A, flat side of grain; A2, longitudinal section of grain, narrow side showing starchy endosperm stained dark with iodine--embryo not stained; A3, longitudinal section of grain, narrow side, showing embryo stained dark with Sudan IV, a dye that is specific for fats and oils. The starchy endosperm is unstained; A4, side view of soaked bean; A5, unstained cotyledon of bean with attached portion of seedcoat; A6, cotyledon of bean stained with iodine to show high starch content epicotyl and hypocotyl low in starch; A7, longitudinal section of soaked bean showing two fleshy cotyledons. B, and B2, germinating grains; B3 and B4, germinating beans; c, cotyledon; d, dent in grain; e, epicotyl; en, endosperm; em, embryo; h, hypocotyl; hi, hilum; p, primary root; s, seedcoat; sr, secondary root.

The seedcoat, or testa, is developed from one or two outer layers or integuments of the ovule. These layers form a covering, whose function is to protect the embryo against drying out, mechanical injury, and attacks by insects, fungi, and bacteria. Usually the outer coat is hard and durable, and the inner one is thin and membranous. Often the two seem to be fused into one layer.

The seedcoat and fruit wall may develop appendages or special structures that adapt the seed to certain ways of dissemination. In cotton, for example,long fibers produced by epidermal cells of the cotton seed cling to passing objects, and the seeds thus are carried from place to place.

THE SEED usually matures at the same time the fruit ripens.

Fruits are classified as simple, aggregate, or multiple. The simple fruits are further classified as dry and fleshy.

In certain fruits, the wall, or pericarp, which sometimes is called the seed vessel, is composed of three layers exocarp, mesocarp, and endocarp.

The peach is a one-seeded fleshy fruit whose pericarp has three layers the outer skin (exocarp); the fleshy edible part (mesocarp); and the hard, bony pit (endocarp), which surrounds the seed.

One has to have some knowledge of fruit classification if he wants to identify seeds correctly as to origin and structure. The fact that a blackberry is not a berry but an aggregate fruit and watermelon is a hard-rinded berry indicates that common terms do not always indicate the true structure of a fruit.

We should remember that the fruit is a device for seed distribution and that its structure is related to the way in which seeds of each species are dispersed.

FROM THE STANDPOINT of function, a seed is a device for the reproduction, preservation, increase, and dissemination of the plant species.

The many different ways in which seeds are dispersed illustrate the complexity of Nature's plan in providing for the perpetuation of plant species. Dryness is a factor of great importance in this connection. At low moisture content, the living embryo respires very slowly, and some seeds remain viable for many years, even if they are subjected to harsh conditions.

The food stored in seeds is also important. The reserves supply energy to the embryo as it resumes growth during germination. The young seedling plant is thus given a start in its new location, often a long distance from the mother plant that produced the seed.

Furthermore, the stored food has a part in Nature's plan of seed distribution, for it is attractive to animals. Squirrels bury many more acorns and nuts in the soil than they ever consume as food. Some of these seeds sprout and grow into young oak and hickory trees to replace overmature individuals in the forest.

Many examples may be cited of the interdependence of plants and animals based upon stored food in the seed. Notable is man's use of seeds for food.

This point leads to a related one. Despite the marvelous mechanisms that produce and disperse seed, many of the improved varieties of crop plants would perish under natural conditions. People, who need the seeds for food, have bred and selected crop plants adapted to their needs. Such improvement and specialization have been possible only because the seed is a product of sexual reproduction in the flower. The sexual reproduction means that the plant, represented by the embryo of the seed, may show traits inherited from either or both parents and consequently be different from either.

That is the key to the improvement of useful, beautiful plants.

THUS, seeds serve us in at least three ways. A large part of agriculture has to do with producing seeds that are used for food and as materials for many other uses. Many seeds are grown for planting, so farmers can have the best seed for the next crop. Seeds are the basis of all procedures to improve plants through breeding.

Plant explorers search all parts of the world for unusual plants that might furnish valuable characters for the plant breeder. Resistance to disease has been bred into many crop plants by use of a wild seed that in itself was worthless. Desert plants frequently provide drought-resistance characters valuable in breeding dry-land crops.

Man's dependence on the crops he has developed through work with seeds behooves us to learn more about their origin, structure, and function.

JOHN W. MCKAY is a horticulturist at the Plant Industry Station, Beltsville, Md. He is a graduate of the University of Texas and University of California at Berkeley.