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The worldwide array of plant life, including plants that have roots in the soil, plants that live on or within other plants and animals, plants that float on or swim in water, and plants that are carried in the air. Fungi used to be included in the plant kindom because they looked more like plants than animals and did not move about. It is now known that fungi are probably closer to animals in terms of their evolutionary relationships. Also once included in plants were the “blue-green algae,” which are now clearly seen to be bacteria, although they are photosynthetic (and presumably the group of organisms from which the chloroplasts present in true plants were derived). The advent of modern methods of phylogenetic DNA analysis has allowed such distinctions, but even so, what remains of the plantlike organisms is still remarkably divergent and difficult to classify.
Plants range in size from unicellular algae to giant redwoods. Some plants complete their life cycles in a matter of hours, whereas the bristlecone pines are known to be over 4000 years old. Plants collectively are among the most poorly understood of all forms of life, with even their most basic functions still inadequately known, including how they sense gravity and protect themselves from infection by bacteria, viruses, and fungi. Furthermore, new species are being recorded every year.
Within the land plants, a great deal of progress has been made in sorting out phylogenetic (evolutionary) relationships of extant taxa based on DNA studies, and the system of classification listed below includes these changes. The angiosperms or flowering plants (Division Magnoliophyta) have recently been reclassified based on phylogenetic studies of DNA sequences. Within the angiosperms, several informal names are indicated in parentheses; these names may at some future point be formalized, but for the present they are indicated in lowercase letters because they have not been formally recognized under the Code of Botanical Nomenclature.
It is known that the bryophytes (Division Bryophyta) are not closely related to each other, but which of the three major groups is closest to the other land plants is not yet clear. Among the extant vascular plants, Lycophyta are the sister group to all the rest, with all of the fernlike groups forming a single monophyletic (natural) group, which is reflected here in the classification by putting them all under Polypodiophyta. This group is the sister to the extant seed plants, within which all gymnosperms form a group that is sister to the angiosperms. Therefore, if Division is taken as the highest category within Embryobionta (the embryo-forming plants), then the following scheme would reflect the present state of knowledge of relationships (an asterisk indicates that a group is known only from fossils).