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- KINGDOM PLANTAEMulticellular photosynthetic eukaryotic organisms containing chlorophylls a and b and carotenoids and forming true starch; chlorophyll contained in stacks of membranes (grana) within chloroplasts; cellulosic cell walls; aerial tissues covered with a waxy cuticle and provided with openings (stomata) in the epidermis bordered by 2 guard cells and serving in gas exchange; alternation of generations with multicellular gametangia and sporangia and possessing a multicellular embryo stage; motile sperm, when present, with whiplash flagellum; sperm with a microtubular cytoskeleton; cell division associated with the dispersal of the nuclear membrane and the formation of a cell plate across the mitotic spindle (phragmoplast).
- Division Bryophyta (mosses)Small, mostly nonvascular, archegoniate plants with a dominant photosynthetic free-living gametophyte; sporophyte has little or no chlorophyll and is dependent on gametophyte; biflagellate sperm. Gametophytes “leafy” and radially symmetrical, with leaflike structures arising spirally from stemlike axis; many chloroplasts per cell; gametophyte with multicellular rhizoids; sporophytes with complex spore-containing capsule with peristome and operculum, columella, and stomata present but elaters absent; between 10,000 and 14,000 species; representative genera include Polytrichum, Mnium, Funaria, and Sphagnum.
- Division Anthocerotophyta (hornworts)Gametophyte thalloid, with a single large chloroplast per cell, mucilage cavities present; sporophytes persistent, erect (hornlike), photosynthetic, arising from upper surface of gametophyte, possessing stomata, columella, basal meristem, and pseudoelaters opening by 2 valves but lacking an operculum; representative genus, Anthoceros.
- Division Marchantiophyta (liverworts)Gametophytes either “leafy” or dorsiventrally flattened (strap-shaped) and thalloid; leafy forms with leaves in 3 rows, 2 lateral and 1 below; leaves usually notched or lobed, with thickened midrib lacking; gametophytes with unicellular rhizoids; many chloroplasts per cell; sporophytes ephemeral, surrounded by gametophytic tissue, lacking opercula, peristome, stomata, and columella; elaters mixed with spores in capsule; capsule opening into 4 or more valves; between 6,000 and 9,000 species; representative genera include Porella, Frullania, Marchantia, Conocephalum, and Riccia.
- Division Lycophyta (club mosses, spike mosses, quillworts)Vascular plants; sporophyte differentiated into stem, leaf, and root; leaves spirally arranged on stem, of the microphyll type; sporangia borne on upper (adaxial) surface of leaves (sporophylls), 1 sporangium per sporophyll; vascular tissue basically forming a central core in stem (protostelic); homosporous or heterosporous; ligulate (having basal leaflike protuberances) or eligulate; strobili (cones) present or lacking; all living genera with primary growth only; gametophytes subterranean or surface-dwelling; motile sperm; about 5 living genera and between 600 and 1,000 species; representative genera include Lycopodium, Selaginella, and Isoetes.
- Ferns, unranked
- Class Polypodiopsida (leptosporangiate ferns)Vascular plants; sporophyte differentiated into stem (rhizome), roots, and leaves (fronds); leaves entire or, more often, divided; arrangement of vascular tissue in stem variable, primary growth only; sporangia usually clustered into sori, often located on the under (abaxial) surface of sporophylls; mostly homosporous; gametophytes (prothallia) either subterranean and nongreen or, more commonly, surface-dwelling and photosynthetic; sperm motile; between 9,000 and 12,000 species; representative genera include Pteridium, Polypodium, Polystichum, Adiantum, and Cyathea.
- Class Equisetopsida (horsetails, scouring rushes)Vascular plants; sporophyte differentiated into stem, leaf, and root; stems ribbed and jointed, monopodial; minute leaves whorled at the nodes; vascular tissue organized into bundles; sole living genus with primary growth only; sporangia borne on specialized stalks (sporangiophores) in strobili; homosporous; gametophytes photosynthetic, surface-dwelling; motile sperm; 1 genus, Equisetum.
- Class Marattiopsida giant fernsVascular plants; mostly massive, fleshy ferns; leaves pinnately divided, pulvinate (enlarged or swollen at attachment point of leaflets) in extant genera, and with well-developed fleshy stipules (appendages at leaf base); sporangia eusporangiate, in sori, or more or less coalescent in synangia (clusters); homosporous; representative genera include Marattia, Angiopteris, and Danaea.
- Class Psilotopsida (whisk ferns)Vascular plants; sporophyte lacking roots and often leaves; stems with small enations, dichotomously branched; vascular tissue forming a central core in stem (protostelic); sporangia fused into synangiate structure, apparently terminal on short stem; homosporous; gametophytes subterranean, with motile sperm; representative genus, Psilotum.
- Division Cycadophyta (cycads)Palmlike gymnospermous plants with typically short, thick, unbranched aerial trunks, sometimes subterranean, and large, divided leaves; leaves usually thick and leathery; cones present, often large and terminal on the stem; pollen (male) and seed (female) cones borne on separate plants; gametophytes reduced, not free-living; sperm motile (flagellated) but taken to vicinity of egg by a pollen tube; 10 genera and about 120 species; representative genera include Zamia and Cycas.
- Division Ginkgophyta (ginkgoes)Gymnospermous plants; tall much-branched tree with well-developed cylinder of wood; resin ducts present; xylem with tracheids only; stem differentiated into long shoots and short spur shoots; simple fan-shaped leaves with open dichotomous venation terminate short shoots; leaves deciduous; sexes on separate trees; distinct cones lacking; gametophytes reduced, not free-living; sperm motile (flagellated); mature seeds with fleshy, foul-smelling outer region; 1 living species, Ginkgo biloba.
- Division Gnetophyta (gnetophytes)Diverse and unusual group of 3 gymnospermous genera; growth habits of all 3 are atypical among gymnosperms in possessing vessel elements in the xylem and reproductive structures that are somewhat flowerlike; gametophytes reduced as in other gymnosperms; sperm nonmotile; extant genera Gnetum, Ephedra, and Welwitschia.
- Division Pinophyta (conifers)Gymnospermous plants; mostly trees with abundant xylem composed of tracheids only; resin ducts present; leaves simple, needlelike, scalelike, with a single vein or, less commonly, strap-shaped with multiple veins; reproduction by well-defined cones; seeds exposed on ovuliferous scales; gametophyte generation reduced, microscopic, not free-living; sperm nonmotile, transported to egg by pollen tube; approximately 68 genera and some 630 species; representative genera include Pinus, Abies, Sequoia, Taxodium, Juniperus, Podocarpus,Cupressus, Taxus, and Agathis.
- Angiosperms, unranked (flowering plants)Vascular plants, xylem typically with vessel elements; reproduction by flowers; ovules or young seeds enclosed by female reproductive structure (carpel); gametophyte generation extremely reduced, consisting of only a few cells; archegonia and antheridia lacking; nonmotile sperm transported to egg by pollen tube; pollen transported to specialized receptive surface (stigma) on carpel; double fertilization, one sperm uniting with the egg to form a zygote, another fusing with the polar nuclei to form the primary endosperm nucleus; nutritive tissue of seed triploid endosperm; seeds enclosed by mature ovary that ripens into a fruit; more than 300,000 species worldwide, more than 300 families.