=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 consequently 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 incident upon the leaf, and in many plants, the difference is very great. According to Haberlandt[16], 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 increasing 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 protoplasm. 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 chloroplasts in the epidermal cells. Such cases merely serve to confirm the view that the perception of light stimuli is localized in the chloroplast. In conformity with this view, the initial response to such stimuli must be sought in the chloroplast, and the explanation of all adaptations due to light must be found in the adjustment shown by the chloroplasts.