Page:The Effect of External Influences upon Development.djvu/14

10 Recent advance in our knowledge of the physiology of plants has, however, resulted in showing that much commoner phenomena of plant—life must depend upon adaptation, and are not mere products of the general constitution of the plants. I cannot help expressing my wonder and admiration at the recent achievements of botanical research in analyzing the reactions of the parts of plants to extemal influences with such a degree of certainty and clearness. Gravity determines the growth of roots vertically downwards and of stems vertically upwards, and this 'geotropic' sensitiveness is so delicate that even on the unsteady foundation of the floating shoots of Utricularia the flower-stalks rise perpendicularly to a height of 4–5 centimeters, and so exactly as to remain standing erect and to display their flowers—even though, with a very slight deviation, they would necessarily turn over and fall into the water.

How exceedingly important, too, are the consequences of the sensitiveness of plants to light;—what infinitely fine gradations in this sensibility occur in the reactions of the parts of plants, and how exactly is it adapted to their needs! A superficial observer might imagine that we are here concerned, not with adaptation, but with an original peculiarity of plants; but it is well known that the modes in which plants or the parts of plants react to light are very different, and these modifications of their general sensitiveness to light depend on differences in the fine molecular structure of their living substance. The shoots of most plants incline towards the light or are 'positively heliotropic';