Page:Popular Science Monthly Volume 20.djvu/682

662 disappear altogether. Thus the persistent, often-fruiting species gain the monopoly on high mountains and in Arctic regions, but with the difference that in some districts they maintain themselves aboveground through the whole year without protection against the climate, while in others they exist through a long period of rest, protected against the injurious effects of the cold by means of their perennial parts under the soil or under the cover of an effective shelter.

Without attempting a closer review of these relations, or of the development of different life-terms in the successive periods of the development of the earth's surface and of the vegetable kingdom, we come to these comprehensive conclusions: A plant has to pass through two phases, one of vegetation, the other of propagation, both of which are conditioned upon the supply of food. In the simplest instances these phases are exhibited in one and the same cell, which, absorbing the food, elaborates it for its own growth, and gathers so much force that two new individuals are produced by division while the mother plant ceases, by this fact, to be. In another yet similar manner is accomplished the life of those plants which in their complicated structure present an extreme contrast to the simple cell. In this case also the whole feeding of the plant is performed with the sole object of adapting it to the purpose of propagation; but the end is reached in various ways and times, while the attainment of it is not always accompanied with the end of life. Some plants proceed in uninterrupted growth from their origin to the end of the propagation, collect forces on forces, and die exhausted as soon as propagation is accomplished. Others can not reach maturity by a short stage, but require a longer time for gathering their forces. This enforcement is gained either in an uninterrupted course, or, as is more frequently the case, by a series of alternate periods of growth and rest, followed by a period of vigor, when maturity of the fruit is at last reached and the plants die from exhaustion. A third group reach the stage of propagation after a shorter or longer period without giving up their whole strength to the formation of seed, but apply a portion of the nourishment they receive to the formation of perennial organs by the aid of which they continue to grow and become fruitful again, and competent to repeat the same processes an indefinite number of times.

Seeing different plants thus attain very different ages, we naturally inquire what are the causes of the diversity. They depend partly on the adaptation to external conditions induced by climate, soil, and environment, all of which, especially climate, determine the life-habits of plants in various ways, and seem also to affect the length of their life. But these external conditions can not of themselves force the plant to adapt itself to them; it must itself have a fitness to react upon influences acting upon it from without. This is really the case in various degrees, for the variability which a species exhibits by its individuals in different directions extends to the length and the manner of its life,