BENDING OF HIGHER PLANTS TOWARD THE LIGHT 7 1 



which produces the curvature, as Loeb's theory quo ted above 

 demands. The mechanism involved is undoubtedly far 

 more complex than this theory indicates. It may be similar 

 to that offered by Pollock to explain the curvatures in roots. 

 He says (1900, p. 59): " The stimulus is transmitted from 

 the sensitive root tip to the curving parts, in the cortical 

 parenchyma. The effect of the stimulus is to increase the 

 normal tension between cortical parenchyma and axial cylin- 

 der on the side that becomes convex, and to decrease or re- 

 verse the normal tension between the cortical parenchyma 

 and the axial cylinder on the side that becomes concave. 

 The change in tension also extends to the different layers of 

 the cortical parenchyma on the concave side, the outer 

 layers becoming negative with respect to the inner ones. 

 So much has been demonstrated. The evidence is in 

 favor of the view that the tensions on the concave side 

 are changed by the protoplasm becoming more permeable 

 to water, some of which passes out into intercellular spaces, 

 possibly to be taken up by the convex cells, which later 

 contain more water than the concave cells. The shorten- 

 ing of the concave side may be masked sometimes by a 

 certain amount of growth." This theory does not account 

 for curvature in structures having but a single cell cavity, 

 like the hyphae of molds, rhizoids of liverw^orts, and some 

 algae, all of which are known to respond to light by bending 

 toward or from its source. That these reactions cannot be 

 accounted for on the basis of osmotic changes was pointed 

 out by Hofmeister as early as 1867. 



Very little is known concerning the fundamental factors 

 involved in orientation in other plant structures than those 

 mentioned, although much work has been done on them, 

 especially on the leaves. Darwin (1881) was the first to 

 attempt to locate the sensitive structure in the leaf. He 

 found that neither quality nor intensity of reaction is 

 affected by shading the blade, and concluded that the 

 petiole perceives the light. V^oechting (1888) came to 

 quite the opposite conclusion in experiments on malva 



