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Seed
I INTRODUCTION
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.
In a few plants, such as the orchids, the embryo is a small, undifferentiated mass of cells until after the seed has parted from the parent plant; during the period between separation from the parent plant and eventual germination, the undifferentiated cells develop into an embryonic root, bud, stalk, and leaf. In most other plants this development occurs prior to seed dispersal: the embryonic root, or radicle, usually grows toward the micropyle; the embryonic bud, called plumule, or epicotyl, is at the end of the embryo opposite to the radicle; the embryonic stem, or hypocotyl, connects the radicle with the seed leaves, or cotyledons. In gymnosperms, several cotyledons are usually present; among angiosperms two great groups of plants exist, one group having but one cotyledon in the seed and known as the monocotyledons, and the other with two cotyledons and known as dicotyledons. The cotyledons serve as centers of absorption and storage, drawing nutritive material from the endosperm. The cotyledons of many plants, such as the sunflower, function as primary photosynthetic organs after germination and before the development of foliage leaves from the plumule.
II SEED VIABILITY
Some seeds, such as those of the willow, are viable (capable of growing into healthy organisms) for only a few days after falling from the parent tree. Other seeds are viable for years—for example, seeds of the Oriental lotus have been known to germinate 3000 years after dispersal. Each species of plant has its specific period of viability; seeds sown after the period of optimum viability may produce weak plants or may not germinate.
III SEED TESTING
In most of the United States, law requires dealers to test seeds for viability and purity before putting them on the market. A specific number of seeds are counted out, and the seeds are placed in an environment favorable to development; the percentage of viable seed in the batch of seed being tested is an index of viability of all seeds of the same lot. Seed testing also ensures the marketing of seed that is true to type—that is, seed that does not differ from the variety of plant desired.
IV SEED DORMANCY
Lack of viability of seed is often confused with seed dormancy. Many seeds require a so-called resting period after falling from the parent plant before they are able to germinate into new plants. Among the members of the orchid family, the seeds complete their maturation during this resting period. In other plants, chemical changes take place during the resting period that make the seed ready for germination. Still other seeds have extremely tough seed coats that must soften or decay before water and oxygen can enter the seed to take part in the growth of the embryo, or before the growing embryo is capable of bursting through the seed coat. Plant growers who wish to shorten the period of seed dormancy in seeds with undeveloped embryos can do little; germination may be induced, however, in seeds having mature embryos by abrasion of the hard coat, by soaking in water or in such chemicals as sulfuric acid, by heating to crack the seed coat, or by alternate freezing and thawing.
V SEED GERMINATION
The term germination is applied to the resumption of the growth of the seed embryo after the period of dormancy. Germination does not take place unless the seed has been transported to a favorable environment by one of the agencies of seed dispersal. The primary conditions of a favorable environment are adequate water and oxygen and suitable temperature. Different species of plants germinate best in different temperatures; as a rule, extremely cold or extremely warm temperatures do not favor germination. Some seeds also require adequate exposure to light before germinating.
During germination, water diffuses through the seed coats into the embryo, which has been almost completely dry during the period of dormancy, causing a swelling of the seed; the swelling is often so great that the seed coat is ruptured. With the absorption of oxygen by the seed, energy is made available for growth. The foodstuffs stored in the endosperm or in the cotyledons are broken down by enzymes into simpler substances that are transported through the embryo to the various centers of growth. The radicle is the first portion of the embryo to break through the seed coat. It develops root hairs that absorb water and attach the embryo to particles of soil. The hypocotyl then lengthens, bringing the plumule and often the cotyledon or cotyledons above the surface of the soil. If the cotyledons are brought into light, they develop chlorophyll and carry on photosynthesis until the true foliage leaves develop from the plumule. In many plants, especially members of the grass family, the cotyledons never appear above the surface of the soil, and photosynthesis does not occur until true leaves develop; the plant meanwhile subsists on food stores in the seed. From the time of germination until the plant is completely independent of food stored in the seed, the plant is known as a seedling.
See also Horticulture; Plant Breeding.
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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