meaning little gate) and there passes from the end of the tube a male cell or germ, which unites with an egg cell in the ovule—this is fertilisation. Fertilisation does not always occur immediately after pollination; for instance, the flowers of witch-hazel are pollinated in the fall, but fertilization does not occur until the following spring.
Before the mutual relations of flowers and Insects for the purpose of ensuring cross-pollination is mentioned, it is essential that something be said about the importance of crossing, both in the animal and vegetable kingdom. The effects of self-fertilization and cross-fertilization were first dearly pointed out by Charles Darwin. He was experimenting with two beds of toadflax, or butter-and-eggs (Linaria vulgaris), one set being the offspring of self-fertilization and the other of cross-fertilization. The plants had been raised for the purpose of studying heredity, and not the results of cross-breeding. To his astonishment he observed that the plants which were the result of cross-breeding were far more vigorous than the others. He temporarily abandoned all his other investigations and devoted himself to making numerous experiments in interbreeding, and to acquiring information as to the practical experience of horticulturists and breeders of livestock.
As the result of his inquiries he laid down the general lav that no species of animal or plant can fertilize itself through numberless generations, and that an occasional cross is indispensable. The evil effects of interbreeding had, indeed, long been known in a general way, and are even instinctively recognized by barbarous races. The deterioration goes on so slowly at first that it is not easily discovered; but finally results in loss of size, vigor, and fertility, and may be accompanied by deformity. The good effects of intercrossing are immediately manifest. When plants were crossed, their offspring were larger in size, grew more vigorously.