72 LIGHT AND THE BEHAVIOR OF ORGANISMS 



and other plants. Krabbe (1889) supported Darwin in his 

 conclusion, as did also Rothert (1894) and Czapek. Haber- 

 landt (1904), on the other hand, maintains not only that 

 the blade is functional in light perception, but also that the 

 curved and thickened outer walls of the epidermal cells act 

 as lenses and focus the light on the protoplasm within, and 

 that orientation is regulated by responses due to the dis- 

 tribution of the intensity of light within the cells of the 

 epidermis. Kneip (1907) covered the upper surface of the 

 blades of Tropaeolum with a thin layer of paraffin oil whose 

 index of refraction is about 0.143 greater than the index of 

 cell sap. The oil consequently inverted the lens effect of 

 the curved walls of the epidermal cells and thus caused a 

 dispersal of the rays within the cell. Kneip found however 

 that the leaves treated thus responded to light much like 

 those not treated, and concluded (p. 136), '' that the lens 

 action is of no importance in the leaves studied." Haber- 

 landt (1909) however does not agree with this conclusion. 

 He claims that the fact that leaves still respond to light 

 after the epidermis is covered in such a way as to neutralize 

 the focusing effect of the curvature of the outer cell walls, 

 merely shows that the effect of these walls can be dispensed 

 with and not that it is useless, and holds that after the lens 

 effect of these walls is neutralized the light intensity is still 

 unequal on the inner surface of the cells, when the light 

 strikes the epidermis obliquely, and that this may cause 

 orienting responses, but that the focusing effect of the 

 curved outer walls of the cells enhances the promptness 

 and precision of the orienting responses. 



Various other experiments aside from those mentioned 

 above have been carried out, but the results obtained lead 

 to no definite conclusions concerning the function of the 

 lens action of the epidermal cells, nor do they give any 

 clear notion as to the mechanism of orientation in plants. 

 About all that can be said is that leaves generally take a 

 position such as to facilitate photosynthesis, and that the 

 chloroplasts within the cells likewise assume what may be 



