Plant Biology: Classification: Plants, Other Organisms

A century ago, botanists recognized four major groups of plants, but within 50 years these groups had been subdivided and rearranged into still further groupings. Table 1 presents the old groupings of plants in considerable detail because some of the names persist as common names for modern plant groups, for example, thallophytes, gymnosperms, and monocots. By and large, the major groups (such as ferns, mosses, and flowering plants) are still recognized as units in today's more recent rearrangements but now appear with new technical names.

TABLE 1

Three Historical Classification Schemes of Organisms

A current lively debate among plant systematists concerns how best to incorporate the recently obtained data from molecular biology into a phylogenetic arrangement of the plant kingdom. (Configurations in the past were based primarily upon morphological and anatomical features.) Of interest as well is the controversy over the ranking of the two clearly different groups of bacteria. Are they “kingdoms,” “superkingdoms,” or “domains”? Should the classification of all living things start by first separating the prokaryotes and eukaryotes into two superior groups? Table 2 summarizes three current ways. Note that the four eukaryote groups are retained as “kingdoms” in all of the classifications.

TABLE 2

Three Ways to Classify Organisms

*3 Domains, 4 Kingdoms*	*6 Kingdoms*	*2 Superkingdoms, 5 Kingdoms*
		Superkingdon Prokarya
		Kingdom Bacteria (Prokaryotae, Procaryotae, Monera)
Domain Archaebacteria (Archaea): bacteria of extreme environments	Kingdon Archaebacteria	Subkingdom Archaea
Domain Bacteria: most of the common bacteria	Kingdom Eubacteria	Subkingdom Eubacteria
Domain Eukarya		Superkingdom Eukarya
Kingdom Fungi: molds, rusts, yeasts, smuts, morels, truffles, mushrooms, lichens	Kingdom Fungi: molds, rusts, yeasts, smuts, morels, truffles, mushrooms, lichens	Kingdom Fungi: molds, rusts, yeasts, smuts, morels, truffles, mushrooms, lichens
Kingdom Protoctista (Protista): algae, slime molds, water molds, protozoans	Kingdom Protoctista: algae, slime molds, water molds, protozoans	Kingdom Protoctista: algae, slime molds, water molds, protozoans
Kingdom Plantae: mosses, liverworts, ferns, fern allies, cycads, ginkgos, conifers, flowering plants	Kingdom Plantae: mosses, liverworts, ferns, fern allies, cycads, ginkgos, conifers, flowering plants	Kingdom Plantae: mosses, liverworts, ferns, fern allies, cycads, ginkgos, conifers, flowering plants
Kingdom Animalia: multicellular animals	Kingdom Animalia: multicellular animals	Kingdom Animalia: multicellular animals

Most plant biology texts mention briefly the multicellular animals and discuss in detail only the most plant-like of the protista. Bacteria and fungi are customarily included, although scientists no longer considered them to be plants. Table 3 gives an overview of the major groups usually studied in introductory plant biology courses. As always, your textbook is the best place to find pictures, more detailed information about plants and definitions of unfamiliar terms.

TABLE 3

A Synopsis of the Organisms Usually Studied in Introductory Plant Biology

*Name*	*Distinctive Features*	*History and Phylogeny*	*Other Significant Features*
Eubacteria (Bacteria): true bacteria, cyanobacteria, spirochetes, purple and green bacteria, pathogens	Prokaryotes; the most metabolically diverse—organisms: autotrophs (photosynthetic and chemosynthetic), hetertophs; anaerobes, and aerobes	The most abundant, smallest, and oldest organisms; present 2 billion years before eukaryotes appeared; modified the environment and made possible eukaryotic life	Recycle organic matter; fix atmospheric nitrogen; significant cause of diseases; used in industry to make cheese, alcohol, antibiotics, and in genetic engineering
Archaebacteria: extremophiles, methanogens, halophiles, thermophiles	Prokaryotes of extreme environments, i.e. very: hot, cold, acid, salty; structurally different from eubacteria and more closely related to the eukaryotes; appeared later than eubacteria	The endosymbiosis theory suggests some prokaryotes were engulfed by others and lived symbiotically within them; over time, these became the organelles of eukaryote cells	Extremophiles responsible for: natural gas reserves, colors in hot-springs, salt flats; live in hydrothermal vents on the ocean floor in high pressures and temperatures
Viruses (Not Organisms): bacteriophage, retrovirus, HIV, polio virus, tobacco mosaic cell; virus, rhinovirus, Ebola virus	Non-cellular, not alive; are molecules of DNA or RNA surrounded by like their own, need specific protein; need energy from a host cell to replicate; non-motile; don't grow nor metabolize	No fossil record; may have caused malformations seen in some fossils of early organisms; probably arose as bits of DNA broken from genomes of cells	Attack hosts with genomes most protein receptors to enter a presence triggers response, i.e. disease in plants, animals, bacteria, protists
Fungi: mushrooms, truffles, rusts, smuts, molds, yeasts, rots. Divided into four or five groups on basis of their sexual reproductive structures: Zygomycota, Ascomycota, Basidiomycota, Deuteromycetes, Fungi Imperfecti. Chytridiomycota often included with Fungi	Not plants; eukaryotic body called mycelium consists of masses of hyphae (filaments); cell walls of chitin; glycogen stored as reserve food; are heterotrophic; obtain food by spores and sexually by zygotic meiosis	Oldest filaments appear in a Lower Cambrian; also some mycorrhizae in stems of an early Devonian plant; mycorrhizae may have facilitated the move of plants from water to land—with fungi substituting for roots of the first land plants	The principal decomposers together with the bacteria; most are saprotrophs (saprobes); some are causal agents of plant, animal, human diseases; many form important symbioses with roots of vascular plants (mycorrhizae); others are symbiotic with algae forming lichens; some edible, others used commercially in brewing, baking, medicine
Protista (Protoctista): algae, amoebae, flagellates, sporozoans, ciliates, water molds, diatoms, slime molds, etc. No completely satisfactory way to classify this extremely diverse group	Not plants; eukaryotes many with plant-like features (chlorophyll, photosynthetic, cellulose in walls), others like fungi (filamentous), some animal-like (ingest food); unicellular, multicellular, colonial; mostly aquatic, both marine and freshwater	Oldest eukaryote may be a brown-alga-like fossil 1.7 billion years old; or another presumed photosynthetic eukaryote found in 2.1-billion-year-old rocks; acritarchs (fossilized shells of shelled amoebas) first found in rocks 1.5 billion years old	Importantance: plants, animals, and fungi all derived from ancient protists; plant ancestor a green alga very much like green algae of today
Plantae– Bryophytes: mosses, liverworts, hornworts. Divided into three groups: Bryophyta (mosses), Hepaticophyta (liverworts), Anthocerotophyta (hornworts)	Small, nonvascular plants; body a thallus; gametophyte is free-living and the prominent plant; the sporophyte small, dependent upon the gametophyte; both sexual and asexual reproduction present; motile (flagellated) sperm require water in which to reach the egg	Ancestors of bryophytes probably derived from a green alga ancestor; oldest fossil bryophytes in 350 million years old rocks (younger than first vascular plant fossils, but probably because bryophytes lack resistant tissues for preservation)	Three groups are of different lineages, liverworts the oldest; mosses important in the ecology of the arctic and subarctic; some commercial use—peat for fuel, Sphagnum as packing material
Plantae–Seedless Vascular Plants: ferns, fern allies, and horsetails. Divided into four groups: Psilotophyta (whisk ferns), Lycophyta (club mosses), Sphenopsida (horsetails), Pterophyta (ferns)	Xylem and phloem present; sporophyte dominant; asexual spores produced in sporangia; some taxa homosporous, others heterosporous	Earliest vascular plants were small, simple, and dominated mid-Silurian to mid-Devonian landscapes (425–370 million years ago); by late Devonian through the Carboniferous (the Coal Age), plants formed swamp forests of large trees; major groups died out by end of the Permian (250 million years ago)	Most living taxa are depauperate remnants of large tree species of the Coal Age flora
Plantae–Gymnosperms: conifers, cycads, gnetophytes, pines, junipers, yews, sequoia; divided into four groups: Cycadophyta (cycads), Ginkgophyta ( Ginkgo), Coniferophyta (conifers), and Gnetophyta (gnetophytes)	Ovules and seeds exposed and not covered by sporophyte tissue at time of pollination; wood lacks fibers and contains only tracheids (except in gnetophytes, which have vessels in addition); some have flagellated sperm carried in a pollen tube toovule; female gametophyte produces several archegonia and embryos, but usually only one embryo survives/ovule	Seed plants probably evolved from seed-bearing progymno-sperms sometime during the Devonian (370 million years ago); current gymnosperms represent a series of separate lineages	Trees and shrubs with no herbaceous representatives (some vine-like Gnetum species are present); distributed world-wide from boreal conifer forests to tropics, some taxa mixed with Angiosperm trees, others in pure stands of Gymnosperms
Plantae–Anthophyta: angiosperms, flowering plants	Ovules enclosed in a carpel and seeds produced in fruits; both gemetophytes highly reduced (female a seven-celled structure, male the germinated pollen grain); sporophyte the dominant plant	First fossil flowers appear in Cretaceous beds 130 million years old; in next 35 million years (Upper Cretaceous), angiosperms spread and became dominant plants in the Northern Hemisphere and, in next 10 million years, in the Southern Hemisphere also	Most successful of all the plant groups; insect-pollinated flowers basic; seeds with embryos enclosed in a resistant seed coat enabled plants to be widely dispersed; physiological adaptations and production of secondary metabolites of importance in their success