NEEDLELEAF FORESTS

Needleleaf forests grow mainly in regions that have long, cold winters. These forests, which are also called softwood forests, stretch across Canada, northern Europe, and Siberia. Many firs, larches, and spruces grow in these northern forests, along with a few broadleaf trees, such as birches and willows. Some willows grow even farther north than needleleaf trees do. But they seldom reach more than shrub size. Needleleaf forests also blanket slopes in such mountain ranges as the Alps and the Rocky Mountains.


The Canadian needleleaf forests extend southward into the Western United States, where they include many of the world's largest trees. Many California redwoods tower over 300 feet (91 meters). Tall Douglas-firs also grow in the Western United States.

A few needleleaf forests grow in warmer regions. For example, the Southeastern United States has large forests of pines, such as loblolly pines and longleaf pines. These forests provide great quantities of wood for lumber and wood pulp.

NEEDLELEAF TRESS ARE ABOUT 500 SPECIES

Needleleaf trees include such familiar trees as firs, hemlocks, pines, redwoods, and spruces. There are about 500 species of needleleaf trees. Most of these trees have narrow, pointed, needlelike leaves. But a few types, such as cedars and junipers, have narrow, scalelike leaves.

Most needleleaf trees are evergreen, though they produce new needles each year. The oldest needles turn yellow or brown and drop, but the youngest needles remain green and do not fall. A few species of needleleaf trees are deciduous. One kind is the larch, which grows in northern forests throughout the world. Another deciduous needleleaf tree is the baldcypress that grows in swamps of the Southeastern United States.

Foresters call needleleaf trees softwoods because most of them have softer wood than broadleaf trees have. But the wood of Douglas-firs, yews, and some other needleleaf trees is hard.

Needleleaf trees belong to a group of plants called gymnosperms. Gymnosperms do not have flowers and their seeds are not enclosed to form fruits. Most gymnosperm trees bear their seeds in cones composed of hard scales. The seeds lie open on the surface of the scales. Botanists call such trees conifers.

Most conifers grow north of the equator. The conifers belong to four families-the pine, yew, cypress, and taxodium families. The pine family is by far the largest. It includes not only pines, but also such trees as firs, hemlocks, larches, and spruces. Pine trees make up a large genus (group of species) within the pine family. Loblolly pines, ponderosa pines, and white pines are a few North American members of this genus. The yew family includes such well-known ornamental trees as English yews and Japanese yews. Although yews are classified as conifers, they do not produce cones but cup-shaped "berries." Many members of the cypress family, such as arborvitae and junipers, have scalelike leaves and give off a spicy fragrance. The taxodium family includes baldcypresses and the largest of all living trees-the redwoods and giant sequoias.

Two conifer families-the podocarpus family and the araucaria family--grow mainly south of the equator. Podocarpus trees are tall evergreens with broader leaves than those of most needleleaf trees. The araucaria family includes the Chile pine. This strange-looking tree has snakelike branches covered with sharp, scaly leaves. It is sometimes called the monkey puzzle tree because its sharp leaves make it difficult to climb.

SPECIALIZED LEAVES

Some leaves have special functions along with or instead of food making. Such specialized leaves include (1) protective leaves, (2) storage leaves, (3) tendrils, (4) bracts, and (5) insect-capturing leaves.

Protective leaves include bud scales, prickles, and spines. As described earlier, bud scales are specialized leaves that protect the young, undeveloped tissues of the bud. Bud scales are short and broad, and they overlap like roof shingles. In many plants, the bud scales have an outer layer of waterproof cells. Prickles and spines are sharp leaf structures that protect the plant from being eaten. For instance, prickles cover the leaves of the Canada thistle. The prickles protect the plant from grazing animals. Many cactuses have clusters of spines. In many species of cactuses, the pointed spines replace the leaves on the mature plants. In these plants, the green stem has the job of photosynthesis.

Storage leaves. Most plants store food in their roots or stems. However, some plants have special leaves that hold extra food. Onion and tulip bulbs, for example, consist mainly of short, fat storage leaves called bulb scales. These leaves cannot make food. Their job is to store food underground during the winter months.

BULB. Many plants that grow in dry places have thick leaves that store water. The mosslike stonecrop plants that grow on rocky cliffs in the Southwestern United States have such leaves.

Tendrils are slender, whiplike structures that help hold climbing plants in place. They wrap around twigs, wires, and other solid objects. Among many climbing plants, specialized leaves serve as tendrils. For example, climbing garden peas have compound leaves in which the upper leaflets are threadlike tendrils. In one kind of sweet pea, a garden flower, the entire leaf blade becomes a tendril. The plant's stipules enlarge and take over the food-making job. In the greenbrier vine, the stipules form long, curving tendrils.





Bracts grow just below the blossoms of certain plants. Most bracts are smaller and simpler in shape than a plant's regular leaves. Many members of the daisy family-including daisies, goldenrods, marigolds, and sunflowers-have bracts. These bracts form a cup beneath the plant's cluster of flowers. A few kinds of plants, such as the flowering dogwood and poinsettia, have large, showy bracts. These bracts look like part of the flower, but they are not.

Insect-capturing leaves. Carnivorous (meat-eating) plants, such as the butterwort, pitcher plant, sundew, and Venus's-flytrap, have leaves that capture insects. These leaves, like other leaves, can make food using sunlight. But they also have features that attract, trap, and then digest insects. Plants with insect-capturing leaves grow in wetlands, where the soil contains little nitrogen. They obtain this necessary nutrient from the captured insects. For a description of these plants and their leaves.  << --- Next>>>

The Importance of Leaves

The chief job of leaves is to make food for plants. This food-making activity, called photosynthesis, occurs mostly in fully grown leaves. But young leaves also are important. They wrap tightly around the tips of growing stems. They thus keep the delicate tips moist and help protect them from insects, cold, and other dangers.

Leaves are also vital to animals. Animals cannot make their own food. They depend on plants for their basic supply of food. Many animals eat leaves. For example, antelope, sheep, and other grazing animals eat grass leaves. People also eat leaves, such as those of cabbage, lettuce, and spinach plants. But even when people and animals eat the fruits, roots, seeds, and stems of plants, they are obtaining food made by leaves.

In the same way, eggs, meat, milk, and all other animal foods can be traced back to food made by photosynthesis.
Leaves help make the air breathable. They release oxygen during photosynthesis. People and animals must have oxygen to live. Without the activities of leaves, the earth's supply of breathable oxygen would probably soon be used up.

People obtain many products from leaves in addition to food. For instance, we use the leaves of the tea plant to make tea. Peppermint and spearmint leaves contain oils used to flavor candy and chewing gum. Such leaves as bay, sage, and thyme are used in cooking to flavor foods. Some drugs come from leaves. For example, the drug digitalis, which is used to treat certain heart diseases, comes from the leaves of the purple foxglove, a common garden flower. Leaves of abaca and sisalana plants provide fiber used in making rope. Finally, the leaves of the tobacco plant are used to make cigarettes, cigars, and other tobacco products.

The life story of a leaf

A leaf begins its life in a bud. Buds are the growing areas of a stem. They form along the sides of the stem, at the point just above where a fully grown leaf is attached. A bud also grows at the tip of the stem. A leaf bud contains undeveloped leaf and stem tissues. Within the bud is a mound slightly larger than the head of a pin. Each leaf starts out as a tiny bump on the side of this mound. The mature bud contains a tightly packed group of tiny leaves. In most soft-stemmed plants, the buds are hard to see. A new leaf becomes noticeable only after it begins to unfold. Most soft-stemmed plants continue to form new leaves until the plants flower or until cold weather sets in. In temperate regions, which have warm summers and cold winters, the aboveground parts of many soft-stemmed plants die after the first hard frost, but the roots live through the winter. Other soft-stemmed plants die completely after the cold weather arrives.

Woody plants, on the other hand, may live many years. They grow several sets of leaves during their lifetime. Most needleleaf trees and shrubs shed old leaves and grow new ones continuously throughout the year. So do most broadleaf trees in the tropics. But in temperate regions, most broadleaf trees and shrubs are deciduous. Deciduous plants of temperate regions shed all their leaves each fall and grow a new set each spring. Deciduous trees and shrubs start growing the next year's leaves even before the present year's leaves have fallen. The new leaves are enclosed in winter buds.

The leaves in the winter buds stop growing during the summer and remain dormant (inactive) throughout the winter. During the winter months, the buds are protected from drying out by special outer leaves called bud scales. In spring, warmth and moisture cause the dormant leaves to become active. The bud scales drop off, and the leaves unfold.