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本帖最后由 阿秦 于 2009-4-19 17:21 编辑
Classification of Plants
The newer classification system lists all of the more than 300,000 known plants
in just two phyla, the Bryophytes and the Tracheophytes. Bryophytes, the mosses
and liverworts, are usually soft and nonwoody in structure, take in water throu-
gh short root-like filaments called rhizoids, and may have stems and simple lea-
ves but, unlike the more complex Tracheophytes, do note have true roots or vasc-
ular tissue whose function it is to circulate water, food, and essential minera-
ls throughout the organism.
Tracheophytes are divided into four sub-phyla: lycopsids, which number some 900
living species; sphenopsids, whose fossil species contributed to coal formation
in the Carboniferous period, but which have few living species; psilopsids, an
extinct group of relatively simple plants, which fossil studies show to have be-
en more advanced than any of the mosses; and pterosids, subdivided into three c-
lasses, the ferns, the gymnosperms, and the angiosperms.
The angiosperms, the most highly developed and complex class of plants, reprodu-
ce by means of single and double seed leaves called cotyledons. Monocots, such
as corn, wheat, lilies, and orchids, have leaves with parallel veins, while dic-
ots, which include oaks, maples, roses and thistles, among others, have net-vei-
ned leaves and stems with annual growth rings. |
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