equal parts, each of which is like the parent plant. The rapidity with which selfdivision takes place is almost incredible. Cohn computed that the descendants of a single hay bacillus in 24 hours would exceed four septillions. In a single night many square miles may be reddened by a one-celled red alga (Protococcus nivalis). No trace of sexual reproduction can be discovered in many common fungi, as the toadstool, mushroom, and puffball. The higher plants, although they have sex organs, may readily be reproduced vegetatively by buds, rootstocks, runners, stolons, offsets, suckers, bulbs, and tubers. “It is probably true,” says Coulter, “that more plant individuals are produced asexually than sexually. Sex is not essential to reproduction. It was the last method of reproduction attained among plants, and it did not replace the older methods but was added to them.”*
Since sex is not necessary for reproduction, what then is its significance? It secures greater variation. When plants multiply vegetatively, or asexually, they as a rule vary very little. When a florist obtains a new flower, or the horticulturist a new fruit, he increases the number of his plants by budding or by cuttings. Asexual reproduction is much more likely to give new plants resembling the parent form than sexual. But plants in a state of nature must vary or they are likely to perish. If the conditions under which they live change, and they can not change to meet the new conditions, they will become extinct. Neither can a plant migrate into new territory with different conditions, unless it can adapt itself to them. But in the case of plants which possess sex, when two individuals are crossed, the offspring will inherit the characters of both, and will of necessity be more variable than either. They will also be more vigorous than the parents, since beneficial characters are more likely to survive than those which are injurious, and the progeny will inherit more of them.
The development of sex made cross-fertilization possible, and greater variation and vigor in the offspring hastened the evolution of new species, which were able to adapt themselves to every part of the earth and to every condition of life. Lotsy, in his Evolution by Means of Hybridization, regards hybridization as the sole cause of the appearance of new species; and in the opinion of Jeffrey hybridism has played a large role in the evolution of flowering plants. It is worthy of note that, in the great work of pollination, plant-life to-day has found a most efficient helper in the beekeeper.* The reader who desires to pursue further the study of the origin and significance of sex in plants will find the two following little books very helpful: Coulter, John M., “The Evolution of Sex in Plants.” Coulter, John M. and N. C., “Plant Genetics.”