Plant

Plants are multicellular eukaryotic organisms that generally do not have sensory organs or voluntary motion and have when complete, a root, stem[?], and leaves, though consisting sometimes only of a single leafy expansion.

Plants find their origins among a group called the green algae, which are paraphyletic to the remaining forms, and are variously included here or among the Protista. Green algae have chloroplasts containing chlorophylls a and b, bound by double membranes, and come in a variety of forms: flagellate, colonial, filamentous, and even primitively multicellular. Many are primarily haploid, but others exhibit alternation of generations between haploid and diploid forms, called the gametophyte and sporophyte[?]

Some time during the Palaeozoic plants began to appear on land. In these new forms, the gametophyte and sporophyte become very different in shape and function, the sporophyte remaining small and dependent on its parent for its whole brief life. Groups at this level of organization, collectively called bryophytes, include:

• Division Bryophyta (mosses)
• Division Anthocerotophyta[?] (hornworts)
• Division Hepaticophyta[?] (liverworts)

All of these forms are small and confined to moist environments, relying on water to disperse spores. In the Silurian, new embryophytes appeared with adaptations enabling them to overcome these constraints, which underwent a massive adaptive radiation in the Devonian period, taking over the land. These groups typically have a cuticle resistant to desiccation, and vascular tissue, which transports water throughout the organism, and are called vascular plants as a result. In many of these the sporophyte acts as a separate individual, with the gametophyte remaining very small. Groups at this level of organization include:

• Division Lycophyta[?] (club mosses)
• Division Sphenophyta (horsetails)
• Division Psilophyta (whisk ferns)
• Division Ophioglossophyta (adders-tongues and grape-ferns)
• Division Pterophyta (ferns)

The vascular plants also include as a subgroup the spermatophytes, or seed plants, which diversified towards the end of the Palaeozoic. In these forms it is the gametophyte that is completely reduced, and the young sporophyte begins life inside an enclosure called a seed, which develops on its parent. Spermatophytes include:

• Division Cycadophyta (Cycads)
• Division Ginkgophyta (Ginkgo)
• Division Pinophyta (Conifers, Coniferophyta)
• Division Gnetophyta (Gnetae)
• Division Magnoliophyta (Flowering plants, Anthophyta)

These are often referred to as gymnosperms, except for the flowering plants, which are referred to as angiosperms. The latter are the last major group of plants to have appeared, arising during the Jurassic and quickly becoming predominant.

See also See also flower, fruit, vegetable, herb, spice, tree, weed, invasive exotic, biota