Plant Kingdom

Plants are so varied that no single feature can sort all of them sensibly. Because of this, botanists have used several different kinds of classification one after another, and each new kind solved a problem that the earlier kind could not.

The earliest schemes were artificial systems. Linnaeus, for example, leaned mainly on vegetative characters together with the androecium, that is the arrangement of stamens. Such a system is easy to use, and that is exactly why it lasted so long. But it has two real weaknesses. First, it treats every character it uses as equally important, so a plant may be judged on a feature that says very little about its ancestry. Second, vegetative characters are strongly affected by the environment: the same species growing in shade and in full sun can look different enough to be filed in two separate boxes. The result is that close relatives get separated and unrelated plants get grouped together simply because they look alike.

Natural systems, such as the one worked out by Bentham and Hooker, were built on natural affinities instead. They still use external form, but they add internal evidence: anatomy, embryology, the fine structure of cells, and the chemicals a plant manufactures. Using several independent lines of evidence makes a wrong grouping much less likely, because an error in one line of evidence is usually contradicted by another.

Phylogenetic systems go one step further. They are based on evolutionary relationships, on the assumption that all the members of a group descend from a common ancestor. Fossils are the direct evidence here, but fossils of soft plant tissue are rare, so botanists supplement them with other data.

Three modern approaches feed into that work. Numerical taxonomy uses computers: every observable character is given a number and a code, hundreds of characters can be handled together, and each one can be given equal weight or a weight of its own. Cytotaxonomy uses cytological information such as chromosome number, chromosome structure and chromosome behaviour during cell division. Chemotaxonomy uses the chemical constituents of the plant to settle cases where structure alone is ambiguous.

In this chapter the plant kingdom is studied as five broad groups, and the order is not random. Algae, bryophytes, pteridophytes, gymnosperms and angiosperms form a sequence of increasing adaptation to life on land. Read the whole chapter as three problems being solved one after another: how to avoid drying out, how to conduct water and food inside a larger body, and how to reproduce without needing a film of outside water for the male gamete to swim through.

Algae are chlorophyll-bearing, simple, thalloid and mostly autotrophic organisms. Thalloid means the body is not divided into a true root, stem and leaf. They are largely aquatic, in fresh water as well as sea water, but they also turn up on moist stones, on soil and on wood. Some live in association with fungi as lichens, and some live in association with animals, growing on the hair of a sloth or inside the tissues of a freshwater sponge. In size and form they range from the microscopic single-celled Chlamydomonas, through colonial forms such as Volvox and filaments such as Ulothrix and Spirogyra, up to the massive marine kelps that grow many metres long.

Algae reproduce in three ways. Vegetative reproduction is usually by fragmentation, in which each fragment grows into a new thallus. Asexual reproduction is by spores, most commonly flagellated zoospores produced in a zoosporangium, which germinate into a new plant. Sexual reproduction is by fusion of two gametes, and the pattern has three grades. When the two fusing gametes are similar in size, and they may both be flagellated or both non-flagellated, the condition is isogamous, as in Ulothrix and Spirogyra. When the two gametes are dissimilar in size the condition is anisogamous, as in some species of Chlamydomonas. When a large non-motile female gamete is fertilised by a small motile male gamete the condition is oogamous, as in Volvox and Fucus.

Algae matter far beyond the exam. At least half of the total carbon dioxide fixation on earth is carried out by algae through photosynthesis, and being primary producers they are the base of most aquatic food chains. Several are eaten directly, such as Porphyra, Laminaria and Sargassum. Hydrocolloids are extracted from them: algin from brown algae, and carrageen and agar from red algae. Agar from Gelidium and Gracilaria is used to grow microbes and to prepare ice creams and jellies. Chlorella, a unicellular green alga rich in protein, is used as a food supplement and has even been used by space travellers.

The three classes are separated by a small set of characters, and these are worth learning exactly, because the three sets look similar at a glance and are easy to swap by accident. Chlorophyceae, the green algae, are usually grass green because chlorophyll a and chlorophyll b dominate, and the pigments sit in definite chloroplasts whose shape varies from cup-shaped to spiral to ribbon-shaped. Most members have one or more storage bodies called pyrenoids inside the chloroplast, and a pyrenoid contains protein besides starch. Stored food is starch. The cell wall has an inner layer of cellulose and an outer layer of pectose. Examples are Chlamydomonas, Volvox, Ulothrix, Spirogyra and Chara.

Phaeophyceae, the brown algae, are mostly marine and vary from olive green to deep brown depending on how much of the xanthophyll pigment fucoxanthin is present. Their pigments are chlorophyll a, chlorophyll c, carotenoids and xanthophylls. Food is stored as complex carbohydrates, either laminarin or mannitol. The cell wall is cellulosic, usually covered on the outside by a gelatinous coating of algin. The plant body is commonly attached to the substratum by a holdfast, above which is a stalk called the stipe and a flattened photosynthetic organ called the frond. Examples are Ectocarpus, Dictyota, Laminaria, Sargassum and Fucus.

Rhodophyceae, the red algae, are predominantly marine and are found in warmer waters, both near the surface and at considerable depths where little light reaches. Their red colour is due to the pigment r-phycoerythrin, and they also contain chlorophyll a and chlorophyll d. Food is stored as floridean starch, which is similar in structure to amylopectin and glycogen. The cell wall contains cellulose, pectin and polysulphate esters. They reproduce asexually by non-motile spores and sexually by non-motile gametes, and sexual reproduction is oogamous with complicated changes after fertilisation. Examples are Polysiphonia, Porphyra, Gracilaria and Gelidium.

Bryophytes are the mosses and liverworts that grow in moist, shaded places, on damp soil, on rocks and on tree trunks. They are called the amphibians of the plant kingdom because they can live on soil, yet they still depend on water for sexual reproduction: the male gamete has to swim to the female gamete through a film of water, exactly as an amphibian must return to water to breed.

The plant body of a bryophyte is more differentiated than that of an alga, but it has no true roots, stem or leaves. It is thallus-like, either prostrate or erect, and it is fixed to the substratum by rhizoids, which may be unicellular or multicellular. There are leaf-like and stem-like structures, but they are not built from the same tissues as the leaves and stems of higher plants.

The important point about the life cycle is that the main plant body of a bryophyte is the haploid gametophyte. This gametophyte produces the sex organs. The male sex organ is the antheridium, which produces biflagellate antherozoids. The female sex organ is the archegonium, which is flask-shaped and produces a single egg. The antherozoids are released into water, reach an archegonium, and one of them fuses with the egg to form a zygote. The zygote does not immediately undergo reduction division. It divides to form a multicellular body called the sporophyte. This sporophyte is not free-living. It stays attached to the gametophyte and draws its nourishment from it. Some cells of the sporophyte then undergo meiosis and produce haploid spores, and these spores germinate to give new gametophytes.

Liverworts grow in damp, shaded habitats such as stream banks, marshy ground, moist soil and the bark of trees. The plant body is thalloid, as in Marchantia, where the thallus is dorsiventral and lies closely pressed to the substratum. In leafy members the leaf-like appendages are arranged in two rows on the stem-like axis. Asexual reproduction takes place by fragmentation of the thalli, and also by specialised structures called gemmae. Gemmae are green, multicellular, asexual buds that develop in small receptacles called gemma cups borne on the thalli. They detach from the parent and grow into new individuals. The sporophyte of a liverwort is divided into a foot, a seta and a capsule, and after meiosis the capsule produces spores that give rise to free-living gametophytes.

Mosses have a life cycle with two clear gametophytic stages. The first is the protonema stage, which develops directly from a spore and is a creeping, green, branched and often filamentous structure. The second is the leafy stage, which develops from the secondary protonema as a lateral bud. It consists of an upright slender axis bearing spirally arranged leaves and is attached to the soil by multicellular and branched rhizoids. This leafy stage bears the sex organs. Vegetative reproduction takes place by fragmentation and by budding in the secondary protonema. In sexual reproduction the sex organs, antheridia and archegonia, are produced at the apex of the leafy shoots. After fertilisation the zygote develops into a sporophyte consisting of a foot, a seta and a capsule, and the moss sporophyte is more elaborate than that of the liverworts. Spores are shed in a specialised way. Common examples are Funaria, Polytrichum and Sphagnum.

Bryophytes are useful in ways worth remembering. Mosses along with lichens are the first organisms to colonise bare rock, and by slowly breaking it down they help form soil for larger plants. Species of Sphagnum provide peat, which has long been used as fuel, and because of its capacity to hold water it is used as packing material for sending live plant material. Dense mats of moss on the ground reduce the impact of falling rain and so check soil erosion.

Pteridophytes are the ferns, horsetails, club mosses and their relatives, and they are the first terrestrial plants to possess vascular tissues, that is xylem and phloem. This is the single most important thing about the group. Once a plant can conduct water upward and food downward through specialised tissue, it is no longer limited to a body only a few cells thick, and it can grow tall. Pteridophytes are used for medicinal purposes, as soil binders and as ornamental plants. They are found mostly in cool, damp, shady places, though some also grow in sandy soil.

The main plant body of a pteridophyte is the sporophyte, and it is differentiated into a true root, a true stem and true leaves. These organs possess well-differentiated vascular tissue. The leaves may be small and scale-like, called microphylls, as in Selaginella, or large, called macrophylls, as in the ferns. The sporophytes bear sporangia that are subtended by leaf-like appendages called sporophylls. In some cases the sporophylls may form compact structures called strobili or cones, as in Selaginella and Equisetum.

Inside a sporangium, the spore mother cells undergo meiosis and produce haploid spores. A spore germinates to give an inconspicuous, small but multicellular, free-living and mostly photosynthetic thalloid gametophyte called a prothallus. It is important to state clearly that the prothallus is the gametophyte, because this is a frequent point of confusion. The prothallus needs cool, damp and shady conditions to grow, and it is precisely this narrow requirement that restricts pteridophytes to such habitats even though their sporophytes are well adapted to land.

The prothallus bears both male and female sex organs, the antheridia and the archegonia. Water is required for the antherozoids to reach the mouth of the archegonium and fuse with the egg. So although the pteridophyte sporophyte has solved the conduction problem, the gametophyte has not solved the water-for-fertilisation problem. The zygote produced after fertilisation develops into a multicellular, well-differentiated sporophyte, which is the dominant phase of the life cycle.

Most pteridophytes produce spores of only one kind and are described as homosporous. A few produce two kinds of spores, large megaspores and small microspores, and are described as heterosporous. Selaginella and Salvinia are the two examples to remember. In heterosporous forms the megaspores germinate into female gametophytes and the microspores into male gametophytes, and the megaspores are retained on the parent sporophyte for varying periods. The development of the zygote into a young embryo takes place within the female gametophyte, which is itself still on the parent plant. This event is treated as the precursor to the seed habit, because a seed is essentially an embryo packed with food and retained on and then released from the parent, and it is considered an important step in evolution.

Pteridophytes are classified into four classes. Psilopsida includes Psilotum. Lycopsida includes Selaginella and Lycopodium. Sphenopsida includes Equisetum. Pteropsida, the ferns proper, includes Dryopteris, Pteris and Adiantum.

The name gymnosperm comes from words meaning naked seed, and it is meant literally. In these plants the ovules are not enclosed by any ovary wall, and they remain exposed both before and after fertilisation. Since the seed develops from the ovule, the seeds that follow are also not covered by a fruit wall. That single fact separates gymnosperms from angiosperms more cleanly than any other.

Gymnosperms are medium-sized to tall trees and shrubs. The giant redwood Sequoia is one of the tallest tree species known. The roots are generally tap roots, but two special root types are worth memorising. In Pinus, the roots of some species form a fungal association called mycorrhiza. In Cycas, small specialised roots called coralloid roots grow near the base and are associated with nitrogen-fixing cyanobacteria. Notice that the fungal partner and the cyanobacterial partner belong to different genera; swapping them is a common slip.

The stems may be unbranched, as in Cycas, or branched, as in Pinus and Cedrus. The leaves may be simple or compound. In Cycas the pinnate leaves persist for a few years. The leaves are well adapted to withstand extremes of temperature, humidity and wind: they have a thick cuticle and sunken stomata, and in conifers the needle-like leaf greatly reduces the surface from which water can be lost.

Gymnosperms are heterosporous: they produce haploid microspores and megaspores. The two kinds of spores are produced within sporangia that are borne on sporophylls, and the sporophylls are arranged spirally along an axis to form compact strobili or cones. Cones bearing microsporophylls and microsporangia are called male or microsporangiate strobili, and those bearing megasporophylls with ovules are called female or macrosporangiate strobili. Both kinds of cones may be borne on the same tree, as in Pinus, or on different trees, as in Cycas.

The megaspore mother cell is differentiated from a tissue called the nucellus inside the ovule, and the nucellus is protected by envelopes called integuments. The whole structure of nucellus plus integuments is the ovule, and the ovules are borne on megasporophylls. The megaspore mother cell divides meiotically to form four megaspores. One of these develops into a multicellular female gametophyte that bears two or more archegonia. This female gametophyte is never released; it stays enclosed within the megasporangium.

The microspore mother cells divide meiotically to form the microspores, and each microspore develops into a male gametophytic generation, which is the pollen grain. Pollen grains are released from the microsporangium and are carried in air currents until they come in contact with the opening of the ovule borne on megasporophylls. The pollen tube then carries the male gametes towards the archegonia in the ovules and discharges its contents near the mouth of an archegonium. Following fertilisation the zygote develops into an embryo, and the ovules develop into seeds. There is no covering over these seeds, and there is no double fertilisation in gymnosperms; the food-storing tissue in a gymnosperm seed is formed from the female gametophyte and is therefore haploid, laid down before fertilisation rather than after it.

Angiosperms are the flowering plants, and in them the pollen grains and the ovules are developed in flowers while the seeds are enclosed by fruits. The group is enormously varied in size. The smallest angiosperm is Wolffia, a floating plant only about a millimetre across, while some Eucalyptus trees grow to more than one hundred metres. Angiosperms supply almost all our food, fibre, timber, medicines and ornamental plants. They are divided into two classes, the dicotyledons and the monocotyledons, named after the number of cotyledons in the seed.

The male sex organ of a flower is the stamen. Each stamen consists of a slender filament with an anther at its tip. The anther is typically bilobed, and each lobe contains microsporangia in which pollen mother cells divide meiotically to form pollen grains, which are the microspores. A pollen grain represents the highly reduced male gametophyte: it germinates to produce a pollen tube that carries two male gametes.

The female sex organ is the pistil, made up of one or more carpels. A carpel has three parts: the swollen basal ovary, the elongated tube-like style, and the receptive tip called the stigma. Inside the ovary is the placenta, which bears the ovules or megasporangia. Each ovule contains a megaspore mother cell, which divides meiotically to form four megaspores. Three of these degenerate and only one remains functional. That single functional megaspore develops into the female gametophyte, called the embryo sac.

A mature embryo sac has eight nuclei distributed among seven cells, and this arrangement must be stated precisely. The embryo sac is an elongated sac with two ends. One end faces the micropyle, the small opening left in the integuments of the ovule through which the pollen tube will later enter; this is the micropylar end, and it holds the egg apparatus, made of one egg cell flanked by two synergids. The opposite end, farthest from the micropyle, is the chalazal end, and it holds three antipodal cells. Between these two groups sits one large central cell containing two polar nuclei. Count them: three plus three plus one central cell gives seven cells, and because the central cell carries two nuclei the total comes to eight nuclei.

Pollination brings pollen to the stigma. The pollen grain germinates there, and the pollen tube grows down through the style, enters the ovule through the micropyle, and finally enters one of the synergids, where it releases the two male gametes. Now comes the event unique to angiosperms. One male gamete fuses with the egg cell to form a diploid zygote. This fusion is called syngamy. The other male gamete fuses with the two polar nuclei of the central cell to form a triploid primary endosperm nucleus. Because three nuclei are involved this second fusion is called triple fusion. Since two fusions take place inside one embryo sac, the whole event is called double fertilisation, and it occurs in angiosperms and nowhere else.

After this, the zygote develops into the embryo, and the cell containing the primary endosperm nucleus develops into the endosperm, the food-storing tissue that nourishes the developing embryo. The synergids and antipodal cells degenerate. The ovule as a whole matures into the seed, and the ovary matures into the fruit that encloses it. Keep the ploidy straight: the embryo is diploid because it came from the zygote, while the endosperm is triploid because it came from one haploid male gamete plus two haploid polar nuclei.

Every plant life cycle swings between two phases. The gametophyte is haploid, carrying one set of chromosomes, and it produces gametes by mitosis. Two gametes fuse to give a diploid zygote, which grows into the sporophyte. Somewhere in the sporophyte, meiosis takes place and produces haploid spores, and those spores start the gametophyte again. This regular swing between a haploid gamete-producing phase and a diploid spore-producing phase is called alternation of generations.

The two events that flip the ploidy are worth naming. Fertilisation doubles the chromosome number, taking the plant from n to 2n. Meiosis halves it again, taking the plant from 2n back to n. What differs between plant groups is not whether these two events occur, because they always do, but where in the life cycle meiosis happens and therefore how much of the plant is haploid and how much is diploid.

In the haplontic pattern, the dominant, photosynthetic, free-living phase is the haploid gametophyte. It produces gametes, the gametes fuse, and the zygote formed is the only diploid cell in the whole life cycle. That zygote immediately undergoes meiosis to produce haploid spores. There is no free-living sporophyte at all; the sporophyte is represented by the single-celled zygote. Many algae follow this pattern, including Volvox, Spirogyra and some species of Chlamydomonas.

In the diplontic pattern the situation is exactly reversed. The diploid sporophyte is the dominant, photosynthetic, independent plant. The gametophyte is represented only by a single-celled or few-celled haploid structure. All seed-bearing plants, that is all gymnosperms and all angiosperms, are diplontic. In an angiosperm the entire visible tree is sporophyte, while the gametophytes are just the pollen grain and the embryo sac hidden inside the flower. Among algae, Fucus is diplontic.

In the haplo-diplontic pattern both phases are multicellular, and this intermediate condition is where bryophytes and pteridophytes sit. The difference between those two groups is which phase is in charge. In bryophytes the dominant, independent, photosynthetic phase is the gametophyte, and the sporophyte is a short-lived multicellular structure that stays physically attached to the gametophyte and is totally or partially dependent on it for food. In pteridophytes the dominant, independent and photosynthetic phase is the sporophyte, and the gametophyte, the prothallus, is small, short-lived and independent but inconspicuous. Some algae such as Ectocarpus, Polysiphonia and the kelps are also haplo-diplontic.

Reading the whole chapter through this one idea gives a clean summary. In green algae the haploid phase runs the show. In bryophytes the haploid gametophyte still dominates, but a dependent diploid sporophyte has appeared. In pteridophytes the diploid sporophyte takes over as the large plant, while the haploid prothallus remains free-living. In gymnosperms and angiosperms the gametophyte shrinks until it is a handful of cells kept inside the sporophyte, which is now the whole visible plant. The trend is a steady reduction of the gametophyte and a steady rise of the sporophyte, and it runs in step with the loss of dependence on external water for fertilisation.