What Seeds Are and Do: An Introduction

VICTOR R. BOSWELL.

SEEDS are many things.

Above all else, they are a way of survival of their species. They are a way by which embryonic life can be almost suspended and then revived to new development, even years after the parents are dead and gone.

Seeds protect and sustain life. They are highly organized fortresses, well stocked with special supplies of food against long siege.

Seeds are vehicles for the spread of new life from place to place by the elements and by animals and people.

Seeds are food for man and animals and other living things.

Seeds are raw material for the fashioning of myriad products by people. Seeds are wealth. They are beauty. They are a symbol a symbol of beginnings. They are carriers of aid, of friendship, of good will.

Seeds are a source of wonder. They are objects of earnest inquiry in man's ceaseless search for understanding of living things.

Seeds of unwanted kinds are as enemies; they are a source of trouble. Seeds are many things, but everything about seeds their numbers and forms and structures has a bearing on their main purpose, to insure continuing life. Seeds are containers of embryonic plants, the embryos of a new generation.

SEEDS are borne by two great and different classes of plants.

One group, less highly developed than the other, produce "naked" seeds that develop from "naked" ovules.

In plants of the more highly developed and much larger class, the ovule and the seed develop within an ovary, the seed vessel. The ovary is the part of the flower that contains the ovule with its egg, or female sex cell. The ovary later becomes a fruit with the developed ovule or ovules seeds-inside. This group of plants we call angiosperms, a word that means vessels for seeds.

Plants of the other group, the gymnosperms, the "naked seed" plants, have no ovaries, no flowers, and no fruits, although they do have seeds. Gymnosperms include the cone-bearing trees, the conifers. Their seeds are borne in pairs at the bases of the scales of the cones.

DEEP WITHIN the ovary of the mother flower (or between the scales of a seed cone) lies the ovule, which contains an embryo sac and its tiny egg. The egg must be fertilized by a sperm cell from a pollen tube before it can start to develop into an embryo and so perpetuate the parent's life.

Along with the embryo there develops a special store of food, the embryo's own special "formula" or diet for its use after it is separated from its mother plant.

Every seed contains carbohydrates, proteins, fats, and minerals to nourish the embryonic plant within. The nature and proportions of each of them differ among the many kinds of seeds. Some seeds, like corn, are predominantly starchy. Seeds of flax and sunflower are oily or fat. Others, such as peas and beans, are notable for their high content of protein.

Some seeds (such as the seeds of orchids, which are like specks of dust) contain only tiny bits of stored food because they are so small. Large seeds may contain a billion times more food than the smallest ones.

Some kinds of seed have most of their reserve supplies packed inside their seed leaves. Others have it packed in tissues developed from the embryo sac, called endosperm, or from the cells of the ovule that surrounded the embryo sac.

THE SEED usually is well protected through its development. This protection differs greatly among different kinds in degree and in the way it is provided.

The ovary and the tissues that are attached to it become the fruit of the plant. The seeds (formed by ovules in the ovary) of plants having large or fleshy fruits are deeply protected therefore so that we never see them before maturity unless we open the fruit to find them.

Although the seeds of gymnosperms are said to be naked, they nearly always have some protection during development. The seeds of the pine tree and other conifers, for example, are hidden at the bases of the scales of the cone. The cone scales of some pines separate to release the seeds as soon as they are mature. Others remain closed for years.

The fruit tissues that enclose some seeds are scanty and are attached to the coat of the seed. A kernel of corn, for example, is more than a seed it is a one-seeded fruit. The kernel is nearly all seed, but a thin layer of ovary tissue surrounds the seed and has grown together with the seedcoat in such a way that we can hardly see the tissue.

Many structures that we call seeds are actually fruits. Most of them, such as the fruits of the cereals and other grasses, lettuce, and spinach, contain only one seed. Members of the carrot family produce two-parted fruits, each with one seed. Some fruits, such as those of beets, have one or several seeds.

Botanists identify the various types of fruits and give them specific names, but our purpose here is served if we deal with the small, dry, one- or few-seeded fruits, which we are accustomed to plant like seeds, as though they were seeds.

Seeds of some species develop in the mother plant with amazing speed. Some others are surprisingly slow. A chickweed plant that is pulled from the garden and thrown aside at the time its flowers first open may form some seeds before it withers and dies.

most familiar plants form their seeds during a period of several days to a few weeks following pollination. Pine trees however, take 2 to 3 years to mature their seeds. The fruit of the sea palm is said to need 7 to 10 years to mature.

ANOTHER ASPECT of the survival of plants is that the seed-bearing species can be perpetuated in two ways.

One, which we have been discussing, is sexual that is, by means of seeds, which develop from fertilized egg cells. The other is asexual, or vegetative, as we usually say, by means of such parts as buds, pieces of root, and pieces of stem with attached buds, bulbs, and tubers.

The seeds of some plants like potatoes, cultivated tree fruits, grapes, berries, and many ornamental garden plants do not come true to variety. Their seeds therefore are worthless for perpetuating the varieties we plant in gardens and orchards.

For them, we must use vegetative propagation. We can grow apple trees, grapevines, potatoes, or strawberry plants from seeds, but the plants and their fruits (or tubers) will be unlike those of the varieties that produced the seeds.

That is because most seeds, as we have seen, develop after the union of male and female reproductive cells. The seeds perpetuate the hereditary characteristics contributed by both the male and female cells. Seeds of plants like potatoes, apples, pears, and tulips fail to come true to variety because their sex cells carry random assortments of mixed-up sets of characters. Among the offspring of the numberless chance unions that occur in ch plants, hardly any two are alike. The plants from seeds of most species come reasonably true to variety if precautions are taken to keep the pollen of undesired types from reaching the flowers of desired types.

We must note a rare exception. A few kinds of plants, such as some species of grasses and of Citrus, produce asexual seeds, whose embryos develop entirely from cells of the ovule outside the egg apparatus. No fertilization of an egg cell is involved. There is no mixture of characters from pollen cells with those of the mother cells. The embryo is formed entirely from mother-plant cells and therefore is identical with the mother plant in its hereditary makeup. Such asexual seeds, therefore, come true to variety and afford the unusual opportunity of accomplishing "vegetative" propagation by means of seeds. Except for such rare instances, however, seed propagation means sexual propagation, and asexual or vegetative propagation means propagation by some means other than seeds.

Plants that do come true to variety from seed of sexual origin can also be propagated asexually from stem cuttings or other appropriate parts of the plant under favorable conditions.

Why, then, do we consider the seeds of such plants of great importance? Why are seeds essential if we can perpetuate the plants without seeds?

The answer is that conditions rarely are favorable or practicable for their vegetative propagation.

A prohibitive amount of work would be required for the vegetative propagation of the billions upon billions of such plants that we need to grow every year. An even greater obstacle is that there is no feasible way to keep these "vegetative" plants alive through periods of great cold, drought, or flood. If such plants are killed before they produce seed, that is the end of their line.

The kinds and varieties of plants that fail to produce viable seeds that is, seeds that can grow or develop must be perpetuated by asexual means. There is no other way. Such diverse plants as certain grasses, bananas, and garlic produce no seeds, but each has an asexual feature (a vegetative structure) by which it can be multiplied.

Sometimes, for a particular reason, growers resort to vegetative propagation of a kind of plant that is normally grown only from seeds.

Small farmers in the hills of Vietnam grow cabbage year after year without the use of seeds. The climate there is not cool enough at any time to induce flowering and seed production in cabbage, and the farmers cannot import seeds for each planting. The farmers therefore make cuttings from the stumps of the cabbage plants after the heads are harvested. They plant the pieces of stump, each of which has one or more side buds. Roots soon develop. The buds grow and produce new cabbage plants that will develop heads. The process is repeated for each crop.

This method of growing cabbage would be impossible where the seasons become too cold, too hot, too wet, or too dry for the continued survival of the vegetative stage of the plant.

THE ENORMOUS NUMBERS of seeds that single plants of some species produce make it feasible to increase seed supplies at almost fantastically rapid rates. Single plants of other species produce few seeds, and the rates of increase are ploddingly slow.

One tobacco plant may produce as many as 1 million seeds. The average is about 200 thousand seeds. The garden pea plant produces a few dozen seeds at best.