chapter  VIII
23 Pages

MODIFIED CIRCUMNUTATION: EXCITED BY LIGHT

MOVEMENTS

Sachs first clearly pointed out the important difference between the action of light in modifying the periodic movements of leaves, and in causing them to bend towards its source,l The latter, or heliotropic movements are determined by the direction of the light, whilst periodic movements are affected by changes in its intensity and not by its direction. The periodicity of the circumnutating movement often continues for some time in darkness, as we have seen in the last chapter; whilst heliotropic bending ceases very quickly when the light fails. Nevertheless, plants which have ceased through long-continued darkness to move periodically, if re-exposed to the light are still, according to Sachs, heliotropic.