Seed

I

Introduction

Seed, term applied to the ripened ovule of a seed plant before germination. Seeds of an angiosperm, or flowering plant, differ from those of a gymnosperm (many woody plants such as the ginkgo and the yew), 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 partly digested during the development of the embryo and endosperm tissue. Surrounding the seed is a hard, tough seed coat, derived from the integument (outer layer) 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 projections from the seed coat that serve to aid the absorption of water when the seed is about to germinate (see below) 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 towards the micropyle; the embryonic bud, called a 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 groups of plants exist, one group having but one cotyledon in the seed and known as the monocotyledons (or monocots), and the other with two cotyledons and known as dicotyledons (or dicots). The cotyledons serve as centres 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 3,000 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 countries, the 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 favourable 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 dilute sulphuric acid, by heating to crack the seed coat, or by alternate freezing and thawing.