PHOTOHARMOSE 131 
temporary concentration die 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 
modifications 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 verschaffelti, 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 
1Physiologische Pflanzenanatomie. 3d ed., 537, 1904. 
