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SPHENOPHYTA. PTEROPHYTA. GYMNOSPERMS. CYCADOPHYTA. GINKGOPHYTA

Another group of vascular plants is Sphenophyta, commonly known as the horsetails because that is what they look like. They appeared during the Devonian, around 360 million years ago, and dominated forests during the late Paleozoic era. About 250 million years ago, they began to decline. Today there are some 25 species left, most of which are relatively small.

They all have true roots, stems, and leaves. Living horsetails lack cambium, though many of the larger extinct forms had a vascular cambium. This is the layer of cells in the trunk that divides, producing the secondary tissues that allow plants to grow more tissues as they increase in height and weight. As certain plants grow older, it is the vascular cambium that accounts for their increase in width (girth).

The most common group of living horsetails, Equisetum, is homosporous; their spores give rise to gametophytes that bear both archegonia and antheridia.

Many botanists feel that the pterophytes, or ferns, evolved from the psilophyte stock. Ferns have true roots, stems, and leaves. They first appeared during the Devonian, around 400 million years ago, and became important during the Carboniferous, about 300 million years ago. They then declined, along with the psilophytes, lycophytes, and sphenophytes. However, the decline wasn’t nearly so drastic for the ferns as it was for their contemporaneous vascular plants; today there are about 11,000 fern species living throughout the world.

Most forms are quite small, though one group, the tree ferns, is the exception. They have a woody trunk with leaves (fronds) and sometimes attain heights of nearly 25 m (82 ft). This is possible because ferns have well-developed vascular systems containing xylem (water-transporting cells) and phloem (sugar-transporting cells). But, unlike the conifers and broad-leaved trees, ferns do not have any secondary growth, so it is not possible for the fern trunk to increase in girth. This creates an upper height limit for most ferns and is the reason why their stems often lie on the ground, with only the leaves growing upward.

The dominant stage in the fern’s life cycle is the leafy diploid sporophyte. The sporangia are often located on the underside of the frond. Those fronds containing sporangia are called sporophylls. The location of the sporangia on each fern depends on the species.

Most ferns have just one type of spore. Under the proper conditions, all the spores develop into gametophytes that bear both archegonia and antheridia. The gametophytes never develop vascular tissue, and they rarely grow very large. Because their free sperm cells are flagellated and require moisture to reach the egg cells inside the archegonia, ferns tend to be restricted to moist environments. When transplanted, however, they have little trouble growing in drier environments, although fertilization will not occur. Where fertilization does occur, the zygote develops into the sporophyte without passing through a protected seedlike stage.

In some classifications, the remaining vascular plants are placed in the group Spermopsida. This contains the gymnosperms and angiosperms. Gymnosperms include cycads, the ginkgoes (just one living member), conifers, and others. The angiosperms include all the grasses, sedges, rushes, and other flowering plants.

It has been found that most of the groups called gymnosperms aren’t closely related. Thus, there has been a tendency to list each category separately without placing them under the gymnosperm heading. Here, each of the groups that used to be classified as gymnosperms has been moved up to a taxonomic level that, among botanists, is often called the division. However, the term phylum is more useful.

A distinctive characteristic of the peculiar gymnosperm phylum known as seed ferns is not that its species don’t produce spores, as true ferns do, but that instead they produce seeds. It is thought that the cycads and angiosperms (flowering plants) evolved from this group.

Another unusual group, the gnetophytes, includes about 70 desert and montane (living on mountains) species with flower-like reproductive structures. These plants are heterosporous. The dominant stage in theirlife cvcle is the sporophyte. They are dioecious (have separate male and female plants), produce pollen, and bear small, naked seeds. The gnetophytes appear to be relatives of the ginkgo and the conifers. Both the seed ferns and the gnetophytes were important components of Carboniferous forests.

The earliest fossils of another ancient group, the cycads, date to about 240 million years ago. They reached their climax during the Triassic and Jurassic, about 200 million years ago, and then declined steadily. Now, only about 100 species survive.

The unbranched, erect trunk of the cycad bears a crown of leaves that resembles a palm tree, except for its large, upright cones. The tallest cycads reach about 20 m (65 feet). Their leaves are pinnately compound (one vein bearing many leaflets). All the cycads are confined to the tropics, except those grown ornamentally.

Each species has separate male and female plants. The males bear staminate cones (with pollen) and the females have seed-bearing, or ovulate, cones. There is one exception; instead of bearing cones, the female cycads in the genus Cycas have seed-bearing megasporophylls that resemble leaves.

Cycad pollen is carried by the wind to the naked ovules. Upon landing, the pollen grain develops a tubular extension called the pollen tube. This produces flagellated sperm cells that swim a short distance to reach the egg cells.

Before most members became extinct, the group that now consists of only the ginkgo was extremely diverse. It was an important constituent of the Mesozoic forests around the world, especially during the Jurassic period, about 150 million years ago. The genus Ginkgo is thought to extend all the way back to this period, which is why ginkgoes are often called “living fossils”. The group began to decline toward the end of the Mesozoic and continued its decline through the Tertiary until reaching its current situation in which there is just one surviving member, Ginkgo biloba. The ginkgo, as it is commonly called, is native to western China, where it has been in cultivation for hundreds of years. During the past hundred years the ginkgo has become a very popular street tree throughout much of the rest of the world.

The fan-shaped leaves are borne on a long stalk (the petiole). The species is dioecious. Male trees have catkin-like pollen-bearing cones that, although not very conspicuous, are similar to the male cones found among the conifers. Being heterosporous, the pollen-bearing cones produce microspores that are carried by the air. Some land on the female tree’s mature ovules. The female gametophytes are buried in the ovule, where the small archegonia are formed. The entrance to the ovule is made through a small opening, the micropyle. The microspores are drawn into the micropyle. They lie next to the ovule and develop into male gametophytes that grow into the ovule. The male gametophyte releases motile sperm cells, one of which fuses with the egg nucleus to form a zygote, from which the embryo develops. Tissues grow around the embryo and form a true seed with a soft outer fleshy layer.

Дата добавления: 2015-11-16