20, 2 Trelease: Foliar Transpiring Power of Coconut 175 
thus result from a small increase in transpiring power and a very 
great increase in the evaporating power of the air. For a fur- 
ther discussion of these features, the reader is referred to the 
literature; our present knowledge of the quantitative water 
relations of plants has been largely developed in the publications 
by Livingston.° 
It should be mentioned, in connection with a consideration 
of the results of these tests, that the best-known methods for 
studying transpiring power may be expected to give somewhat 
different results when used on a plant such as coconut, in which 
the exposure of the leaf surfaces undergoes diurnal alterations. 
Only one of these methods (the one depending upon the power of 
the leaf surface to give off water vapor to a standard water- 
absorbing surface) was used in the present experiments. The 
other method, also devised by Livingston," involves a comparison 
between absolute rates of transpiration and rates of water loss 
from some form of atmometer, the white spherical instrument 
being generally satisfactory; by this method the quotient of the 
transpiration rate divided by the evaporation rate from the 
standardized atmometer, for the same time-period and exposure, 
is used as a measure of transpiring power. It is not the purpose 
of this discussion to give a detailed comparison of these two 
methods, since only one of them was used in these tests; but one 
difference, which has not been emphasized in the literature, 
appears worthy of mention when coconut is considered. This 
dissimilarity is connected with the well-known fact that the 
position, or direction of exposure, of the leaf surface influences 
its transpiring power; thus it is generally recognized that a 
leaf, so placed that the rays. of sunlight fall on it at right angles, 
is of course likely to transpire more rapidly than a similar leaf 
so placed that the rays strike it at a smaller angle; also, it has 
long been known that a leaf, freely exposed to the movements 
of the air, has a tendency to transpire more rapidly than a 
Similar leaf that is protected from air currents. It is evident, 
as was pointed out by Copeland, (5) that the wings of the coconut 
pinna, when folding together in the middle of the day, alter 
their position in such a way as to tend to receive the sunlight 
obliquely and also to protect the lower, transpiring surface 
from air currents—the alteration in exposure thus tending to 
decrease transpiring power in these two ways. The two methods 
for measuring transpiring power may thus be expected to give 
“For citations of literature, see Livingston and Hawkins. (9) 
“See Livingston and Hawkins. (9) 
