How Flowers Reproduce
( Originally Published 1943 )
We shall describe the phenomenon of plant reproduction because in it, a major role is played by our enemy "pollen grain," the villain of our hay fever drama. (This is not to be confused with Noel Coward's comedy play entitled Hay Fever.) We feel that a knowledge of the biological development of the offending plants may help some of us to originate an idea on how to impede their growth, kill them, or prevent their very origin.
The plant kingdom is by no means simple. It comprises hundreds of thousands of species with a variety of methods for reproduction. For our purposes we are interested in the sub-class of the plant kingdom known as the angiosperms. These plants are characterized by the fact that they bear seeds and flowers. This group includes the broad-leaved or hard-wood trees, such as ash, oak, elm and hickory; the flowers, such as rose, goldenrod, lily, coffee, and tobacco; the grasses and cereals, including rice, wheat and barley; and the weeds, including prairie sage, wormwood, Russian thistle, and ragweed.
In the process of reproduction the flowering plants depend on several methods because of their variations in structure. In most species of flowers the male and female reproductive organs are contained in the one flower as in the rose, ragweed, violet, peony and clover. In some species of flowers there is a complete separation of the sexes. For instance, the flowers of the ash and willow tree are either male or female.
Regardless of the method of reproduction or variation in structure, every aspect of a flower is designed to assist in effecting the continuation of its kind. The female part of a flower is the pistil, it is composed of three parts; the stigma, style, and ovary in which is contained the ovules or eggs. The male part of a flower is the stamen, it consists of a filament attached to a pollen producing sac known as the anther. See Fig. i. The pistil and stamens are attached to a flower stalk and are surrounded by leaf-like parts. These leaf-like parts are the petals and sepals which form, the floral envelope. It is the floral envelope that gives form and attractiveness to most flowers.
Flowers are considered to be in bloom or to be matured when the stamens begin producing pollen and the ovaries produce eggs. At this stage the plants are ready for reproduction or pollination as it is called. This consists of transferring the pollens of the male plants to the stigmas of the female plants. When the pollen grain is on the stigma it bursts its outer coat and extends a pollen tube through the tissues of the stigma into the ovary. There it discharges two male cells, one of which fuses with an egg. From this fertilized egg there develops the embryo of a seed. The other male cell fuses with nuclear material in the ovule to provide food for the developing seed. As the seed develops the flowering part of the plant withers away, leaving the seed with its reserves of food material to await the return of favorable conditions to develop a new plant.
These seeds are well prepared to withstand the rigors of the climatic conditions in which they grow. Internally they are supplied with a store of food material and externally they form a coat adequate for their own protection. This seed coat may be brittle and paper-like, as is the peanut shell; leathery, as in the squash; or very hard, as in the hazel nut. Thus plants are known to insure perpetuation of their species by a well planned process from beginning to end. This process is sometimes upset by man when the seeds are used as food. But to insure a continuation of this food source, man cultivates the seeds and enhances their growth processes. In the case of hay fever, our aim would be to upset the process and inhibit the growth of the responsible plants. Although many hay fever projects have dedicated themselves to this task, none has yet been successful.