A Civic Biology, Presented in Problems

by George W. Hunter

A wild orchid, a flower of the type from which Charles Darwin worked out his theory of cross-pollination by insects.

History of the Discoveries regarding Pollination of Flowers.—Although the ancient Greek and Roman naturalists had some vague ideas on the subject of pollination, it was not until the first part of the nineteenth century that a book appeared in which a German named Conrad Sprengel worked out the facts that the structure of certain flowers seemed to be adapted to the visits of insects. Certain facilities were offered to an insect in the way of easy foothold, sweet odor, and especially food in the shape of pollen and nectar, the latter a sweet-tasting substance manufactured by certain parts of the flower known as the nectar glands. Sprengel further discovered the fact that pollen could be and was carried by the insect visitors from the anthers of the flower to its stigma. It was not until the middle of the nineteenth century, however, that an Englishman, Charles Darwin, applied Sprengel's discoveries on the relation of insects to flowers by his investigations upon cross-pollination. The growth of the pollen on the stigma of the flower results eventually in the production of seeds, and thus new plants. Many species of flowers are self-pollinated and do not do so well in seed production if cross-pollinated, but Charles Darwin found that some flowers which were self-pollinated did not produce so many seeds, and that the plants which grew from their seeds were smaller and weaker than plants from seeds produced by cross-pollinated flowers of the same kind. He also found that plants grown from cross-pollinated seeds tended to vary more than those grown from self-pollinated seed. This has an important bearing, as we shall see later, in the production of new varieties of plants. Microscopic examination of the stigma at the time of pollination also shows that the pollen from another flower usually germinates before the pollen which has fallen from the anthers of the same flower. This latter fact alone in most cases renders it unlikely for a flower to produce seeds by its own pollen. Darwin worked for years on the pollination of many insect-visited flowers, and discovered in almost every case that showy, sweet-scented, or otherwise attractive flowers were adapted or fitted to be cross-pollinated by insects. He also found that, in the case of flowers that were inconspicuous in appearance, often a compensation appeared in the odor which rendered them attractive to certain insects. The so-called carrion flowers, pollinated by flies, are examples, the odor in this case being like decayed flesh. Other flowers open at night, are white, and provided with a powerful scent. Thus they attract night-flying moths and other insects.

Other Examples of Mutual Aid between Flowers and Insects.—Many other examples of adaptations to secure cross-pollination by means of the visits of insects might be given. The mountain laurel, which makes our hillsides so beautiful in late spring, shows a remarkable adaptation in having the anthers of the stamens caught in little pockets of the corolla. The weight of the visiting insect on the corolla releases the anther from the pocket in which it rests so that it springs up, dusting the body of the visitor with pollen.

The condition of stamens and pistils on the spiked loosestrife (Lythrum salicaria).

In some flowers, as shown by the primroses or primula of our hothouses, the stamens and pistils are of different lengths in different flowers. Short styles and long or high-placed filaments are found in one flower, and long styles with short or low-placed filaments in the other. Pollination will be effected only when some of the pollen from a low-placed anther reaches the stigma of a short-styled flower, or when the pollen from a high anther is placed upon a long-styled pistil. There are, as in the case of the loosestrife, flowers having pistils and stamens of three lengths. Pollen only grows on pistils of the same length as the stamens from which it came.

The milkweed or butterfly weed already mentioned is another example of a flower adapted to insect pollination.[1]

The pronuba moth within the yucca flower.

A very remarkable instance of insect help is found in the pollination of the yucca, a semitropical lily which lives in deserts (to be seen in most botanic gardens). In this flower the stigmatic surface is above the anther, and the pollen is sticky and cannot be transferred except by insect aid. This is accomplished in a remarkable manner. A little moth, called the pronuba, after gathering pollen from an anther, deposits an egg in the ovary of the pistil, and then rubs its load of pollen over the stigma of the flower. The young hatch out and feed on the young seeds which have grown because of the pollen placed on the stigma by the mother. The baby caterpillars eat some of the developing seeds and later bore out of the seed pod and escape to the ground, leaving the plant to develop the remaining seeds without further molestation.