Several features are shared by plants and distinguish plants from other eukaryotes[1][2]:
Growth from apical meristems
Meristems are regions of undifferentiated cells in plants, and the sole regions of active mitosis[3][4]. In contrast to stem cells in animals (for example in bone marrow), apical meristems are on the extremities of plants, so new cells are produced at the boundary rather than pushing old cells out from the inside.
Alternation between a haploid generation and dependent diploid generation[1, p600-3]
(with walled spores and multicellular gametangia)
(with walled spores and multicellular gametangia)
Land plants have both diploid and haploid forms of each species. The details of reproduction vary between different divisions of plants, but all groups share the basic life cycle: an adult dipoid (the sporophyte) releases spores, adapted with tough walls to survive in the open as they're dispersed by wind. The spores are produced by meiosis, and therefor haploid. In contrast to animals, these haploid cells grow into entire plants without fusing with eachother, resulting in a haploid generation (called gametophytes). The gametophytes have multicellular gametangia (a synapomorphy separating land plants from green algae) which produce gametes (by mitosis, since the plant is already haploid). The gametes fertilize and the diploid embryo is born. In bryophytes, the sporophyte grows directly from the gamtophye, and is dependent on it for nutrients[2]. Even in vascular plants however, the sporophyte embryo is packaged with nutrients from the parent gametophyte.
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Divisions of land plants
Bryophytes
Bryophytes are characterized by a dominant gametophyte, and a lack of lignified vascular tissue[5]. They are the oldest (first evolved) groups of land plants, and at one time they were considered a single phylum, but currently they are understood to be paraphyletic[6]. |
Seedless Vascular
Vascular plants are ones that have evolved lignified xylem tissue to transport water. This allows them to grow higher, an advantage that lets them out-compete shorter plants for sunlight. In contrast to Bryophytes, the sporophyte phase is the larger, more conspiculous in vasular plants. |
Gymnosperms
In seed plants, the advent of a protected embryo replaces the spore as a dispersal mechanism. Since seeds are better protected and contain a supply of nutrients for the embro, they are usually able to safely remain dormant for longer than spores[1, p620-621][20]. |
Angiosperms
Angiosperms are seed plants with a few unique reproductive adaptations, emerging about 140 Mya[22]. Flowers attract pollenators to help the plants fertilize eachother, and fruits help to disperse the seeds (for example by being eaten by an animal and then transported, since the nutritious fruit would be digested but the seed too protected to be broken down)[1, p625-626]. They also contain a second cellular component of xylem, vessel elements, opposed to lower plants that contain only tracheids[21]. |
References
[1]Campbell & Reece, Biology 8th Edition
[2]https://www.cabrillo.edu/~ncrane/bio1c/botPDFs/Bryophytesseedlessvas.pdf
[3]http://www2.mcdaniel.edu/Biology/botf99/tissimages/meristematic.html
[4]http://www.mhhe.com/biosci/pae/botany/histology/html/apimeris.htm
[5]http://protein.bio.msu.ru/biokhimiya/contents/v72/full/72121675.html
[6]http://bryophytes.plant.siu.edu/bryojustified.html
[20]http://www.seedbiology.de/evolution.asp
[2]https://www.cabrillo.edu/~ncrane/bio1c/botPDFs/Bryophytesseedlessvas.pdf
[3]http://www2.mcdaniel.edu/Biology/botf99/tissimages/meristematic.html
[4]http://www.mhhe.com/biosci/pae/botany/histology/html/apimeris.htm
[5]http://protein.bio.msu.ru/biokhimiya/contents/v72/full/72121675.html
[6]http://bryophytes.plant.siu.edu/bryojustified.html
[20]http://www.seedbiology.de/evolution.asp