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 mitosis.
The gametophytes of bryophytes are nutritionally independent and remain
green. The sporophytes of bryophytes are usually nutritionally dependent on The
gametophytes and mostly are brown or straw-colored at maturity. In ferns,
sporophytes and gametophytes usually are nutritionally independent; both are
green. Among The gymnosperms and angiosperms, The gametophytes are
nutritionally dependent on the sporophytes.
In all seed plants--gymnosperms and angiosperms--and in certain lycopods
and a few ferns, the gametophytes are either female (megagametophytes) or male
(microgametophytes). Megagametophytes produce only eggs; microgametophytes
produce only sperm. These are produced, respectively, from megaspores, which
are formed as a result of meiosis within megasporangia, and microspores, which
are formed in a similar fashion within microsporangia.
In gymnosperms, the ovules are exposed directly to pollen at the time of
pollination; in angiosperms, the ovules are enclosed within a carpel, and a
pollen tube grows through the carpel to the ovule.
The nutritive tissue in gymnosperm seeds is derived from the expanded,
food-rich gametophyte. In angiosperm seeds, the nutritive tissue, endosperm, is
unique and is formed from a cell that results from the fusion of the polar
nuclei of the embryo sac with a sperm cell.
The pollen of gymnosperms is usually blown about by the wind; although
some angiosperms are also wind-pollinated, in many the pollen is carried from
flower to flower by various insects and other animals. The ripened carpels of
angiosperm grow into fruits, structures that are as characteristic of members of
the division as flowers are.


GYMNOSPERMS AND ANGIOSPERMS

Gymnosperms

Gymnosperms are non-flowering plants. They also make up four of the
five divisions of the living seed plants, with angiosperms being the fifth.
In gymnosperms, the ovules are not completely enclosed by the tissues of
the sporophytic individual on which they are borne at the time of pollination.
Common examples are conifers, cycads, ginkgo, and gnetophytes. Fertilization of
gymnosperms is unique.
The cycad sperm, for example, swim by means of their numerous, spirally
arranged flagella. Among the seed plants, only the cycads and Ginkgo have
motile sperm. The sperm are transported to the vicinity of the egg within a
pollen tube, which bursts, releasing them; they then swim to the egg, and
fertilize it.

Angiosperms

The flowering plants dominate every spot on land except for the polar
regions, the high mountains, and the driest deserts. Despite their overwhelming
success, they are a group of relatively recent origin. Although they may be
about 150 million years old as a group, the oldest definite angiosperm fossils
are from about 123 million years ago.
Among the