Germination and Growth

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Germination and Growth

A seed is an enclosed, protected package of cells surrounding a miniature plant, the embryo, that can grow to form a copy of the plant bearing it. Seeds thus serve to spread and propagate plants, but they can also help the plant survive unfavorable climatic conditions, such as a freezing winter, in the form of the protected embryo.

A typical seed consists of three main parts. The outer layer, or seed coat, protects the interior contents against drying out, infection, attack by predators, and noxious chemicals in the environment; it may also bear hooks or other structures that attach the seed to passing animals, aiding dissemination. The innermost structure is the embryo, complete with root tip, stem tip, and specialized seed leaves called cotyledons. Between these two structures lies the endosperm, cells containing stored food that the embryo digests and uses as an energy source when it starts to grow. Sometimes, as in beans, the stored food is in the cotyledons of the embryo rather than in separate endosperm cells.

Seeds are drier than growing plant cells, and as a result are dormant . Germination begins when the seed absorbs water and the cells of the embryo elongate, pushing the root tip beyond the seed coat. To accomplish this, the embryo mobilizes the food reserves of the endosperm by secreting the hormone gibberellin, which travels to the endosperm. When gibberellin reaches its target cells in the endosperm, it stimulates gene activity, causing the production of digestive enzymes that break down stored starch, proteins, and fats into simpler molecules that can be burned (oxidized), thus furnishing energy for growth.

Seeds are formed from ovules located in the ovary in the center of the flower. These highly hydrated cells become partially dehydrated and dormant in response to accumulation of another hormone, abscisic acid (ABA), produced by the mother plant and transported into the seed. Before becoming active and germinating, the seed must first destroy part of its ABA content. Without ABA to confer dormancy, the seed might germinate prematurely on the mother plant, negating its function of dissemination. Without the water-removing action of ABA, seeds could not withstand freezing temperatures and other unfavorable climatic conditions.

Some seeds can lie dormant for years before germinating. A Chinese lotus seed is known to have germinated after at least three centuries of dormancy, and even longer dormancies have been claimed. Such deep dormancy requires an especially rugged and nonporous seed coat, made of hard, strong materials, and covered by water-resisting waxes, resins, and lacquerlike materials. Before such seeds can germinate, the coats must be made permeable, either by mechanical force or by chemical or microbial action. Some dormant seeds germinate when treated with gibberellin or another plant hormone, cytokinin; these hormones work against the effects of ABA. Certain seeds are sensitive to light, requiring some illumination before becoming active. This is due to activation of a pigment called phytochrome, which also regulates seedling growth and development. The light requirement ensures that the seed will not germinate if it is buried too deeply in the soil to be able to reach the surface before its stored food supply runs out.

Germination occurs when the root protrudes from the seed coat. Root emergence is followed shortly by the emergence of the stem from the other end of the seed. The root grows downward, towards the center of gravity of Earth, while the stem grows upward. It is not yet understood why these organs behave oppositely toward gravity, but it appears that in both, gravity is sensed by the falling of heavy particles in the cells. Their opposite behavior toward gravity makes it likely that roots will find their way to water and minerals in the soil, while leaves will find the light needed for food synthesis. The orientation of plant organs is also affected by light, which generally causes stems to turn towards the light, roots away from the light, and leaf blades to become perpendicular to the source of light.

Growth and Development

The embryo has a bipolar axis, with a region of cell division retained at each end. These regions, called meristems , maintain this activity throughout the life of the plant. The meristem at the stem apex includes an outer layer, whose cells divide in only one plane to produce more surface area, and an inner layer whose cells divide in all directions to produce the interior bulk. From these layers are produced more stem, leaves, and ultimately flowers. By contrast, the root meristem produces only root tissue, covered at its apex by a root cap, whose hardy cells protect the delicate internal meristem region as the root pushes into the soil.

Whenever leaves are formed, lateral buds are also formed in the angle between leaf base and its insertion into the stem.

Each lateral bud contains a meristem, which is generally kept inactive by the downward diffusion of the hormone auxin , which is produced by the dominant bud at the apex. If this apical bud is injured or removed, or if there is an unusually large supply of cytokinin from the roots, a lateral bud may become active or even dominant.

When a seed germinates in darkness, the seedling stem is long, slender, and unpigmented, and may terminate in an apical hook that protects the delicate meristem during upward growth through the soil. The hook is formed in response to the gaseous hormone ethylene. When the seedling is illuminated, activation of phytochrome turns off ethylene production, opens the hook, inhibits further stem elongation, promotes leaf blade expansion, and initiates the synthesis of chlorophyll. In both stem and root, the area just behind the tip elongates the most, as a result of the elongation of young preexisting cells.

see also Differentiation and Development; Embryogenesis; Germination; Hormones; Hormonal Control and Development; Meristems; Phytochrome; Seed Dispersal; Seeds; Tropisms and Nastic Movements.

Arthur W. Galston