=179. Modification of the epidermis.= The development of epidermal chloroplasts in diffuse light is the only change which is due to the direct effect of light. This does not often occur in the shade ecads of sun species, but chloroplasts are regularly present in the epidermis of woodland ferns and of submerged plants. The slight development of hairs in sciophilous plants is an advantage, but it must be referred to the factors that determine water loss. The significance of epidermal papillae in increasing the absorption of light by shade plants has already been discussed. The questions as to what factor has called forth these papillae and what purpose they serve must still be regarded as unsettled. The increased size of the epidermal cells, which is a fairly constant feature of shade ecads, seems to be for the purpose of increasing translocation and transpiration, and to bear no relation to light. The extreme development of the cells of the epidermis in _Streptopus_ and _Limnorchis_, which grow at the edge of mountain brooks, has been plausibly explained by E. S. Clements as a contrivance to increase water loss. The presence of a waxy coating, such as that found upon the leaves of _Impatiens aurea_ and _I. pallida_, is clearly to prevent the wetting of the leaf and the consequent stoppage of the stomata. In regard to the latter, different observers have noted that the number of the stomata is greater in sun than in shade leaves. This holds generally for sun and shade species, but it is most clearly indicated by different ecads of the same species. In _Scutellaria brittonii_, the sun form possesses 100 stomata per square millimeter, but in the shade these are reduced to 40 per square millimeter; the sun leaf of _Allionia linearis_ has 180 stomata to the square millimeter, the shade leaf 90. In the stable leaf of _Erigeron speciosus_, however, the number of stomata is the same, 180 per square millimeter, for sunlight and for diffuse light. The presence of the larger number of stomata in the plant exposed to greater loss, which at first thought seems startling, is readily explained by the more intense photosynthetic activity in the sun. Since the absorption of gases is the primary function of the stomata, and transpiration merely secondary, it is evident that sun plants must have more stomata than shade plants. This is further explained by the fact that the small air passages of sun leaves necessitate frequent inlets, which are less necessary in shade leaves with their larger air spaces. In shade plants, moreover, the decrease in the number is compensated in some measure by the ability of the epidermal cells to absorb gases directly from the air. [Illustration: Fig. 42. Diphotophylls of _Quercus novimexicana_: 1, sun leaf; 2, shade leaf of the same tree (light .06). × 130.]