The table below shows some species count estimates of different green plant (Viridiplantae) divisions. It suggests there are about 300,000 species of living Viridiplantae, of which 85–90% are flowering plants. (Note: as these are from different sources and different dates, they are not necessarily comparable, and like all species counts, are subject to a degree of uncertainty in some cases.)
Diversity of living green plant (Viridiplantae) divisions Informal group Division name Common name No. of living species Approximate No. in informal group Green algae Chlorophyta green algae (chlorophytes) 3,800–4,300 Van den Hoek, C., D. G. Mann, & H. M. Jahns, 1995. Algae: An Introduction to Phycology. pages 343, 350, 392, 413, 425, 439, & 448 (Cambridge: Cambridge University Press). ISBN 0-521-30419-9 8,500 (6,600–10,300) Charophyta green algae (e.g. desmids & stoneworts) 2,800–6,000 Van den Hoek, C., D. G. Mann, & H. M. Jahns, 1995. Algae: An Introduction to Phycology. pages 457, 463, & 476. (Cambridge: Cambridge University Press). ISBN 0-521-30419-9 Bryophytes Marchantiophyta liverworts 6,000–8,000 Crandall-Stotler, Barbara. & Stotler, Raymond E., 2000. "Morphology and classification of the Marchantiophyta". page 21 in A. Jonathan Shaw & Bernard Goffinet (Eds.), Bryophyte Biology. (Cambridge: Cambridge University Press). ISBN 0-521-66097-1 19,000 (18,100–20,200) Anthocerotophyta hornworts 100–200 Schuster, Rudolf M., The Hepaticae and Anthocerotae of North America, volume VI, pages 712–713. (Chicago: Field Museum of Natural History, 1992). ISBN 0-914868-21-7. Bryophyta mosses 12,000 Goffinet, Bernard (2004): Systematics of the Bryophyta (Mosses): From molecules to a revised classification, Monographs in Systematic Botany, 98: pp. 205–239. Missouri Botanical Garden Press Pteridophytes Lycopodiophyta club mosses 1,200 Raven, Peter H.; Evert, Ray F.; Eichhorn, Susan E. (2005): Biology of Plants(7th). W. H. Freeman and Company 12,000 (12,200) Pteridophyta ferns, whisk ferns & horsetails 11,000 Seed plants Cycadophyta cycads 160 Gifford, Ernest M.; Foster, Adriance S. (1988): Morphology and Evolution of Vascular Plants(3rd): pp. 358. W. H. Freeman and Company 260,000 (259,511) Ginkgophyta ginkgo 1 Taylor, Thomas N.; Taylor, Edith L. (1993): The Biology and Evolution of Fossil Plants: pp. 636. Prentice-Hall Pinophyta conifers 630 Gnetophyta gnetophytes 70 Magnoliophyta flowering plants 258,650 International Union for Conservation of Nature and Natural Resources, 2006. IUCN Red List of Threatened Species:Summary Statistics The naming of plants is governed by the International Code of Botanical Nomenclature and International Code of Nomenclature for Cultivated Plants (see cultivated plant taxonomy). Evolution
The evolution of plants has resulted in increasing levels of complexity, from the earliest algal mats, through bryophytes, lycopods, ferns to the complex gymnosperms and angiosperms of today. The groups which appeared earlier continue to thrive, especially in the environments in which they evolved.Evidence suggests that an algal scum formed on the land , but it was not until the Ordovician Period, around , that land plants appeared."The oldest fossils reveal evolution of non-vascular plants by the middle to late Ordovician Period (~450-440 m.y.a.) on the basis of fossil spores" Transition of plants to land However, new evidence from the study of carbon isotope ratios in Precambrian rocks has suggested that complex photosynthetic plants developed on the earth over 1000 m.y.a."The apparent dominance of eukaryotes in non-marine settings by 1 Gyr ago indicates that eukaryotic evolution on land may have commenced far earlier than previously thought." Earth’s earliest non-marine eukaryotes These began to diversify in the late Silurian Period, around , and the fruits of their diversification are displayed in remarkable detail in an early Devonian fossil assemblage from the Rhynie chert. This chert preserved early plants in cellular detail, petrified in volcanic springs. By the middle of the Devonian Period most of the features recognised in plants today are present, including roots, leaves and secondary wood, and by late Devonian times seeds had evolved.Rothwell, G. W.; Scheckler, S. E.; Gillespie, W. H. (1989): Elkinsia gen. nov., a Late Devonian gymnosperm with cupulate ovules, Botanical Gazette, 150(2): pp. 170–189. University of Chicago Press Late Devonian plants had thereby reached a degree of sophistication that allowed them to form forests of tall trees. Evolutionary innovation continued after the Devonian period. Most plant groups were relatively unscathed by the Permo-Triassic extinction event, although the structures of communities changed. This may have set the scene for the evolution of flowering plants in the Triassic (~), which exploded in the Cretaceous and Tertiary. The latest major group of plants to evolve were the grasses, which became important in the mid Tertiary, from around . The grasses, as well as many other groups, evolved new mechanisms of metabolism to survive the low and warm, dry conditions of the tropics over the last . A proposed phylogenetic tree of Plantae, after Kenrick and Crane,Kenrick, Paul & Peter R. Crane. 1997. The Origin and Early Diversification of Land Plants: A Cladistic Study. (Washington, D.C., Smithsonian Institution Press.) ISBN 1-56098-730-8. is as follows, with modification to the Pteridophyta from Smith et al.Smith Alan R., Pryer Kathleen M., Schuettpelz E., Korall P., Schneider H., Wolf Paul G. (2006): A classification for extant ferns, Taxon, 55(3): pp. 705–731 The Prasinophyceae are a paraphyletic assemblage of early diverging green algal lineages.
The plants that are likely most familiar to us are the multicellular land plants, called embryophytes. Embryophytes include the vascular plants, such as ferns, conifers and flowering plants. They also include the bryophytes, of which mosses and liverworts are the most common.All of these plants have eukaryotic cells with cell walls composed of cellulose, and most obtain their energy through photosynthesis, using light, water and carbon dioxide to synthesize food. About three hundred plant species do not photosynthesize but are parasites on other species of photosynthetic plants. Plants are distinguished from green algae, which represent a mode of photosynthetic life similar to the kind modern plants are believed to have evolved from, by having specialized reproductive organs protected by non-reproductive tissues.Bryophytes first appeared during the early Paleozoic. They can only survive where moisture is available for significant periods, although some species are desiccation-tolerant. Most species of bryophytes remain small throughout their life-cycle. This involves an alternation between two generations: a haploid stage, called the gametophyte, and a diploid stage, called the sporophyte. In bryophytes, the sporophyte is always unbranched and remains nutritionally dependent on its parent gametophyte. The bryophytes have the ability to secrete a cuticle on their outer surface, a waxy layer that confers resistant to desiccation. In the mosses and hornworts a cuticle is usually only produced on the sporophyte. Stomata are absent from liverworts, but occur on the sporangia of mosses and hornworts, allowing gas exchange while controlling water loss. Vascular plants first appeared during the Silurian period, and by the Devonian had diversified and spread into many different terrestrial environments. They developed a number of adaptations that allowed them to spread into increasingly more arid places, notably the vascular tissues xylem and phloem, that transport water and food throughout the organism. Root systems capable of obtaining soil water and nutrients also evolved during the Devonian. In modern vascular plants, the sporophyte is typically large, branched, nutritionally independent and long-lived, but there is increasing evidence that Paleozoic gametophytes were just as complex as the sporophytes. The gametophytes of all vascular plant groups evolved to become reduced in size and prominence in the life cycle.The first seed plants, Pteridosperms (seed ferns), now extinct, appeared in the Devonian and diversified through the Carboniferous. In these the microgametophyte is reduced to pollen and the megagametophyte remains inside the megasporangium, attached to the parent plant. A megasporangium invested in protective layer called an integument is known as an ovule. After fertilisation by means of sperm deposited by pollen grains, an embryo develops inside the ovule. The integument becomes a seed coat, and the ovule develops into a seed. Seed plants can survive and reproduce in extremely arid conditions, because they are not dependent on free water for the movement of sperm, or the development of free living gametophytes.Early seed plants are gymnosperms, as the ovules and subsequent seeds are not enclosed in a protective structure (carpels or fruit), but are found naked, typically on cone scales. Pollen typically lands directly on the ovule. Four surviving groups remain widespread now, particularly the conifers, which are dominant trees in several biomes. Fossils Plant fossils include roots, wood, leaves, seeds, fruit, pollen, spores, phytoliths, and amber (the fossilized resin produced by some plants). Fossil land plants are recorded in terrestrial, lacustrine, fluvial and nearshore marine sediments. Pollen, spores and algae (dinoflagellates and acritarchs) are used for dating sedimentary rock sequences. The remains of fossil plants are not as common as fossil animals, although plant fossils are locally abundant in many regions worldwide.The earliest fossils clearly assignable to Kingdom Plantae are fossil green algae from the Cambrian. These fossils resemble calcified multicellular members of the Dasycladales. Earlier Precambrian fossils are known which resemble single-cell green algae, but definitive identity with that group of algae is uncertain.The oldest known fossils of embryophytes date from the Ordovician, though such fossils are fragmentary. By the Silurian, fossils of whole plants are preserved, including the lycophyte Baragwanathia longifolia. From the Devonian, detailed fossils of rhyniophytes have been found. Early fossils of these ancient plants show the individual cells within the plant tissue. The Devonian period also saw the evolution of what many believe to be the first modern tree, Archaeopteris. This fern-like tree combined a woody trunk with the fronds of a fern, but produced no seeds.The Coal measures are a major source of Paleozoic plant fossils, with many groups of plants in existence at this time. The spoil heaps of coal mines are the best places to collect; coal itself is the remains of fossilised plants, though structural detail of the plant fossils is rarely visible in coal. In the Fossil Forest at Victoria Park in Glasgow, Scotland, the stumps of Lepidodendron trees are found in their original growth positions.The fossilized remains of conifer and angiosperm roots, stems and branches may be locally abundant in lake and inshore sedimentary rocks from the Mesozoic and Cenozoic eras. Sequoia and its allies, magnolia, oak, and palms are often found.Petrified wood is common in some parts of the world, and is most frequently found in arid or desert areas where it is more readily exposed by erosion. Petrified wood is often heavily silicified (the organic material replaced by silicon dioxide), and the impregnated tissue is often preserved in fine detail. Such specimens may be cut and polished using lapidary equipment. Fossil forests of petrified wood have been found in all continents.Fossils of seed ferns such as Glossopteris are widely distributed throughout several continents of the Southern Hemisphere, a fact that gave support to Alfred Wegener's early ideas regarding Continental drift theory.The earliest fossils attributed to green algae date from the Precambrian (ca. 1200 mya).Knoll, Andrew H (2003): Life on a Young Planet: The First Three Billion Years of Evolution on Earth. Princeton University PressTappan, H (1980): Palaeobiology of Plant Protists. Freeman, San Francisco The resistant outer walls of prasinophyte cysts (known as phycomata) are well preserved in fossil deposits of the Paleozoic (ca. 250-540 mya). A filamentous fossil (Proterocladus) from middle Neoproterozoic deposits (ca. 750 mya) has been attributed to the Cladophorales, while the oldest reliable records of the Bryopsidales, Dasycladales) and stoneworts are from the Paleozoic.Butterfield, Nicholas J.; Knoll, Andrew H.; Swett, Keene (1994): Paleobiology of the Neoproterozoic Svanbergfjellet Formation, Spitsbergen, Lethaia, 27(1): pp. 76–76Leliaert, F., Smith, D.R., Moreau, H., Herron, M.D., Verbruggen, H., Delwiche, C.F. & De Clerck, O. (2012): Phylogeny and molecular evolution of the green algae, Critical Reviews in Plant Sciences, 31: pp. 1–46