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Diversity 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 ater- 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 orm 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 nd 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.

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