Life cycle of ferns
The ferns life cycle is typical of seedless vascular plants. Gametes form the gametophyte generation fuse to form the zygotes. The sporophyte generation grows from the zygote. As the sporophyte matures, it forms a tightly curled leaf (fiddlehead). When the fiddlehead is exposed to sunlight, it gradually opens into a new frond. Haploid fern spores develop in sporangia on the underside of fronds. Sporangia usually occur in clusters called sori. Mature spores are released from the sporangia and dispersed by the wind.
When a spore falls on damp, shaded soil, it begins to grow. It grows into a tiny, heart-shaped plant. This plant is called a prothallus. The prothallus is only about one centimeter across. This plant lives only a few weeks. Since the prothallus is so small, chances are that you have never seen one.
In the life of a fern, the prothallus is the gamete-forming stage, or gametophyte. Eggs and sperm form on the underside of the prothallus. Archegonia and antheridia develop on the underside of the prothallus. Like mosses, ferns must have water to reproduce. When water is present, sperm can swim to the eggs and fertilize them.
Ferns can be cloned by cutting a section containing rhizome (the underground stem) from a mature fern and replanting it in soil. Cloned ferns are genetically identical.
As winter approaches in areas that have cold winter, people notice the plants that stay green. These plants, called evergreens, may be the only sign of plant life in a winter landscape. Other kinds of plants may loose their leaves in the fall. But evergreens stay green because they do not loose all their leaves at the same time. They loose them throughout the year. Many evergreens have leaves that stay on the plant for several years.
At one time there were many more kinds of gymnosperms living on the earth than there are today. Scientists have found the shapes of their leaves, stems, roots, and seeds preserved in rocks.
Four groups of gymnosperms have living representatives.
There are about 700 different kinds of gymnosperms living today. More than 500 of these are Conifers. The second largest group of gymnosperms is the Cycads with about 100 different kinds and the order Gnetales has approximately 70 species. One interesting species belongs to this group, welwitschia. Ginkgoes, the rarest of gymnosperms, have only one species, Ginkgo biloba.
Like conifers the other kinds of gymnosperms have woody stems. All but the ginkgo form their seeds in cones.
Gymnosperms are seed plants. But the seeds of gymnosperms are not covered. That is, their seeds are not protected by any kind of fruit. Instead, the seeds of many gymnosperms grow on cones or conelike parts. So many gymnosperms are also called cone-bearing plants.
Conifers means “cone-border”. Pines, spruces, firs and other conifers are characterized by their stiff cones and needlelike leaves. Coniferous forests were once common in temperate zones. Now, conifers are mostly found in the North Temperate Zone and other arid regions with sandy soil, cold winters, and moderate rainfall.
Most conifers are tall trees, but some of them are bushes. The tallest-known trees are conifers.
Conifers have special adaptations. Their needles have hard, waxy cuticle and little exposed surface area. As a result, needles retain moisture through hot, dry summers and the coldest of winters. Conifers send roots out a wide area rather than deep into the soil. This shallow root system holds the tree stable even where soil is scarce.
Sexual reproduction in most conifers involves separate male and female cones that grow on the same tree. The male pollen cones produce microspores that develop into pollen grains. Each pollen grain is a male gametophyte. Within the female seed cones, megaspores develop into female gametophytes that contain egg cells. When the egg cells mature, the female cones secrete a sticky sap that traps pollen drifting in the wind. As the sap dries, it draws the pollen toward the egg cells. The pollen grains then produce mature female gametes or sperm, which fertilize the egg cells. After the fertilization the resulting diploid zygote develops into a conifer embryo contained within the seed.
Conifers do not need water to carry out sexual reproduction because pollen grains containing the sperm are dispersed by the wind. Conifers can therefore reproduce in areas where nonvascular plants and seedless vascular plants cannot. Pines, spruces and firs, for example, can grow in places that are dry, in places that are cold or in places that have poor soil. Cypresses and larches grow well in wet places such as swamps and bogs.
Families of conifers
There are seven main groups or families of conifers. But you are probably familiar with members of only four families: pines, redwoods, cypresses and the yews.
Importance of conifers
The most important uses for conifers are for lumber and for making paper and other wood products. The seeds of some conifers are large enough to be used for food.
Another use for conifers is as ornamental plants. In general conifers are desirable plants for landscaping homes or other buildings. Many different kinds of conifers are planted outside the homes, offices and schools.
People plant conifers for many reasons. In some places the soil is too poor or too wet for other finds of trees. Conifers may grow well in such soil. But conifers are also easy to care for. As a rule, they can be trimmed if a certain shape in size needed. Their leaves usually do not have to be raked from the ground.
At one time there were more cycads than are living today. Their fossils are found in many different parts of the earth. A large number of there fossils have been found in the Black Hills of South Dakota.
Almost all the cycads now living are found in the tropics. Most look like the palm trees. However, palms are flowering plants. Cycads from their seeds in large cones.
Zamiz is the only kind of cycad that is native to the United States. It is found in parts of Florida. However, even there it does not grow in large numbers. Zamia is a small plant. Its thick, stem grows mostly underground.
The ginkgo is the last species of a once widespread family of trees. They are called "living fossils" because virtually no ginkgoes are now known to live in the wild. Ginkgoes have unique fan-shaped leaves. Pollen is produced in catkins, or small conelike structures that dangle from the tree branches. Only male ginkgoes produce catkins. After fertilization, female trees produce fleshy seeds that look like pale berries.
The order Gnetales includes trees and woody vines that have traits of gymnosperms, but some species have reproductive structures like those found in angiosperms. One species produces an edible plumlike fruit. Another, Welwitschia, grows only in the deserts of southern Africa.
Angiosperms are flowering plants. They produce seeds enclosed in fruits, as opposed to the uncovered seeds of the gymnosperms. To botanists, the history of angiosperms is still a mystery, because the flowering plants appeared so suddenly in the fossil record about 130 million years ago. Of the more than 250,000 species of vascular plants, about 235,000 are angiosperms. They include most green plants. Oaks, birches, vegetables and grasses are all angiosperms.
Angiosperms are classified according to the number of cotyledons in their seeds. Plants with one cotyledon are called monocots; those with two are called dicots. Monocots include about 89,000 species; dicots have about 1700,000 (Table 4.1).
Angiosperms are also commonly classified by the characteristics of their stem tissues. In woody plants, the xylem and phloem produce cumulative layer of new plant tissue that increase the width of the stem and make it strong and hard. This cumulative growth is called secondary growth. Woody plants often live for many years and tend to produce relatively few seeds. Most woody plants are dicots, such as maple, walnut and chestnut trees. Almost all monocots are herbaceous plants. Their stems are usually green and lack secondary growth. Herbaceous plants typically have shorter lives and produce more seeds than woody plants. Corn and orchids are examples of herbaceous monocot plants. Tomatoes and spinach are herbaceous dicots.
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