Cross-Fertilization

in botany and horticulture, the fertilization of the ovules of one plant by pollen from another plant. Darwin, the investigator of fertilization, claims that plants raised from cuttings of the same parent, as potato plants raised from parts of the same potato, are practically the same plant, and that true cross-fertilization can be secured only when the pollen and the ovules belong to plants raised from seeds.

In many plants the pistils are ready for pollen before or else after the pollen of the same plant is ready to fall. In this case the pistil may receive pollen brought by the wind from another plant, perhaps miles away. Some flowers are so peculiarly constructed that there is no opportunity for the pollen to come in contact with the pistil. In this case the pistil depends on pollen brought to it by the legs of bees and other insects in search of sweets. In the case of the pea family the anthers so envelop the style of the pistil that cross-fertilization is unlikely to occur. The wind and insects are the chief agencies of cross-fertilization. Plant breeders protect the pistil from wind and insect by paper bags, and powder with pollen from the particular plant desired. Darwin established the principle that seeds obtained by cross-fertilization are more vigorous, and produce stronger plants, than seeds obtained by close-fertilization. There are, of course, exceptions to the rule. He experimented with the common morning glory. He found that cross-fertilization and close-fertilization for one generation gave plants having average heights of 100 and 76 respectively; the fifth generation gave 100 and 75. Seeds produced by ten successive cross-fertilizations, and seeds produced by ten successive close-fertilization produced plants having heights of 100 and 54 respectively. If the prevailing wind be from the west when pollen is flying, it is evident that a row of corn on the west side of a field must be close-fertilized, or not at all. Under the same wind conditions, the east row has slight chance of close-fertilization; but the silk is likely to catch pollen from tassels growing in rows further west. Windy weather favors cross-fertilization. The skillful corn breeder chooses his seed from rows that are likely to be cross-fertilized.

The beneficial effect of cross-fertilization upon morning glories and corn has been described. On the other hand, close-fertilization or inbreeding is best for some plants. Experiments with tobacco, for instance, have convinced growers that the best seed may be obtained by tying paper bags over the whole flowers stalk in such a way as to shut out insects and exclude pollen carried by the wind. Seed obtained in this way is heavier, and produces not only more vigorous, but more uniform plants than seed obtained by miscellaneous cross-fertilization.