444 BOTANICAL GAZETTE [JUNE 
hairy or all cutinized, or even that hairy and smooth individuals 
of the same species should not be found occasionally side by side. 
If we go more into the details of distribution than into the considera- 
tion of the general trends of vegetation and the general examples 
already given, we must then consider the individual plants; for it 
often happens that species growing side by side may in one case have 
a deep root system reaching ground water, in the other have a root 
system confined to the driest superficial layers of the soil. It is also 
conceivable that there may be inherent physiological differences 
between species, and even between individuals of the same species, 
calling for variations in the amount and kind of protection. The 
life economy must be studied out in each individual case. We must 
not fail to remember, also, that the whole seasonal cycle, as well as 
the life cycle of the individual, must be considered before a full 
conclusion may be drawn; for a hairy covering which seems not 
to fit into the general scheme may have done so at an earlier period 
of development or in an earlier season. 
General conclusions 
The evaporation experiments outlined in this paper tend to show 
that porous coverings like cotton, wool, or hair must be very thick 
to produce any appreciable effect in retarding evaporation if the 
surrounding atmosphere is quiet, but become very efficient even in 
thin layers when the air is in motion. On the other hand, a waxy 
covering is effective at all times, though of course somewhat more 
so in wind. In sunshine, also, the hairy covering shows a greater in- 
crease in efficiency than does wax. 
It seems probable that those plants employ a hairy covering to Te 
tard transpiration that live in situations where a moderate water supply 
is available, but where transpiration must be reduced in excessively 
dry times, but not interfered with when the surrounding air is damp 
and transpiration therefore difficult. Cutin, on the other hand, 
is probably employed when there is considerable danger of too great 
desiccation at all.times. 
A contemplation of the general occurrence of excessively hairy 
plants lends probability to this view. 
WELLESLEY COLLEGE 
