PHOTOHARMOSE 141 
of epidermal papillae in increasing the absorption of light by shade plants i 
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 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’ 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 Im- 
patiens 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 at 
it is most clearly indicated by Bl 
different ecads of the same 
species. In Scutellaria brit- 
toni, 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 Fig. 41, Isophotophyll of Helianthus pumilus, 
showing isophotic ecad: 1, sun leaf; 2, shade leaf 
shade leaf 90: In the stable (ignt 012). 130. 
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 
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