Because of the internal heat of the plants, heat being a production of the vegetable as well as animal body, though in a much lower degree in the former than the latter. Mr. Hunter appears to have detected this heat by a thermometer applied in frosty weather to the internal parts of vegetables newly opened. It is evident that a certain appropriate portion of heat is a necessary stimulus to the constitution of every plant, without which its living principle is destroyed.—Smith.
Why is fructification so important to plants?
Because it continues them by seeds, and, according to Sir James Smith, "all other modes of propagation are but the extension of an individual, and, sooner or later, terminate in its total extinction." Dr. Drummond is of a contrary opinion, and quotes the following fact:—"In South America there is a species of bamboo which forms forests in the marshes of many leagues in extent, and yet Mutis, who botanized for nearly twenty years in the parts where it grows, was never able to detect the fructifications."—Humboldt.
The produce of vegetable seeds in a hundred-fold degree is common, and many trees and shrubs bring forth their fruit by thousands. A single plant of the poppy will produce above 30,000 seeds; and, of tobacco, above 40,000; and Buffon remarks, that from the seeds of a single elm-tree, one hundred thousand young elms may be raised from the product of one year. Some ferns, it is said, produce their seeds by millions.
Why should seeds be uniformly kept dry before sown?
Because the least damp will cause an attempt at vegetation, when the seeds necessarily die, as the process cannot, as they are situated, go on.
Why, in summer, is continued watering required to newly sown seeds?
Because, if the soil is only moistened at the time of sowing, it induces the projection of the radicle, or first root, which, in very parching weather, and in clayey cutting soil, withers away, and the crop is consequently lost, for want of a continued supply of moisture.
Why is selection important for procuring abundance of genuine seeds?
Because we may then choose the most vigorous plants, which naturally prove of greater fecundity. Thus, in 1823, Mr. Shirreff marked one vigorous wheat plant, near the centre of a field, which produced him 2,473 grains. These were dibbled in the autumn of the same year, the produce sown broadcast the second and third years, and the fourth harvest produced forty quarters of sound grain. A fine purple-topped Swedish turnip produced 100,296 grains, which was seed enough for five imperial acres, and thus, in three years, one turnip would produce seed enough for Great Britain for a year.—Quarterly Journal of Agriculture.