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 onducting 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 nique 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 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 features that have contributed to the success of angiosperms are their unique reproductive features, which include the flower and the fruit.

Angiosperms are characterized primarily by features of their reproductive system. The unique structure known as the carpel encloses the vules and matures into the fruit. Since the ovules are enclosed, pollination is indirect. History The ancestor of angiosperms was a seed-bearing plant that was probably already pollinated by insects to some degree. No living group of plants has the correct combination of characteristics to be this ancestor, but seeds have originated a number of times during the history of the vascular plant.

Although angiosperms are probably at least 150 million years old as a group, the oldest definite fossil evidence of this division is pollen from the early Cretaceous Period. By 80 or 90 million years ago, angiosperms were more ommon worldwide that other plant groups. They became abundant and diverse as drier habitats became widespread during the last 30 million years or so. Flowers and Fruits Flowers make possible the precise transfer of pollen, and therefore, outcrossing, even when the stationary individual plants are widely separated.

Fruits, with their complex adaptations, facilitate the wide dispersal of angiosperms. The flowers are primitive angiosperms had numerous, separate, spirally arranged flower parts, as we know from the correlation of flowers of this kind with primitive pollen, wood, and other features. Sepals are homologous with eaves, the petals of most angiosperms appear to be homologous with stamens, although some appear to have originated from sepals; and stamens and carpels probably are modified branch systems whose spore-producing organs were incorporated into the flower during the course of evolution.

Bees are the most frequent and constant visitors of flowers. They often have morphological and physiological adaptations related to their specialization in visiting the flowers of particular plants. Flowers visited regularly by birds must produce abundant nectar to provide the birds with enough energy so theat they will continue to be attracted o them. The nectar visited plants tends to be well protected by the structure of the flowers. Fruits, which are characteristic of angiosperms, are extremely diverse.

The evolution of structures in particular fruits that have improved their possibilities for dispersal in some special way has produced many examples of parallel evolution. Fruits and seeds are highly diverse in terms of their dispersal, often displaying wings, barbs, or other structures that aid their dispersal. Means of fruit dispersal are especially important in the colonization of islands or other distant patches of suitable habitat.

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