1910] WIEGAND—TRANSPIRATION 439 
to the young leaf. It now remains to determine what relation the 
experiments bear to the solution of the last two questions. 
From a consideration of the thin unprotected epidermis of hygro- 
phytic plants, guttation through water pores, root pressure, and 
possibly other methods of increasing the transpiration stream, we 
may reasonably infer that most if not all plants find it advantageous 
to maintain as great a transpiration stream at all times as is con- 
sistent with their water supply. From this point of view, plants 
living in a highly desiccating atmosphere may be divided into two 
groups. In one group the water supply is very limited; in the other, 
through the presence of water in the soil, the supply is much greater. 
It would seem highly advantageous, therefore, for plants of the latter 
group to possess some mechanism by which the transpiration could 
be retarded when tending to be so excessive as to exceed the water 
supply, as for instance in strong dry winds or bright sunshine; but 
which would allow almost uninterrupted transpiration when the 
transpiration tends to be less, as at night. Plants of the former 
group would find continued protection desirable. This they would 
find in the waxy and resinous coatings, while those of the second group 
would find hairy coverings better adapted to their needs. If our 
interpretation is correct, we should expect desert plants with very 
Scanty water supply to be highly cutinized, instead of hairy, while 
the hairy desert plants would be found in some way connected with 
a greater supply of subterranean water. Such a hairy plant in the 
desert might be supposed to act as follows. During the day the 
hot dry winds blow and the sun shining upon the leaflets tends still 
further to increase the transpiration. The requisite protection is 
now afforded by the hairy covering; but at night the winds die down, 
the atmosphere becomes humid, dew falls, and transpiration becomes 
more difficult. At such times the hairy covering does not materially 
impede the transpiration. 
Known facts do not seem opposed to this interpretation, but 
rather in its favor. The writer has had no opportunity to determine 
the available water supply of desert plants, but an inspection of 
Covittr’s Death Valley report shows that the habitat of a great 
majority of the characteristically hairy shrubs cited (p. 52) is given as 
either dry river bottoms, lake shores, or high on the mountains. 
