Evolution and Systematics
Plants are eukaryotes and find their origins among a group called the green algae. These forms are often classed among the Protista as the phylum Chlorophyta, but in that case form a group paraphyletic to the kingdom Plantae. Green algae have chloroplasts containing chlorophylls a and b, bound by double membranes, and come in a variety of forms: flagellate, colonial, filamentous, or even primitively multicellular.
Some time during the Palaeozoic plants began to appear on land. In these new forms, called embryophytes, the haploid and diploid individuals (called the gametophyte and sporophyte) become very different in shape and function, the sporophyte remaining small and dependent on its parent for its whole life. Groups at this level of organization include:
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 radition in the Devonian, taking over the land. These groups typically have a cuticle resistant to dessication 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. Groups at this level of organization include:
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 is completely reduced, and the young sporophyte begins life inside an enclosure called a seed, which develops on its parent. Spermatophytes include:
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.
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