A Civic Biology, Presented in Problems

by George W. Hunter

Development of Ovule into Seed.—The primary reason for the existence of a flower is that it may produce seeds from which future plants will grow. After fertilization the ovule grows into a seed. The first beginning of the growth of the seed takes place at the moment of fertilization. From that time on there is a growth of the fertilized egg within the ovule which makes a baby plant called the embryo. The embryo will give rise to the adult plant.

The fruit of the locust, a bean-like fruit. p, the attachment to the placenta; s, the stigma.

A Typical Fruit,—the Pea or Bean Pod.—If a withered flower of any one of the pea or bean family is examined carefully, it will be found that the pistil of the flower continues to grow after the rest of the flower withers. If we remove the pistil from such a flower and examine it carefully, we find that it is the ovary that has enlarged. The space within the ovary has become nearly filled with a number of nearly ovoid bodies, attached along one edge of the inner wall. These we recognize as the young seeds.

The pod of a bean, pea, or locust illustrates well the growth from the flower. The pod, which is in reality a ripened ovary with other parts of the pistil attached to it, is considered as a fruit. By definition, a fruit is a ripened ovary and its contents together with any parts of the flower that may be attached to it. The chief use of the fruit to the flower is to hold and to protect the seeds; it may ultimately distribute them where they can reproduce young plants.

The Necessity of Fruit and Seed Dispersal to a Plant.—We have seen that the chief reason for flowers, from the plant's standpoint, is to produce fruits which contain seeds. Reproduction and the ultimate scattering of fruits and seeds are absolutely necessary in order that colonies of plants may reach new localities. It is evident that plants best fitted to scatter their seeds, or place fruits containing the seeds some little distance from the parent plants, are the ones which will spread most rapidly. A plant, if it is to advance into new territory, must get its seeds there first. Plants which are best fitted to do this are the most widely distributed on the earth.

The development of an apple. Notice that in this fruit additional parts besides the ovary (o) become part of the fruit. Certain outer parts of the flower, the sepals (s) and receptacle, become the fleshy part of the fruit, while the ovary becomes the core. Stages numbered 1 to 7 are in the order of development.

How Seeds and Fruits are Scattered.—Seed dispersal is accomplished in many different ways. Some plants produce enormous numbers of seeds which may or may not have special devices to aid in their scattering. Most weeds are thus started "in pastures new." Some prolific plants, like the milkweed, have seeds with a little tuft of hairlike down which allows them to be carried by the wind. Others, as the omnipresent dandelion, have their fruits provided with a similar structure, the pappus. Some plants, as the burdock and clotbur, have fruits provided with tiny hooks which stick to the hair of animals, thus proving a means of transportation. Most fleshy fruits contain indigestible seeds, so that when the fruits are eaten by animals the seeds are passed off from the body unharmed and may, if favorably placed, grow. Nuts of various kinds are often carried off by animals, buried, and forgotten, to grow later. Such are a few of the ways in which seeds are scattered. All other things being equal, the plants best equipped to scatter seeds or fruits are those which will drive out other plants in a given locality. Because of their adaptations they are likely to be very numerous, and when unfavorable conditions come, for that reason, if for no other, are likely to survive. Such plants are best exemplified in the weeds of the grassplots and gardens.

[2] To the Teacher.—Any simple plant or animal tissue can be used to demonstrate the cell. Epidermal cells may be stripped from the body of the frog or obtained by scraping the inside of one's mouth. The thin skin from an onion stained with tincture of iodine shows well, as do thin sections of a young stem, as the bean or pea. One of the best places to study a tissue and the cells of which it is composed is in the leaf of a green water plant, Elodea. In this plant the cells are large, and not only their outline, but the movement of the living matter within the cells, may easily be seen, and the parts described in the next paragraph can be demonstrated.