Conifers

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.

Cycads

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.

Date: 2014-12-22; view: 905