Diverrsity Of Plants

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Term Paper Title	Diverrsity Of Plants
# of Words	2685
# of Pages (250 words per page double spaced)	10.74

Diverrsity of Plants

Diverrsity of Plants

     Plants evolved more than 430 million years ago from multicellular green
algae.  By 300 million years ago, trees had evolved and formed forests, within
which the diversification of vertebrates, insects, and fungi occurred.  Roughly
266,000 species of plants are now living.
     The two major groups of plants are the bryophytes and the vascular
plants; the latter group consists of nine divisions that have living members.
Bryophytes and ferns require free water so that sperm can swim between the male
and female sex organs; most other plants do not.  Vascular plants have elaborate
water- and food conducting strands of cells, cuticles, and stomata; many of
these plants are much larger that any bryophyte.
     Seeds evolved between the vascular plants and provided a means to
protect young individuals.  Flowers, which are the most obvious characteristic
of angiosperms, guide the activities of insects and other pollinators so that
pollen is dispersed rapidly and precisely from one flower to another of The same
species, thus promoting out crossing.  Many angiosperms display other modes of
pollination, including self-pollination.

Evolutionary Origins

     Plants derived from an aquatic ancestor, but the evolution of their
conducting tissues, cuticle, stomata, and seeds has made them progressively less
dependent on water.  The oldest plant fossils date from the Silurian Period,
some 430 million years ago.
     The common ancestor of plants was a green alga.  The similarity of the
members of these two groups can be demonstrated by their photosynthetic pigments
(chlorophyll a and b,) carotenoids); chief storage product (starch); cellulose-
rich cell walls (in some green algae only); and cell division by means of a cell
plate (in certain green algae only).

Major Groups

     As mentioned earlier, The two major groups of plants are The bryophytes-
-mosses, liverworts, and hornworts--and The vascular plants, which make up nine
other divisions.  Vascular plants have two kinds of well-defined conducting
strands: xylem, which is specialized to conduct water and dissolved minerals,
and phloem, which is specialized to conduct The food molecules The plants
manufacture.

Gametophytes and Sporophytes

     All plants have an alternation of generations, in which haploid
gametophytes alternate with diploid sporophytes.  The spores that sporophytes
form as a result of meiosis grow into gametophytes, which produce gametes--sperm
and eggs--as a result of mitos...

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