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of the

tender apex may sometimes mechanically prevent its growth; or the of thick gum-water more than once to the same side may and then checked growth on this side with continued growth on the opposite and unaffected side would account for the reversed of the apex.

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. When a plant which is strongly heliotropic is exposed to a bright lateral light, it bends quickly towards it, and the course pursued by the stem is quite or nearly straight. When a plant is exposed to a dim lateral light and continues during the whole day bending towards it, receding late in the evening, the movement unquestionably is one of heliotropism. The common view seems to be that heliotropism is a quite distinct kind of movement from circumnutation; and it may be urged that in the foregoing diagrams see heliotropism merely combined with, or superimposed on, circumnutation. The conclusion explains that diaheliotropic movements cannot be fully explained by the direct action of light, gravitation, weight, and so on., any more / than can the nyctitropic movements of cotyledons and leaves.