gametes differ in that the female eggs have 5,500 particles in (page 21)p. 21the outer membrane of the eyespot chloroplast while the male gametes have 4,900. Having found the gamete of their dreams, the now quadriflagellate zygote is negatively phototaxic, meaning that it swims away from the water's surface to the rocks at the bottom of the water, where they can grow a holdfast and then a new thallus, only this thallus and holdfast are diploid.

When mature, the diploid thallus of the sporophyte produces haploid spores from the margin. These are the result of meiotic division that halves the chromosome number. These zoospores (like the zygotes) are both quadriflagellate and negatively phototaxic. Also like the zygote, the eyespot membrane has 11,300 particles, and this is thought by some to have a role to play in phototaxis. The zoospore will then grow into a holdfast and a male or female gametophyte. Ulva is collected from beaches in Scotland and eaten in soups and salads, and in Japan it is used in some sushi dishes.

The charophytes are a much smaller group than the chlorophytes in terms of the number of species. One may be familiar from school days. Spirogyra is a filamentous algae found in freshwater pools and ponds. It normally lives below water, but in warmer weather the rapid growth rate and lots of oxygenic photosynthesis results in a frothy, slimy mass of tangled filaments rising out of the water. At any time of the year, it is easily identified by the fact that the chloroplasts are arranged in a pattern that resembles a stretched spring. The cylindrical cells are joined end to end and individual filaments may be many centimetres long. The cell wall has two layers: an outer coat of cellulose and an inner wall of pectin. The filaments can break, but this is essentially asexual reproduction as the two halves can each grow into a new plant.

Each cell of an adult plant of Spirogyra is haploid, so it is a gametophyte. Sexual reproduction is simple and comes in two scenarios. In the first, two different filaments come to lie alongside each other. Tubes grow out from cells in each filament (page 22)p. 22and fuse at their tips to create a conjugation canal between two cells, one from each filament. The contents of the male cell migrates into the female cell, the nuclei fuse, and a diploid zygote is formed and released as a zoospore. This is known as scalariform conjugation, like a ladder. The other scenario is known as lateral conjugation. This is when a filament forms conjugation tubes between adjacent cells in the same filament. This is followed by the migration of the male contents into the female cell and the formation and release of the diploid zoospore as happens in scalariform conjugation. This spore then divides meiotically to give four haploid cells, from which new gametophyte filaments form.

The final example for this chapter and the second charophyte is Chara itself. This is a multicellular plant that is found in freshwater pools in temperate regions of the northern hemisphere. The plants look similar in general terms to other water plants such as Ceratophyllum (see Chapter 5) and to some land plants such as horsetails or goosegrass, yet they are closely related to neither. The plant consists of a central filamentous stem from which whorls of branches are produced at regularly spaced nodes. The plants may be found floating freely, but they do grow into the mud at the bottom of ponds with the production of rhizomes. The cells at the apex of the stem divide, with the upper daughter cell retaining the function of apical cell. The apical cell in a plant is simply the cell at a tip. The lower daughter cell develops into either a nodal or inter-nodal cell. It is from the nodal cell that the whorl of branches grows. The branches are either short and of determinate growth, or long and of indeterminate growth.

This plant is the haploid gametophyte. The plant produces motile sperm that are released into the water. The female gametes are not released but retained in structures on the gametophyte. The diploid zygote that is formed could, and perhaps through our eyes should, grow into a diploid sporophyte, as happens in Ulva. However, Chara is not us (that is, not human) and the zygote goes (page 23)p. 23straight into meiosis to produce four haploid spores. These spores drift away and develop first into a filamentous protonema and then into more haploid adult gametophytes.

So the green algae consist of two separate groups: the chlorophytes and the charophytes. These are not one monophyletic group sharing one unique ancestor; rather, they are adjacent branches on this limb of the tree of life. There is one more big group left on this limb and it shares a unique common ancestor with the charophytes. This big group is very big. It consists of about 400,000 species. This group of plants is very familiar to us because they are the land plants. These plants can be traced back to one unique, unidentified ancestor that had accumulated a new combination of traits that enabled it to survive for most of the time out of water. This ancestor was the first land plant, and without it there would be no terrestrial ecosystems as we know them, and there would certainly be no Homo sapiens and no Oxford University Press and no Very Short Introduction to plants. The next chapter is about the land plants.