The major parts of a treeThe major parts of a tree are its roots, trunk, leaves, flowers, and seeds. These components play vital roles in a tree’s growth, development, and reproduction.
A. Roots
Trees are held in place by anchoring organs called
roots. In addition to anchoring the tree, roots also absorb water and minerals through tiny structures called root hairs. From the roots the water and mineral nutrients are carried upward through the wood cells to the leaves. Although the internal structure of most kinds of roots is similar, there are often external differences. Pines, for example, have a strongly developed taproot, or main root, in addition to branching side roots. In maples, on the other hand, there is little or no central taproot, and the other roots are produced in great numbers near the surface of the soil.
Roots grow constantly, and at the growing tip of each root is a region called the meristem, which is composed of special rapidly dividing cells. Just behind the meristem the cells become elongated, and farther from the tip the cells become differentiated into various kinds of plant tissue. In rapidly growing roots the root tip is covered by a root cap, a protective coat of loose cells that are constantly being rubbed off and replaced as the root grows.
B. Trunk
Bark is the outer protective covering of tree trunks. Because bark varies so widely in color, texture, and thickness, its characteristics provide one of the most important means of identifying species of trees. Most of the total thickness of bark consists of outer bark, which is made up of dead cells. Outer bark may be very thick, as in the cork oak, or quite thin, as in young birches and maples. Openings in the outer bark allow the movement of carbon dioxide and oxygen to and from the inner tissues.
The inner bark layer, called the
phloem, consists of a thin layer of living cells. These cells act together to transport food in the form of sugars, which are made in the tree’s leaves, through the trunk and stems to other parts of the tree. Phloem cells have thin walls, and their living contents are so interconnected that the sugar solutions can pass easily and rapidly from one end of the plant to the other. As old layers of outer bark are sloughed off, new ones are constantly being added from the inside, where new phloem is always being created.
Most of a tree trunk is occupied by the wood, or
xylem layer, which consists almost entirely of dead cells. The living xylem cells, however, act as the tree’s plumbing system by transporting water and dissolved food through the trunk and stems. A layer of cells called the cambium separates the living xylem cells from the phloem. As the tree grows and develops, the cambium forms new phloem and xylem cells. The layers of xylem cells form rings; these rings can be counted to determine the age of the tree in areas with distinct growing seasons.
C. Leaves
In trees, as in other green plants, the principal function of the leaves is the manufacture of sugars by the process of
photosynthesis. In this process, sugars are formed when carbon dioxide (from the air) and water (from the leaf cells) are combined in the presence of light and the green pigment
chlorophyll. Oxygen is produced as a byproduct. Some of the newly formed sugar is used by the
leaf cells for energy, but most is carried to other parts of the tree to provide energy for growth and development in those areas.
The leaves are also the chief organs involved in the loss of water from the plant, called
transpiration. Many of the tree’s tissues cannot function without a constant supply of water, and water is necessary to prevent overheating or wilting of the leaves. Transpiration is responsible for the movement of water from the roots of the tree up to the top. As water is lost through the leaves, water that enters the roots is pulled upward through the xylem tissue to replace the lost moisture, ensuring a constant circulation of water through the tissues of the tree.
D. Flowers
All angiosperms bear flowers, the trees’ reproductive structures. In some trees, such as dogwoods, cherries, and some magnolias, the flowers are large and colorful. Oaks, willows, and other temperate forest trees, on the other hand, often bear small, pale, and inconspicuous flowers.
In maples and many other trees the male and female reproductive parts are carried in separate flowers on the same tree. This arrangement is known as monoecism, and such trees are called
monoecious. In oaks, for example, the male pollen-producing flowers are borne in long hanging tassels, and the short-stalked or stalkless female flowers are located on the twigs. In some trees, such as the hollies and willows, the male and female flowers are borne on separate trees. This is known as
dioecism, and these trees are called dioecious.
E. Seeds
Seeds, the ripened ovules of the plant that are capable of germination, are the product of fertilized flowers and are distributed in various ways. In pines, for example, each seed is surrounded by a winglike structure. As the winged seed falls from the cone, it floats down to the ground, riding air currents. Oak seeds are enclosed in acorns, which are either planted by squirrels or merely fall to the ground near the parent tree. Willow trees produce thin-walled, flask-shaped fruits that burst open, releasing the seeds. Each seed has a tuft of downy fibers, which enables it to be picked up by air currents and travel for considerable distances. Seeds of other tree species are dispersed by water, mammals, birds, and ants.
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