438 BOTANICAL GAZETTE [JUNE 
saturated layer of air will be somewhat increased and evaporation 
will be correspondingly retarded. 
In wind, however, molecules are immediately blown away from 
the naked or wax-covered surface (d), while in the hairy covering 
(c) there is retained a partially saturated atmosphere. Air currents 
do not easily penetrate the interstices between the hairs, and the 
vapor molecules probably find more difficulty in rapidly passing 
through the hairs. Since evaporation from a surface is in propor- 
tion to the saturation of the adjacent atmosphere, it follows that 
evaporation in diagrams a and 0 will differ but slightly, while between 
c and d there will be a much greater difference. 
Evaporation is also proportional to temperature. It is reasonable, 
therefore, to expect an increase when the blotter is exposed to the 
heat of the sun’s rays. Hairy coverings act as a screen to the heat 
rays, and therefore the cooler blotters beneath them evaporate less. 
It is probable, however, that the effect of sunshine in nature is rarely 
if ever as great as in our experiments, except possibly in the case of 
leaves lying flat on the ground, because of the absorbing effect of 
the black background of the laboratory tables used. 
Significance of the results 
We are now able to answer at once the first two questions pro- 
pounded in the introduction to this paper. A protective action of 
35-40 per cent in wind with coverings of the thickness of outing 
flannel certainly warrants us in saying that hairy coverings are suffi- 
ciently efficient in retarding loss of water to justify their maintenance 
on the basis of natural selection. A protection of 5.6 per cent in 
wind shows also that strigose coverings may materially affect the 
loss of water. In the case of strigose plants the hairs are usually 
developed very early, and therefore while the leaves and shoots are 
small. The hairy covering is therefore more concentrated an 
denser at this time, but becomes more scattered as the surface increases. 
As Covitte® has suggested, such strigose hairs probably function 
mainly while the organs are young. Hairs which on the mature 
leaves are scattered far apart may have been of considerable service 
§ CoviLtE, F. V., Botany of the Death Valley Expedition. Contr. U.S. Nat. 
Herb. 4:53. 1893. 
