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CONIFEROPHYTA: CONIFERS

Usually referred to as the conifers, this group includes pine, spruce, fir, hemlock, cypress, redwood, larch, juniper, and yew, as well as others that aren’t as well-known. The conifers include about 50 genera, comprising over 600 species. Their fossil history dates back to the late Carboniferous, some 300 million years ago. Although the cone is a conspicuous feature distinguishing many members of this group, it does not appear on all conifers. For example, juniper berries are actually small cones with fleshy scales. And yews have seeds surrounded at the base by a fleshy, berry-like pulp. In biological terms, the distinction between cones and more berry-like forms of reproduction is significant in terms of the mode of dispersal used by the conifers involved. The pinoids are adapted for wind dispersal of their seeds. The others are adapted to animal dispersal.

Trees are the diploid sporophyte stage of the life cycle. The cones are actually tight clusters of modified leaves known as the sporophylls, which, in the case of the pines, are also known as cone scales. Each sporophyll contains two sporangia in which haploid spores are produced through meiosis. These trees are heterosporous. The large female cones contain the sporangia that produce megaspores; the small male cones contain sporangia that produce microspores. All seed plants, both the gymnosperms and angiosperms, are heterosporous.

Each pine scale contains two sporangia; each sporangium has asmallopening, the micropyle. Meiosis occurs inside the sporangium, producing four haploid megaspores, three of which disintegrate. The remaining megaspore, through repeated mitotic divisions, becomes the female gametophyte, which, unlike that in the ferns, is considerably reduced in size. It is located within the cone. The gametophyte is not free-living, nor does it contain chlorophyll. Each female gametophyte produces several archegonia in which egg cells develop. Together the entire structure consisting of the integument, the sporangium, and the female gametophyte is called an ovule.

Within the male cones are sporangia that produce microspores. These become pollen grains, which develop a thick coating, resistant to desiccation. They have small wing-like structures that help them along when carried aloft by the wind. Inside the pollen grains, the haploid nucleus divides mitotically and the pollen grains become four-celled. Two of these cells degenerate. When the sporangia burst, millions of mature pollen grains are released. This is the male gametophyte stage cycle.

Most of the pollen grains that land on the female cones fall between the scales. Some of these land near the opening of the micropyle. When a pollen grain lands touching the end of the sporangium inside the micropyle, it grows a pollen tube. The germinated pollen grain, which is now the pollen tube, grows down through the sporangium and penetrates one of the archegonia of the female gametophyte. There the tube releases its nuclei, which, in this case, are sperm that developed when the cells in the pollen tube were dividing. The sperm fertilize the egg and the resulting zygote produces an embryo sporophyte. This is still contained within the female gametophyte inside the pine cone. Finally, the cone sheds its ovules, more commonly known as seeds, which grow into adult sporophyte trees.

ANTHOPHYTA / ANGIOSPERMS: FLOWERING PLANTS

The angiosperms, or flowering plants, also known as the anthophytes, are by far the most successful group of living plants, totalling some 250,000 described species. It is their flowers and fruits that differentiate them from all other plants. The earliest known angiosperm-like plants first appear in the fossil record during the Jurassic, about 150 million years ago. But there is some speculation that angiosperms may have existed as long ago as the Permian, about 250 million years ago. It was not until the Cretaceous, 100 million years ago, that there was a rapid decline in the dominance of the gymnosperms. Then, suddenly, and from that time on, the fossil record reflects the diverse spectrum of angiosperms dominating most flora throughout the world.

Among all angiosperms, the diploid sporophyte, retaining and nourishing the gametophyte, dominates the life cycle. And it is the small gametophyte that retains and nourishes the immature eggs and non-motile sperm during development. Flowering plants are heterosporous, having two different sized spores, and oogamous, having sperm and eggs. Their zygotes develop into seeds and fruit that are highly evolved for protection and dispersal. One of their most distinguishing characteristics is the unique reproductive system involving flowers. The floral structures coevolved with the plant’s pollinating vectors, which are modes of transferring pollen from one flower to another, such as the wind, insects, and other animals. These vectors appear to have helped angiosperm dispersal into habitats where other plants might not have reached so readily.

Early angiosperms were probably pollinated by the wind. The ovule of modern gymnosperms exudes a sticky substance that traps wind-borne pollen grains. Similarly, the first angiosperms probably had such a pollinating mechanism.

Some insects, such as certain beetles, may have become dependent on readily accessible sticky, sugary droplets, produced by ovules. Insects traveling from one ovule to another may have inadvertently carried pollen with them, conferring a reproductive advantage to some plants, helping to pass on the genes of plants that had larger nectar-secreting organs (nectaries) and other structures that lured insects. Plants with certain scents and brightly colored flower parts, arranged in ways that signalled potential pollinators, had selective advantages. These are all insects that specialize in nectar consumption. Because certain insects carried pollen from flower to flower, many became vital to their host plants. Both the plants and the insects benefited from this relationship, and they coevolved. By the early Cenozoic, about 65 million years ago, many modern groups of flowers, as well as the bees, wasps, moths, and butterflies that pollinated them, had already evolved.

Дата добавления: 2015-11-16