How Seeds Are Formed

JOHN W. MCKAY.

A FLOWER exists to produce seed.

For that, two organs are essential.

The stamen produces pollen grains that later form the male cells, or sperms. The stamen has a stalk, or filament, at the tip of which is the pollen sac, or anther.

The pistil, usually in the center of the flower, is the female organ. Normally it has three fairly distinct parts: The ovary, which contains one or more immature seeds, called ovules; above the ovary, a slender style, or tube; at the tip of the style, the stigma, on which the pollen is deposited.

The stamen and pistil are called essential organs because they are necessary if there is to be seed.

Two other organs sepals and petals are not directly involved in sexual reproduction. Some flowers do not have them. We call them accessory organs. A flower that has all four organs is a complete flower.

The sepals are the lowermost of the four organs. Usually they look like leaves. Their main function is to protect the bud until it has developed into a flower. The sepals collectively are called the calyx.

Above and inside the calyx are the petals, known collectively as the corolla. In many flowers they are brightly colored. The petals of many flowers have glands nectaries in which a sweet liquid, called nectar, is secreted.

The colors, the nectar, and the odors of essential oils produced by the petals of many flowers attract insects, hummingbirds, people, and other creatures, which, with wind and gravity, may transfer the pollen from the anther to the stigma to fertilize the ovules.

Sometimes the reproductive organs are formed in separate flowers on the same plant. Such plants are said to be monoecious. The watermelon, cucumber, and other members of the gourd family are examples.

Individual plants of some species have one-sexed flowers that is, flowers on a plant may have only stamens, and the flowers of another plant of the species may have only pistils. To such plants the term dioecious is applied. The holly is an example.

A SEED is a ripened ovule containing an embryo.

A fruit, in terms of origin, is a ripened ovary containing the seeds.

Both ovule and ovary are in the pistil of the flower. To identify seeds and fruits correctly, one has to follow the development of these parts to maturity.

The botanical definition of a fruit is much broader than the popular meaning of the word. For example, the mature bean pod is the fruit of the bean plant, and bean seeds are ripened ovules. The bean pod originates in the bean flower as a minute ovary, which contains ovules so small as to be scarcely visible without a magnifying glass.

Fruit and seeds are present in miniature form in the flower as ovary and ovules, hence the importance of the flower in the development of the seed.

Each female flower in the corn plant has an ovary containing a single ovule, and the mature grain, or fruit, is single seeded, and the ovary wall and the seedcoat are united to form a single covering. Many ovaries are arranged together on a common receptacle, which later becomes the cob of the ear of corn. The long, slender silk is the style through which the pollen tube grows to reach the ovule. A grain of corn is a familiar example of a single-seeded fruit that is commonly called a seed.

All the important cereals are members of the grass family, and have one-seeded fruits.

The large sunflower and other members of the aster family, such as lettuce, dandelion, and aster, produce one-seeded fruits.

By contrast, the ovary of the watermelon flower contains many ovules, which mature to produce the many-seeded watermelon (fruit).

The plants that produce seed are in two natural divisions.

Gymnosperms, the "naked-seeded" plants, bear cones and produce seeds on the surface of cone scales. Among them are such trees as pine, fir, cypress, cedar, and redwood. In the gymnosperms, ovules are produced in female cones, and the pollen is produced in male cones. Pollen is carried to the ovule-bearing female cones by the wind, and each ovule matures into a "naked" seed. This group has no structure comparable with the fruit of the angiosperms, the other division, which are the true flowering plants.

THE SEED-PRODUCING plants spermatophytes originated from lower forms of the plant kingdom through a long series of evolutionary changes in both the reproductive and vegetative structures.

The four great divisions of the plant kingdom, beginning with the simplest and ending with the most complex, are algae and fungi; liverworts and mosses; ferns and fern allies; and the seed plants, the spermatophytes.

Algae and fungi lack differentiation of the plant body into true roots, stems, and leaves, although some of the forms may have structures that resemble those organs. Sexual reproduction makes its appearance in this group, but the sex organs and spore-producing structures are usually one celled and primitive. Most of the algae live in water, and the simple, reproductive structures seem to depend on water for functioning and distribution.

Liverworts and mosses are essentially land plants. The zygote the fertilized egg is retained in the female sex organs for some time, during which it divides to form a mass of cells, the young sporophyte or asexual phase of the plant's life cycle. During this period, the sporophyte gets its food, water, and other essentials of life from the gametophyte the part of the plant which produces gametes, or sex cells and thus is parasitic on it.

A Typical Flower a a a, petals (corolla); b b, sepals (calyx); c, receptacle; d, pistil (composed of: e, stigma; f, style; g, ovary); h, stamen (composed of: i, anther; j, filament); k, pollen, kk, pollen tube; l, sperm nuclei; m, egg cell; n, ovule.

The many-celled and differentiated sporophyte is structurally adapted to withstand drying out and is linked with the beginning of the existence of plants on land. In water plants, such as algae, the zygote is protected against drying, since it is bathed in water at all times.

Mosses have sporophytes that are about equal in size to the gametophyte but depend on the latter for nourishment.

The ferns and fern allies are the first great group of plants that develop an independent sporophyte with true roots, stems, and leaves.

This is one of the most important steps in the evolution of the plant kingdom. The development of a vessel system, which allows water and food to be conducted rapidly through stems, roots, and leaves, was mainly responsible for this advance.

The sporophyte is the dominant plant body in this group, and the gametophyte is usually quite small, though still independent and self-sustaining. Spores are disseminated as asexual bodies when conditions are favorable and develop into small, separate gametophytes.

In seed plants, the last big step in plant evolution, the sporophyte is completely dominant over the gametophyte. Spores are retained in special organs of the sporophyte (essential parts of the flower), and male and female gametophytes are formed within these organs.

The fertilized egg in all lower plants develops immediately into the mature sporophyte, but in seed plants it grows for a time and then goes into a dormant condition to form the seed.

Some of the lower algae multiply only by mitotic cell division a process of exact splitting of the chromosomes, resulting in two identical cells where one existed before.

Beginning with the higher algae and extending through the rest of the plant kingdom, a process known as alternation of generations is the framework of evolutionary change.

That means that a sexual generation (gametophyte) alternates with an asexual phase (sporophyte). In algae and fungi, the gametophyte generation is the dominant plant body, and the sporophyte, usually quite small, is parasitic on the gametophyte.

Evolutionary differentiation from this point on involved a gradual reduction in size of the gametophyte and consequent increase of the sporophyte.

In the seed plants, reduction of the gametophyte generation has reached the point at which the male gametophyte (pollen tube) and the female gametophyte (embryo sac) are much reduced and are parasitic on the sporophyte an exact reversal of the relation of the two phases in algae.

Special cells of the ovule and anther differentiate into embryo-sac-mother-cells and pollen-mother-cells, all having the diploid or sporophytic number of chromosomes.

The mother cells divide by a process called meiosis, which results in the daughter cells receiving half the number of chromosomes characteristic of the species. Male and female gametophytes derived from the daughter cells pollen grains and embryo sacs thus have nuclei with the reduced or gametophytic number of chromosomes. Fusion of sperm and egg at fertilization restores the double, or diploid, chromosome number in the new embryo or sporophyte.