HOW A TREE GROWS - ( 2 )

How trunks and branches grow thicker. The trunk and branches of a broadleaf or needleleaf tree grow thicker as long as the tree lives. The cambium tissue just underneath the inner bark causes this thickening. It uses the sugar produced by the leaves to make new plant tissue. On its outside, the cambium makes new phloem, or inner bark, and on its inside, new xylem, or wood. Wood consists largely of cellulose, a tough substance made from sugar. The xylem has two kinds of wood--sapwood and heartwood. The wood nearest the cambium is the sapwood. It is living wood and contains the tiny pipelines that carry sap. In tropical climates, the sapwood thickens all year.

In cooler climates, a new layer of sapwood usually forms only in early summer. As a tree ages, the wood nearest the center dies. This dead wood is the heartwood. It helps support the tree.

In regions where trees make a new layer of wood once a year, the layers form a series of annual rings. Each ring represents one year's growth. After such a tree has been cut down, a person can count the rings to determine the tree's age. Scientists have also found that slight changes in the composition of a tree's cellulose reveal the kind of weather that a tree experienced.

How trees reproduce. Most trees reproduce sexually. That is, seeds are produced only after sperm unite with eggs. Sperm are produced by pollen, which forms in the tree's male reproductive parts-either the male part of the flower or the male cone. Eggs form in the female part of the flower or in the female cone. Among many angiosperm species, the flowers have both male and female parts. The pollen from the male part can simply drop onto the female part. Other angiosperms and all gymnosperms have separate male and female flowers or cones, which may grow on the same tree or on separate trees. The pollen of these species is carried to the female flower or cone by insects, the wind, or other means. After contacting the female flower or cone, pollen produces sperm. The sperm then unite with eggs, and one or more seeds develop within a fruit or cone.

When the fruit or cone has ripened, the seeds are ready to leave the tree. The wind scatters the seeds of needleleaf trees and the winglike seeds or fruits of such broadleaf trees as ashes, maples, poplars, and willows. Birds, squirrels, and other animals scatter seeds contained in nuts or fleshy fruits. Ocean currents sometimes carry the seeds of coconut palms and mangroves.