Page:Popular Science Monthly Volume 20.djvu/681

Rh accommodate itself to the change, and must thereby, directly or indirectly, suffer a modification in the length and kind of its life. The soil also, and changes in its condition, have their influence, and surrounding plants and animals have their effects, all of which the plant may feel and show very plainly.

Evidence that the causes which we have only briefly touched upon here play a part in determining the duration of the life of different plants is afforded by cultivation and by the geographical distribution of species. It is well known that when plants are taken into cultivation, and exposed to different temperatures, different degrees of moisture, and different soils from those they have been accustomed to, and direct attacks are made upon their growth, they often suffer changes of one kind or another, and the duration of their life especially is affected. Modifications of external influences may be brought about partly by sowing the seed at different seasons from the usual one, partly by artificial stimulation, pruning, manuring, withholding moisture, etc. The object in these processes has generally been to make the plant go through its course faster, so as to secure an earlier blooming or fruiting; and its life has been shortened without that being the real intention. The other object, of prolonging the life, is also desirable; and there are numerous examples in which the life of plants may be lengthened or shortened by different modes of cultivation.

Particularly interesting examples of the evolution of different life-terms are exhibited in the geographical distribution of plants. If we consider the whole earth as to its climate, we shall observe that in a few regions near the equator, that have a uniform climate, plants will grow all the year through without manifesting any periodical preferences. This is the case, for example, east of the Andes in Northern Brazil, in Guiana, and in Java, where the vegetation is green and blooms continuously, where most species become woody, and nearly all live long and bear fruit often; while the short-lived, once-fruiting species retire to the background. In other tropical regions, where a periodical climate is produced by differences in the moisture of the atmosphere, the long-lived plants prevail, and the ground is so occupied with them till the coming on of the dry season that the short-lived kinds can not find room upon it. The case is different in those regions where spots become barren of vegetation in consequence of the parching heat. Then, when the rainy season sets in, the annuals quickly spring up between the bulbous and tuberous herbs that are able to keep their places through the drought. The short-lived species are of most importance where a warm season alternates with a cold one, and the warm season lasts long enough for them to go within its term through the whole cycle of their life, from their seed-time to the ripening of their fruit. As the warm season becomes shorter the number of annuals is reduced, until finally, when the summer is not long enough for any of them to perfect their seed, they