Showing posts with label seed. Show all posts
Showing posts with label seed. Show all posts

Major Parts of a Tree

The major parts of a tree
The 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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Seed

Seed, term applied to the ripened ovule of a seed plant before germination. Seeds of the angiosperm, or flowering plant, differ from those of the gymnosperm, or conifer and related plants, in being enclosed in the ovary that later forms a fruit; gymnosperm seeds lie exposed on the scales of the cones.

During the process of fertilization the pollen tube enters the ovule through a small opening known as the micropyle. One of the two sperm nuclei in the pollen tube unites with the egg cell in the ovule to form a zygote, which develops into the embryo. In flowering plants the other sperm nucleus unites with two polar nuclei present in the embryo sac to form an endosperm nucleus, which later produces the nutritive endosperm tissue surrounding the embryo in the seed. In gymnosperms, the endosperm is formed from the tissue of the embryo sac itself. The nucellus, or megasporangium, is the tissue composing the main part of the ovule; it is partially digested during the development of the embryo and endosperm tissue. Surrounding the seed is a hard, tough seed coat, derived from the integument of the ovule and known as the testa. In flowering plants a second seed coat occurs within the testa; this second coat is thin and membranous and is known as the tegmen. Some seeds, in addition, have projects from the seed coat that serve to aid in the absorption of water when the seed is about to germinate or that merely form an additional protective coating about the seed. In almost every seed, the micropyle through which the pollen tube entered the ovule persists as a small opening in the seed coat. Close to the micropyle in flowering plants, a stalk, or funiculus, attaches the seed to the placenta on the inside of the fruit wall. When the seed is removed, a small scar, known as the hilum, marks the former attachment of the stalk.

See also Gymnosperm and Angiosperm Seeds; Monocot and Dicot Seeds.

SEED GERMINATION

Most seeds begin to germinate only with the warming days of spring, months after they have fallen to the ground. As the embryo inside expands, the seed cracks, and a root emerges to provide the seedling with both stability and nutrients from the soil. While the root continues to grow and branch downward, the embryonic stem sprouts upward. Nourished from this point by the cotyledons, or seed leaves previously folded within the seed coat, the seedling will develop a shoot with adult leaves.

Monocot and Dicot Seeds


Monocotyledons (monocots) and dicotyledons (dicots) make up the two large groups of flowering plants, differentiated by their seed structures. Monocot seeds contain one cotyledon, or embryonic leaf. When these seeds germinate, the cotyledon remains below ground, absorbing nutrients from the endosperm, the starchy food supply in the seed. The coytledon transports these nutrients to the developing seedling. Dicot seeds contain two coytledons, which absorb and store the nutrients from the endosperm before the seed germinates. The cotyledons, thick with stored nutrients, emerge above ground during germination, and then transport the stored nutrients to the developing seedling. For a brief time, the cotyledons also serve as the first photosynthesizing leaves, but they wither and die when the true leaves emerge.


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Gymnosperm and Angiosperm Seeds

The term gymnosperm is derived from two Greek words: gymnos, meaning naked, and sperma, meaning seed. The term refers to plants, such as pine and spruce, in which seeds mature on the surface of cone scales. In contrast, the term angiosperm means a seed contained in a vessel and refers to flowering plants, in which seeds mature within a fruit.



The seeds of angiosperms develop in an ovary, a part of the carpel that surrounds and protects the egg-containing ovules. Seeds develop from the ovules after pollination and fertilization of the eggs. Ovules and seeds are not exclusive to angiosperms. The “naked-seed” plants (see Gymnosperm), which include the conifers, cycads, and ginkgo, have ovules that lie exposed on the surface of specialized, scalelike leaves arranged into cones. The development of seeds from ovules enclosed in an ovary, which enlarges into a fruit as the fertilized seeds grow, is a feature unique to the angiosperms.

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