ABSTRACT
Heliotropism prevails so extensively among the higher plants, that there are extremely few, of which some part, either the stem, flower-peduncle, petiole, or leaf, does not bend towards a lateral light. Climbing plants circumnutate much more widely than any other plants, yet they are not at all heliotropic. This chapter illustrates the hygroscopic movements of plants: if the tissues on one side of an organ permit of rapid evaporation, they dry quickly and contract, causing the part to bend to this side. The retina, after being stimulated by a bright light, feels the effect for some time; and Phalaris continued to bend for nearly half an hour towards the side which had been illuminated. The extreme sensitiveness to light of the upper part of the sheath-like cotyledons of the Gramineae, and their power of transmitting its effects to the lower part, are specialized arrangements for finding the shortest path to the light.
