Kingdom Plantae
Systematics

Evolution of Plants: The chloroplasts of green plants are surrounded by two membranes, supporting the theory of an endosymbiosis, with cyanobacteria a common ancestor. Embryophytes descended from green algae during the Palaeozoic era, and are also called the land plants (though some live in water) and include all the trees, flowers, ferns, mosses with which we are all familiar. All are complex multicellular organisms with specialized reproductive organs and, with rare exception, use photosynthesis for their energy.

For some 1.5 billion years, photosynthetic organisms remained in the sea, protected from ultraviolent radiation. The earliest photosynthetic organisms to invade land probably resembled modern algae, cyanobacteria, and lichens, and were followed by the bryophyte, including liverworts & mosses that descended from the charophyte group of green algae.

After evolving from primitive green algae, plants probably began their terrestrial invasion some 450 million years ago during the late Ordovician, with a first major evolutionary task being desiccation resistance. Vascular structures emerged to transport water and nutrients to tissue that was above the water. Symbiotic relationships with fungi emerged to assist in the uptake of nutrients from moist soil, as opposed to flooded soil. Later success was fostered by the evolution of means for gamete and progeny dispersal in flowers and seeds among some, but not all, plant lineages.

Land colonization by plants is believed to have been achieved around 425 Ma during the Silurian with the appearance of the vascular plants. Cooksonia is usually considered the earliest fossil of a vascular land plant; it was a small plant, only a few centimeters high. Its leafless stems had sporangia (spore-producing structures) at their tips. No specimen has been found with roots suggesting it either connected to the ground with very fine root hairs.

As plants evolved and spread across terrestrial landscapes, and themselves adapted to changing environments, they were a major selective influence in paleobiology and evolution of animals (co-evolution) from the time of their prominence in the Paleozoic, and through the Mesozoic and Cenozoic. Gymnosperms, and especially the cycads, were the dominant land plants in the Jurassic, but during the Cretaceous saw, the rise of the flowering plants (angiosperms) and their associated insect pollinators (an example of coevolution) became the dominant plant form that continues to present day.

In the most general context, plants are at the bottom of the majority of terrestrial food chains, and through photosynthesis, produce the energy for most of earth’s life, directly or indirectly. For example, the amphibians followed the "amphibious" plants onto land, as did the reptiles follow plants onto land. Interestingly, the rise of mammals coincided with the rise of plants that, like mammals, utilize internal development by enclosing embryos in a protective and nourishing shell, the seed in the case of plants.

See Some Representative Plant Fossils Across Geological Time

Kingdom Plantae Systematics:

Green Plants
Algae (Kingdon Protista - see note 1)
Embryophytes (Land Plants)	Subkingdom	.		Division
	Bryophytes (non-vascular plants)	.		Marchantiophyta - liverworts
				Anthocerotophyta - hornworts
				Bryophyta - mosses
	Tracheophytes (vascular plants - note 2)	.		Lycopodiophyta - clubmosses
				Equisetophyta - horsetails
				Pteridophyta - true ferns
				Psilotophyta - whisk ferns
		Superdivision Spermatophytes (seed plants - note 3)	Gymnosperm (?)	Pteridospermatophyta - seed ferns
			Gymnosperms Gymnospermae (note 4)	Pinophyta - conifers
				Cycadophyta - cycads
				Ginkgophyta - ginkgo
				Gnetophyta - gnetae
			Angiosperms - flowering plants (note 5)	Magnoliophyta - flowering plants (also called Angiospermopsida)
Notes: Algae are not within Kingdom Plantae, but instead are mostly in Kingdom Protista. There are several different groups of organisms that are predominently photosynthetic. Most natable are seaweeds that are multicellular algae and appear much like land plants, but are classified among the green, red, and brown algae. These and other algal groups also include various single-celled organisms. Importantly, the embryophytes (or land plants) are believed to have evolved from green algae and together are often referred to as the green plants or Viridiplantae. Phylogeny for modern Spermatophyta (seed plants) and some related vascular plant groups is shown to the right (click the image to enlarge). The spore-bearing vascular plants are paraphyletic with respect to the seed plants, with ferns (Pteridophyta) more closely related to seed plants than they are to clubmosses (Lycopodiophyta) Seed plants span the fossil record with the seed ferns, or (Pteridospermae) once dominating and building entire forests especially in the Upper Paleozoic. By the Triassic period, seed ferns were largely replaced by gymnosperms until the Cretaceous, when the Angiosperms became dominant. Gymnosperm (Gymnospermae) is a name derived from the Greek word for naked seed. They comprise a group of seed-bearing and hence vascular plants whose seeds appear “naked” on scales of a cone or, as opposed being formed within an ovule and developing fruit, as is the case for the angiosperms. The Pteridospermatophyta, or seed ferns, is an extinct gymnosperm division that were predominant in the Devonian. The systematic position of the Pteridospermatophyta is currently in debate. The angiosperms are the vast and major group of flowering plants whose seeds are covered in a true fruit. The reproductive organs are in a flower and the ovule is enclosed within a carpel that will lead to a fruit.