PHOTOHARMOSE I3I 



temporary concentration due to drops of dew, rain, etc. It seems improba- 

 ble that the concentrating effect of epidermal papillae can do much more 

 than compensate for the reflection and absorption of the epidermis. Ex- 

 perimental study has shown that the maximum intensity in nature may be 

 increased several, if not many times, without injurious results and without 

 an appreciable increase in the photosynthetic response, thus indicat- 

 ing that the efficient difference increases toward the maximum as well as 

 toward the minimum. 



173. The reception of light stimuli. Rays of light are received by the 

 epidermis, by which they are more or less modified. Part of the light is 

 reflected by the outer wall or by the cuticle, particularly when these present 

 a shining surface. Hairs diffract the light rays, and hairy coverings con- 

 sequently have a profound influence in determining stimuli. The walls 

 and contents of epidermal cells furthermore absorb some of the light, 

 especially when the cell sap is colored. In consequence of these effects, the 

 amount of light that reaches the chlorenchym is always less than that inci- 

 dent upon the leaf, and in many plants, the difference is very great. 

 According to Haberlandt', the epidermal cells of some shade plants show 

 niodifications designed to concentrate the light rays. Of such devices, he 

 distinguishes two types : one in which the outer epidermal wall is arched, 

 another in which the inner wall is deeply concave. Although there can be 

 no question of the effect of lens-shaped epidermal cells, their occurrence 

 does not altogether support Haberlandt's view. Arched and papillate 

 epidermal cells are found in sun plants where they are unnecessary for in- 

 creasing illumination, to say the least, A large number of shade plants 

 show cells of this character, but in many the outer wall is practically a 

 plane. Shade forms of a species usually have the outer wall more arched 

 or papillate, but this is not always true, and, in a few cases, it is the lower 

 epidermis alone that shows this feature. Finally, a localization of this 

 function in certain two-celled papillae, such as Haberlandt indicates for 

 Fittonia verschaffeiti, does not appear to be plausible. 



The epidermis merely receives the light; the perception of the stimulus 

 normally occurs in those cells that contain chloroplasts. The cytoplasm of 

 the epidermal cells, as well as that of the chlorenchym cells, is sensitive to 

 light, but the response produced by the latter is hardly discernible in the 

 absence of plastids, except in those plants which possess streaming pro- 

 toplasm. The daily opening and closing of the stomata, which is due to 

 light, is evidently connected with the presence of chloroplasts in the guard 

 cells. Naturally, the perception of light and the corresponding response 

 occur in the epidermis of many shade and submerged plants which have 



^ Physiologische Pflanzenanatomie. 3d ed., 537. 1904. 



