20, 2 Trelease: Foliar Transpiring Power of Coconut 173 
From the generally known facts briefly outlined’ above, the 
explanation may be advanced that the high transpiring power 
of coconut observed during the early hours of sunshine resulted 
from the fact that the stomata were wide open and that the 
leaf tissues were nearly saturated with water. The marked 
decrease beginning at about 11 a. m. was probably due to par- 
tial closure of the stomata and decreased partial pressure of 
water vapor in the leaf, both of these conditions probably depend- 
ing upon a diminished water content of the leaf tissues. That 
such a diminution did occur is suggested by the observed de- 
crease in the apparent pinna width. But it will be noted that 
the decrease in transpiring power continued during the after- 
noon, after the apparent pinna width had begun to increase— 
that is, after the water content of the hinge cells had apparently 
begun to increase. This may be taken to mean that the hinge 
cells respond more quickly (and with different critical values) 
to changes in water content than do the other leaf cells, which 
control transpiring power; apparently, although the water con- 
tent of the hinge cells begins to increase in the early afternoon, 
the other leaf cells may still exhibit marked and increasing 
incipient drying, which may maintain a low partial pressure of 
water vapor in the sub-stomatal spaces of the leaf, or a reduced 
stomatal aperture, or both conditions together. The rest of the 
leaf tissues (or those controlling transpiring power) appear to 
go on drying out long after the hinge cells have passed their 
minimum of turgidity, and have begun to increase in size, thus 
lifting the pinna wings. The continued reduction in transpir- 
ing power in the very late afternoon may be regarded as due to 
stomatal closure in response to the reduced light intensity ac- 
companying the setting of the sun; and the fall in transpiring 
power to the very low night values no doubt resulted from 
partial stomatal closure accompanying darkness. Such partial 
closure during the night is, of course, generally observed in 
plants having active stomata. The low values were maintained 
during the night until 3 a. m., and the very rapid increase in 
transpiring power from 4 to 6 a. m. was apparently due to the 
well-known rapid opening of the stomata during the period 
about sunrise, 
The present tests were made upon only the lower surfaces 
of the leaves, because the stomata of coconut are limited to the 
lower surface; and preliminary tests showed that the transpir- 
ing power of the upper surface is extremely low, almost zero. 
Copeland(5) states that at least 98 per cent of the water trans- 
