Plants

Abstract

Plants have been recognized since ancient times as a distinct lineage of organisms, but the precise boundaries of the group have only recently come into sharp focus. Land plants, or embryophytes, are immediately familiar as the organisms that constitute the terrestrial vegetation, dominate agriculture, and play a wide range of cultural and economic roles in human society. It is more difficult to identify the boundaries of plants as a whole because there is a vast diversity of photosynthetic eukaryotes that have affinities with land plants but are less conspicuous or lack the structural features of land plants. These organisms, algae, were classically treated as a subset of plants, but include both close relatives of plants and quite distantly related organisms: the term ‘alga’ is a form-classification, not a natural one. Some algae are naturally classified with land plants, while others are not. Furthermore, the original diversification of many algal lineages greatly predates that of land plants, so although it is tempting to think of algae as a kind of plant, it is really more accurate to think of land plants as a kind of alga. Plants have been recognized since ancient times as a distinct lineage of organisms, but the precise boundaries of the group have only recently come into sharp focus. Land plants, or embryophytes, are immediately familiar as the organisms that constitute the terrestrial vegetation, dominate agriculture, and play a wide range of cultural and economic roles in human society. It is more difficult to identify the boundaries of plants as a whole because there is a vast diversity of photosynthetic eukaryotes that have affinities with land plants but are less conspicuous or lack the structural features of land plants. These organisms, algae, were classically treated as a subset of plants, but include both close relatives of plants and quite distantly related organisms: the term ‘alga’ is a form-classification, not a natural one. Some algae are naturally classified with land plants, while others are not. Furthermore, the original diversification of many algal lineages greatly predates that of land plants, so although it is tempting to think of algae as a kind of plant, it is really more accurate to think of land plants as a kind of alga. There is a monophyletic group of eukaryotes marked by the presence of primary plastids (plastids that are directly derived from cyanobacteria). This lineage, composed of glaucocystophytes, rhodophytes, and chlorophytes, including land plants, has been given several names (among them ‘Archaeplastida’ and ‘Chlorobiota’), but because it is the most inclusive monophyletic group of phototrophs that includes land plants, and because it accounts for the bulk of the organisms known both classically and informally as plants, we are comfortable with referring to this group simply as ‘plants’. This is generally consistent with the term's usage colloquially and with the scientific literature going back to Ingenhousz and earlier. Some photosynthetic eukaryotes are not plants in the sense used here. These organisms rely on secondary plastids — plastids acquired either directly or indirectly by ingesting a rhodophyte or chlorophyte — and are not evolutionarily related to plants except in the context of this symbiosis. Plants, in the sense outlined above, are the dominant photosynthetic organisms in the terrestrial and some freshwater habitats, and they play a key role in both onshore and offshore marine environments. As a result these organisms are hugely important in the global carbon cycle and provide the basis for agriculture, timber, natural fiber, and other industries. Plants occupy a vast fraction of the Earth's surface and have had a profound impact on the biogeochemistry and evolution of the planet (Figure 1). The colonization of the terrestrial environment occurred in the late Ordovician or early Devonian period, but red and green algae are far more ancient. The three ancient lineages of plants are glaucocystophytes, red algae, and green algae (including embryophytes). Glaucocystophytes are inconspicuous and relatively uncommon freshwater algae. They would probably have received little attention except that their plastids have both pigmentation and ultrastructure that are strongly reminiscent of cyanobacteria. They were considered extremely puzzling before it was understood that plastids (including the chloroplasts of green algae and plants) are of endosymbiotic origin and are derived from free-living cyanobacterial ancestors. In this context, the resemblance between cyanobacteria and glaucocystophyte plastids is easy to understand; the glaucocystophyte plastids retain some ancestral properties, including light-harvesting proteins organized into structures called phycobilisomes, and bacterial peptidoglycan cell walls surrounding the plastid. Despite these primitive features, glaucocystophyte plastids are clearly organelles that are highly modified from their free-living ancestors, and they have undergone reduction in genome size comparable to that of red algal plastids (green algal plastid genomes are even smaller). There are only three known genera of glaucocystophytes. All are microscopic, and none is particularly common, but these seemingly unimportant organisms have had a profound impact on the understanding of the endosymbiotic origin of organelles. Unfortunately, freshwater habitats are among the most severely degraded on Earth, and there are relatively few scientists studying freshwater eukaryotic microbial diversity. There is consequently sparse information on the diversity and distribution of glaucocystophytes, but a few species are fairly well studied. The phylogenetic placement of glaucocystophytes is still a matter of discussion, and it is possible that they form a monophyletic group with red algae, in which case they might be viewed as the only rhodophytes with flagella. Red algae are typically macroscopic — with thalli measuring centimeters in length and grams in weight — and structurally complex, multicellular organisms. Like cyanobacteria and glaucocystophytes, red algae harvest light with phycobilisomes; this pigmentation gives them a red, brown, or black color and is responsible for their name. They completely lack flagella at any life stage and, perhaps as a consequence of their minimal motility, have evolved extremely complex life histories. Fascinating life histories emerged from limiting fertilization and dispersal strategies, such that some species indulge in a triphasic alternation of generations — with two diploid phases and one haploid phase, all multicellular, alternating in an elaborate chromosomal dance that maximizes the products of each fertilization event — while others undergo intercallary meiosis and simply reduce from diploid to haploid without abandoning their position attached to a rock, and still others have miniaturized females or are parasitic nuclei. The great majority of red algae are marine, where they play key roles in coastal and coral communities. In temperate waters they are conspicuous members of rocky intertidal and subtidal shoreline communities. They display a wide variety of morphologies ranging from fine branching filaments to fleshy blades and spreading crusts. Some species are calcified; these ‘coralline’ red algae may form crusts or complex branching structures linked by flexible but remarkably tough joints. In tropical waters, red algae facilitate the formation of coral reefs. Because they are macroscopic and frequently calcified, red algae are well represented in the fossil record. Some of the oldest known fossils of eukaryotes are often assigned to the red algae (although the oldest are not calcified). The cell walls of red algae are composed of a wide variety of complex polysaccharides, extracted for such economically important products as carageenan, agar and agarose. Many red algae are eaten in coastal areas around the world, most notably in Asia. The widespread popularity of Japanese sushi has popularized the wrapper nori (Porphyra japonica) to the West. Despite their importance in marine environments, it is entirely possible that the red algae arose in freshwater. At least three of the basal-branching lineages of red algae are wholly or in part freshwater organisms. The hypothesis that red algae originated in the freshwater environment is particularly interesting given that the same seems likely to be true of both glaucocystophytes and green algae. The implication is that the fundamental diversification of plants took place in freshwater, challenging the axiom that the seas are the cradle of diversity. The green algae are profoundly important organisms by any measure. Even if one insists upon artificially excluding the land plants, they are found in almost every environment, including marine and fresh waters, in terrestrial habitats from the rainforest to severe deserts, and from the equator to the extreme polar regions. They are not responsible for as much marine primary productivity as are diatoms or dinoflagellates, but when land plants are included these organisms dominate global primary productivity. There is an extremely old fossil record for green algae, probably comparable in depth to that of the red algae, although the identity of the oldest macrofossils is open to considerable interpretation. This is complicated by their tremendous morphological diversity, which ranges from minimalist unicells and rococo coenocytes to highly complex, branched multicellular forms and the elaborate morphologies of land plants. Although the ancestral condition appears to have been a flagellate unicell, structural complexity and multicellularity have evolved independently multiple times. There are two great lineages of green algae — the chlorophytes and the charophytes — both of which contain a wide range of morphologies. The majority of species are found among the chlorophytes, but the charophytes hold pride of phylogenetic position with their close relationship to land plants (Figure 2B ). The bulk of green algal diversity is found among the chlorophytes. They occur in both marine and freshwater environments, with freshwater taxa extending to subaerial and essentially terrestrial habitats. In most cases, the species occupying very dry (xeric) habitats do so by virtue of desiccation tolerance and reliance on transient wet conditions, rather than by true exploitation of dry conditions, and thus are generally regarded as aquatic organisms. There is so much structural diversity in this lineage that it almost defies description. The classical interpretation of the evolution of morphology in green algae and plants envisioned a series of steps of increasing complexity starting with unwalled unicellular flagellates, then moving to walled single cells, disorganized clusters of cells, filaments and multicellular spheres (considered more advanced because these morphologies require coordination of cell division), branched and pluriseriate filaments, and finally complex, three-dimensionally organized tissues. In each of these categories there may be a larger or smaller number of distinct cell types, and cells may or may not communicate and share nutrients across the thallus. These principles seem to be fundamentally sound, but most of these developmental conditions are now known to have evolved independently multiple times, and there are forms of development that are difficult to categorize in this context. Ostreococcus is among the smallest eukaryotic cells known: it is a minute spherical cell, roughly 1 μm in diameter, found in the marine plankton. This places the cell in a size-class more typically associated with bacteria, and yet the cell manages to pack in typical eukaryotic features, including a nucleus, chloroplast, mitochondria, and endomembrane system, although flagella seem to be completely lacking. Ostreococcus does branch near the base of the green algal tree, but other unicells, such as the important model system Chlamydomonas and the candidate biofuel organism Dunaliella, are much more derived. At the opposite extreme are a number of large and structurally complex genera that could easily be mistaken for land plants. Noteworthy genera include the coenocytic Caulerpa; this organism has stolons, assimilators, and rhizoids corresponding to the stems, leaves, and roots of land plants without the benefit of multicellularity. An individual thallus does contain many nuclei and other organelles, but it is organized into a single continuous cell. This may simply be an accident of its evolutionary history, but it has been suggested that this coenocytic organization may confer an advantage in environments with heavy growth of epiphytes by permitting rapid resource reallocation to newly growing tissue. Acetabularia is nearly as complex, but is a single cell, several centimeters long, with a single nucleus; it was in this organism that mRNA was first identified. Between these extremes there is a vast diversity of colonial, filamentous, or tissue-like chlorophytes that form an important part of the flora of marine and freshwater environments, and are surprisingly common in terrestrial environments. The charophytes are a well-defined, strictly freshwater lineage with seven distinct sub-lineages. The nomenclature for the group becomes a bit confused, probably largely because the embryophytes, which are often given the rank of Kingdom, are embedded within the group, but there is no doubt that they are a natural group. Like the chlorophytes, the charophytes display tremendous morphological diversity. The earliest branching lineages of charophytes include two apparently monotypic lineages: the Mesostigmatales, which consists solely of the scaly green unicellular flagellate Mesostigma viride, and the Chlorokybales, represented only by Chlorokybus atmosphyticus. Mesostigma has a curious shape, reminiscent of an oval pillow with a depression in one side and two flagella inserted in that depression. Not surprisingly, this produces a distinctive, tumbling motion when swimming. Chlorokybus is non-motile in the vegetative state and consists of packets of cells embedded in concentric layers of mucilage. The Klebsormidiales, an unbranched filamentous order, are more diverse, comprising at least 15 species of Klebsormidium, which is a common alga on sidewalks, walls, and roofs in wetter climates, as well as the rare genus Entransia. Zoospores of Klebsormidium and Chlorokybus are scaly cells with two laterally or subapically inserted flagella. The remaining four lineages almost certainly constitute a monophyletic group (the ‘Phragmoplastophyta’), although the relationships among them are an area of active research. Of these, the Zygnematophyceae are either filamentous or unicellular, the Coleochaetales and Charales are branched filaments (albeit of markedly different sizes), and the embryophytes are composed of complex, three-dimensional tissue. Although the zygnematophytes include unicellular species, it is likely that the ancestral condition for the group is branched filaments, with most if not all of the unicellular species being a case of secondary reduction. The most striking attribute of the zygnematophytes is the complete lack of any flagellate stages, with the concomitant evolution of a distinctive form of sexual reproduction termed ‘conjugation’. Many of the unicellular species of zygnematophytes (particularly those in the Desmidiaceae) have extraordinarily complex and beautiful cells. Filamentous zygnematophytes, such as Spirogyra, Mougeotia, and Zygnema, appear to be unbranched filaments, but many produce holdfasts (specialized attachment structures) composed of complex cell wall extensions, and some species do produce a single basal branch. The zygnematophytes are abundant in a wide range of freshwater environments and are quite diverse, with thousands of species. The second key lineage in the Phragmoplastophyta are the Coleochaetales, two or three genera of moderately common to exceptionally rare freshwater species. These microscopic organisms are branching filaments equipped with distinctive ‘sheathed’ hairs. The life cycle of Coleochaete is primarily haploid, but up to 32 swimming spores can be released from a single zygote. This is somewhat reminiscent of the life cycle of embryophytes, which involves an alternation of multicellular haploid and multicellular diploid generations, and led Bower to propose a close relationship between Coleochaete and embryophytes as early as 1906. This was reinforced with ultrastructural studies starting in the 1960s showing that cell division in Coleochaete involves the formation of a cell plate with plasmodesmata. Such phragmoplastic cell division is characteristic of embrophytes and lends its name to the group of four lineages in which this kind of cell division is observed. A number of other structural and biochemical properties in Coleochaetales strongly resemble those of embryophytes. Like the Coleochaetales, the Charales are branched filaments, but the thalli can be much larger — typically centimeters to meters in length at maturity — and are substantially more complex. Their body plan consists of whorls of branches at nodes. Because of this, the thalli appear superficially to be plant-like, but the apical meristem consists of a single cell, and the development is fundamentally different from that of embryophytes. Like embryophytes, however, members of both the Coleochaetales and Charales clearly control their developmental processes in three dimensions, and have coordinated structures that are composed of multiple cells. The Charales have a rich and deep fossil record. The oldest unambiguous fossil is over 400 million years old, and the slightly younger but exquisitely preserved Paleonitella from the Windyfield and Rhynie Cherts is remarkably similar to modern species. This is only slightly less ancient than the earliest fossils found in the embryophytes, and is consistent with the belief that the Charales are a lineage comparable in age to the embryophytes. Neither the zygnematophytes nor the Coleochaetales has an extensive fossil record, but studies of DNA sequence divergence suggest that these are also very old lineages (DNA sequence divergence between two genera in the Coleochaetales exceeds that between the gymnosperm Ginkgo and the flowering plant Arabidopsis). Although it is clear that the four phragmoplastic lineages form a monophyletic group, there is considerable uncertainty regarding the branching order among them. In fact, almost every possible combination of branching sequence among these four groups has been observed in one or another recent and credible analysis. We regard the identity of the sister taxon to embryophytes as an open question, although it is almost certain to be some combination of the zygnematophytes, Charales, and Coleochaetales. In any case there is little doubt that the common ancestor was a branched, filamentous organism, very likely with oogamous sexual reproduction and a haplontic life history. Embryophytes constitute the dominant terrestrial vegetation. They are a monophyletic group that colonized the land roughly 450 million years ago and then underwent rapid diversification, so that by 400 million years ago there was a well-developed terrestrial flora (Figure 2). The land was not devoid of vegetation prior to the origin of embryophytes; several lineages of cyanobacteria and green algae were almost certainly resident on land far earlier. Fifty million years provided ample time for evolution, but it is striking that the terrestrial vegetation went from simple (and probably localized) crusts to complex communities of large, strongly three-dimensional organisms in a tiny fraction of Earth's history. This transition had a profound effect on the entire biosphere, not least of which was the evolution of a new type of rock — coal — that sequesters many billions of tons of carbon worldwide. There are several distinctive developmental, biochemical, and life-history properties of embryophytes that provide adaptation to the terrestrial environment, and clearly demarcate them as a monophyletic group. The organization of the plant body into multicellular, three dimensional tissue made it possible for plants to anchor themselves in the soil and grow tall. This facilitated the competition for light and space, and permitted the exploitation of new resources. A key biochemical innovation was the evolution of lignin; this complex phenolic polymer is nearly unique to the embryophytes and is crucial in compression-resistant and decay-resistant cell walls. These allow plants to grow tall both by providing structural support and by facilitating the transport of water by evapotranspiration. Protection of gametes by sterile jacket cells, control of hydration with a waterproof surface layer (the cuticle) and stomata, and later the evolution of specialized structures for the acquisition and transport of water and nutrients also played key roles in the colonization of the land. The plant life cycle can be particularly disorienting. All of the charophytes other than embryophytes have a haplontic life cycle, meaning that all stages of the life cycle other than the zygote are haploid. In embryophytes the ancestral haplontic life cycle has been modified by mitotic cell division in the zygote, producing a multicellular diploid generation (called the‘sporophyte’ because it produces spores), which grows, at least initially, in place and dependent upon the haploid parent (called the ‘gametophyte’, because it produces gametes). The presence of a multicellular diploid embryo is the basis for the name ‘embryophyte’. This life cycle permits a single fertilization event to result in millions of spores, so it is thought to be a terrestrial adaptation. There also appears to be an advantage to diploidy, and a broad trend in the evolution of embryophytes is the increasing dominance of the sporophyte generation at the expense of the gametophyte. In the higher seed plants the gametophytes are reduced to just a few cells. The most structurally simple embryophytes are three distinct lineages often (and possibly naturally) linked as the ‘bryophytes’. All three have life cycles that are dominated by the gametophyte, with the sporophyte functioning primarily as a mechanism for spore dispersal. They are all small, with most species being no more than a few centimeters in height. Most grow and develop in relatively damp conditions, although many are desiccation tolerant and some can endure extreme heat and near-complete drying during dormancy. Hornworts are relatively uncommon and consist of a sheet-like gametophyte that in most species is not organized into an organized branching structure. Their sporophyte is a narrow column that grows from the base and dries and splits open at the tip to release spores. The sporophyte can become at least partially independent of the gametophyte, but it never branches and the gametophyte is considered to be the dominant phase. Liverworts are found more frequently than are hornworts, and include both the ‘thalloid’ liverworts, which have two-dimensional, sheetlike gametophytes, and the ‘leafy’ liverworts, whose gametophytes are organized into distinctly stem-like and leaf-like structures. Although tiny, the sporophytes have clearly distinct sections (the sporangium, column, and foot). Sporophyte growth is determinate and entirely dependent upon the gametophyte. The mosses are by far the most widespread and abundant of the bryophytes. Although often inconspicuous, they constitute an important component of the vegetation, and are dominant in some arctic and alpine regions. In most species the gametophytes are organized into stems and leaves with differentiated conducting tissue supporting this arrangement. The sporophyte is unbranched, but is equipped with a sporangium with a specialized lid and aperture for spore release. The lycopods are a classic ‘relict’ group, the sparse living remnants of a previously widespread and ecologically important lineage. Although still widely distributed, they never dominate the vegetation. During the Carboniferous, however, relatives of the living lycopods dominated vast tracts of low-lying wetland marsh, and it is largely the remains of these organisms that resulted in the formation of coal. Lycopods are true vascular plants, with well-developed water and photosynthate transport systems (xylem and phloem, respectively). The dominant generation is the sporophyte, and the gametophyte is (depending upon the species) inconspicuous, subterranean and nonphotosynthetic, or else housed entirely within the walls of the parental spore. This latter case is considered to be of evolutionary importance because ‘heterospory’ — a syndrome that includes the production of morphologically distinct spores which give rise to separate male and female gametophytes and development of the gametophyte entirely within the spore walls — is thought to reflect independent evolution of a syndrome similar to that which ultimately led to the evolution of seed plants. The pteridophytes are the true ferns and their relatives. They are vascular plants with dominant sporophytes and well-developed leaves and roots, and are widespread and important members of the vegetation. With 10,000 or more species they are highly diverse, ranging from minute understory herbs, to holoaquatic plants, to spectacular tree ferns. Dispersal in ferns is largely as spores, which are small and highly abundant. Consequently, ferns easily disperse long distances and are often early colonizers of new habitat, becoming disproportionately important in the vegetation of remote islands and other isolated regions. The pteridophytes display vast diversity of habitat, morphology, and biology, and they present a splendid independent contrast to the diversification of seed plants. Several organisms, notably the ‘whisk ferns’ Psilotum and Tmesipteris and the ‘horsetail’ Equisetum are so different from typical ferns that many historical classifications separated them from the other pteridophytes and gave them their own high-level groups. The advent of molecular systematic data provided clear evidence for their inclusion in a monophyletic group with other pteridophytes. The gymnosperms are a diverse assemblage of ancient and ecologically significant plants, although they are not a species-rich group comparatively speaking. Considerable seed plant diversity is known only from the fossil record, and a comprehensive discussion of gymnosperms would require a great deal of paleobotany. Together with angiosperms, gymnosperms constitute the seed plants, which are marked by a key development in the life cycle of embryophytes. Although seed plants retain the classic alternation of generations, in seed plants the megaspores (female gametophyte-producing spores) are not dispersed from the parent sporophyte. They develop in place, which opens up the opportunity for contribution of nutrients from the parent sporophyte to the developing gametophyte and, following fertilization, embryo. Pollen, which is a microspore that has developed into a partially mature male gametophyte, can be transferred to the female gametophyte, after which it produces sperm and effects fertilization. This all seems a bit rococo, but can be understood in the context of the ancestral life history. Because the embryo of the next sporophyte generation develops in (indirect) communication with the sporophyte of the previous generation, it can receive far more nutritive support, and seeds can develop into durable and long-lived dispersal structures with vast nutrient reserves compared to the ephemeral nature of spores. There are four main lineages of gymnosperms. The conifers (Pinophyta) are the most familiar and species-rich lineage. Although there are only about 600 species, the subarctic region of North America and Eurasia is covered by almost continuous stands of pines and other conifers. Many species of gymnosperms produce high-quality wood; such ‘softwoods’ form the basis for major timber and pulp industries. The second most diverse and widespread lineage of gymnosperms are the cycads. Cycads have a dense whorl of typically leathery leaves near the top of a minimally extended stem with little or no branching. They are very distinctive plants, but in some cases could be mistaken for a small palm tree or coarse fern, at least until the heavy seed-bearing cones become apparent. Cycads also retain a remarkable ancestral trait: the pollen matures after transmission to the receptive female cone and produces an elaborate swimming sperm with thousands of flagella that swims a millimeter or two across a tiny puddle of liquid between the male and female gametophytes. The third major lineage consists in living species solely of the tree Ginkgo. Ginkgo is widely planted as an urban street tree because of its ability to tolerate air pollution and enforced crowding and disruption of its roots. It was also abundant in the fossil record in Europe and North America, but survived to modern times only in Asia. It produces swimming sperm in a manner similar to cycads. The fourth and final lineage consists of the gnetophytes. These are three utterly distinctive plants, Ephedra, Gnetum, and Welwitschia. Welwitschia, once described by the eminent plant morphologist Donald Kaplan as “the plant from Mars”, survives for decades or centuries with only two photosynthetic leaves that grow continuously from the base. Like conifers, gnetophytes do not produce any flagellate stage, and transfer sperm nuclei via a specialized cellular extension called a pollen tube. Despite several striking similarities between gnetophytes and flowering plants, most molecular studies have shown the gymnosperms to be monophyletic. If this ‘gnetifer’ hypothesis is correct and the gymnosperms are the sister taxon to angiosperms, then this suggests a real puzzle. The gymnosperms probably originated in the Carboniferous Period, easily 300 million years ago; however, angiosperms do not appear in the fossil record until nearly 200 million years later. Darwin called the sudden appearance of angiosperms an “abominable mystery”, and although substantial progress has been made, it remains perplexing. The answer may yet be found by study of the many ancient, extinct lineages of seed plants. Angiosperms, or flowering plants, are a relatively recent, but profoundly important, part of the terrestrial vegetation. After bursting on the scene about 130 million years ago, they underwent remarkably rapid spread and diversification, such that by 100 million years ago they were widespread and diverse. There are now more than 223,000 species, dwarfing all other plant lineages in terms of diversity. They also dominate the terrestrial vegetation in coverage. The grasses alone are responsible for the bulk of human nutrition; rice, wheat, corn, barley, rye, and oats are all grass fruits. One key to angiosperm success is their life cycle, which is remarkably fast compared with those of gymnosperms. Gymnosperms often take a year or more from pollination to fertilization, and the entire life cycle (from seed to seed) for some species is measured in centuries. By contrast, the ‘model’ angiosperm Arabidopsis has a generation time of about five weeks. But even more important than speed may be the selectivity of angiosperm pollination and seed dispersal. Angiosperms are the vegetable masters of animals. The flower presents and protects the young female gametophyte while facilitating dispersal of the pollen. There is a vast diversity of pollination syndromes, including wind and water dispersal, but a key innovation in angiosperms is the reliance on animal vectors for dispersal (insects, birds, and bats, to name just three). Flowers are finely tuned mechanisms to attract, guide, deceive, and reward potential pollinators. And because of this angiosperms can live quite far from their potential mates, so they are not restricted to dense conspecific stands as are most wind pollinated plants. Following fertilization, while the zygote develops into an embryo inside of a seed, the ovary surrounding the seed (often along with other flower parts) will develop into a fruit. Fruits are nearly as exquisitely adapted to the dispersal of seeds as flowers are to the dispersal of pollen. Like pollen, wind and water can provide important dispersal mechanisms, but the most compelling vectors are animals. Many fruits have sweet or savory outer portions that entice animals such as ourselves to eat the fruit only to encounter a tough core, and if you are so unwise as to bite into the pit of that peach you will find a bitter and potentially toxic seed inside. It is far easier to eat the peach and toss the pit aside, and in so doing you have functioned as the dispersal agent for that plant. Other plants have hooks, barbs, or adhesives that cause their seeds to stick to the fur of passing animals or even stealthy stinging hairs that shock the unsuspecting monkey into dropping its meal. The plants — defined here as a major eukaryotic lineage with primary plastids and encompassing the groups traditionally referred to as glaucocystophytes, red algae, green algae and embryophytes — are an ancient evolutionary lineage accounting for some of the oldest known fossils of eukaryotes, and are probably at least two billion years old. Because of this, there is vast biological diversity among the deeper branching lineages within the group. The terrestrial flora consists primarily of a single clade, the embryophytes, which arose roughly 450 million years ago. One sub-clade, the angiosperms, appears in the fossil record only about 130 million years ago, but it underwent such rapid diversification and spread that these organisms now dominate the land. Thus, understanding the diversity of plants requires attention to both the riot of diversity within the angiosperms and the profound divergences among the most ancient clades of algae.