1910] WIEGAND—TRANSPIRATION : 443 
is great. This is a fine picture of just such conditions as we should 
expect would demand hairy plants; absorption somewhat below 
normal, transpiration at times excessive, and at other times, as in 
foggy weather, very slight. 
The use by epiphytic plants of cutin rather than hair for retard- 
ing transpiration is well known. Even though the air is humid, 
the scantiness and uncertainty of the water supply actually obtain- 
able by the plant warrants the employment of cutin. The most 
notable exception is the Florida moss (Tillandsia usneoides), the 
ecological relations of which I have not been able yet to fully make 
out. It seems probable, however, that the excessive development 
of the scaly absorbing glands of the leaves is for still further 
increasing the efficiency of absorption, rather than primarily to 
retard transpiration. 
Hairs are provided on growing shoots and unfolding leaves to 
retard the transpiration during windy and sunny times in spring, 
before the cutin has become fully developed. Some plants make 
use of resin for this purpose instead (e.g., Larrea in the desert, and 
Gaylussacia in New England). It is probable that in these cases 
a more efficient covering is here desired. Such resinous coverings 
may be superior to cutin in that they may be easily shed or be much 
interrupted when their early protective action is no longer desired. 
This actually takes place in both of the above-named examples. 
In regard to the last question propounded in the introduction, 
whether the relation to light or to loss of water has been the principal 
factor in the evolution of hairy coverings, the following facts may 
be cited. In practically all cases where one leaf surface is devoid 
of hairy covering it is the upper. Such cases are very common, €.g., 
Ledum, Antennaria, Quercus, etc. This would not be true if the 
primary function was as a light screen. Instead, the covering is 
maintained over the stomate-bearing surface. The multitude of 
such cases makes it seem almost obvious that the main function of 
hairy coverings lies in their relation to transpiration, not to the 
intensity of the light. 
From the foregoing statements it is not to be understood that 
wherever conditions are as described only hairy plants are to be 
expected, and that in a given locality plants should be either all 
