30 LIGHT AND THE BEHAVIOR OF ORGANISMS 



the assumption that the hehotropic reactions are caused by 

 a chemical effect of light; in all such reactions time plays 

 a role. We assume, furthermore, that if light strikes the 

 two sides of a symmetrical organism with unequal inten- 

 sity, the velocity or the character of the chemical reactions 

 in the photosensitive elements of both sides of the body is 

 different." This and the following quotation show that 

 he now considers orientation to be controlled by difference 

 of intensity on opposite sides, the very idea which Sachs 

 in his theory opposed. 



In the following quotation he also brings out his idea as 

 to the direct effect of the agent on the reacting tissue with 

 reference to plants. Orientation in animals is supposed to 

 be just like this in principle; in animals, the agent is sup- 

 posed to affect the locomotor organs directly or through a 

 direct reflex arc (1906, p. 118) : '' How can light bring about 

 heliotropic curvatures? Let us suppose that light strikes 

 a plant on one side only, or more strongly on one side than 

 on the opposite side, and that it be absorbed in the super- 

 ficial layers of tissue of that side. In this case we assume 

 that on that side certain chemical reactions occur with 

 greater velocity than on the opposite side. What these 

 reactions are is unknown; we may think provisionally of 

 oxidations. This change in the velocity of chemical re- 

 actions either produces a tendency of the soft elements on 

 that side to contract a little more than on the opposite 

 side, or creates otherwise a greater resistance to those 

 forces which have a tendency to elongate or stretch the 

 plant, e.g., hydrostatic pressure inside the cells, or imbibi- 

 tion of certain tissue elements. The outcome will be that 

 one side of the stem will be stretched more than the oppo- 

 site side, and this will bring about a curvature of the stem. 

 Where the latter is soft at the tip, the bending will occur 

 only, or chiefly, in that region; and as the degree of softness 

 decreases rapidly from the tip downward, the result will 

 be that the tip will bend toward the source of light. This 

 result may possibly be aided by a greater photosensitive- 



