PHOTOHARMOSE 



141 



of epidermal papillae in increasing the absorption of light by shade plants 

 has already been discussed. The Cjuestions as to what factor has called forth 

 these papillae and what purpose they serve must still be regarded as un- 

 settled. The increased size of the epidermal cells, which is a fairly constant 

 feature of shade ecads, seems to be for the purpose of increasing transloca- 

 tion and transpiration, and to bear no relation to light. The extreme 

 development of the cells of the epidermis in Streptopns and Liuinorchis, 

 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 Im- 

 pafiens aurca and /. 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 brit- 

 tonii, the sun form possesses 

 100 stomata per square milli- 

 meter, but in the shade these 

 are reduced to 40 per square 

 millimeter; the sun leaf of Al- 

 lionia 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 



Fig. 41. Isophotophyll of Helianthus pumilus, 

 showing isophotic ecad: 1, sun leaf; 2, shade leaf 

 (light .012). X 130. 



