172 The Philippine Journal of Science 1922 
until nearly sunrise. After a period of rapid transpiration, due 
to high evaporation rates, Livingston and Brown(7) found 
that the water content of many kinds of leaves is markedly lower 
than it is after a period of slow transpiration (soil-moisture 
conditions remaining about the same), and that the diminished 
water content or incipient drying is in a general way propor- 
tional to the evaporation rates. Since apparent pinna width 
decreases with increased evaporation rates and increases with 
decreased evaporation rates, this inverse relationship may be 
taken as evidence of the truth of the assumption made by Cope- 
land(5) that apparent pinna width may be used as an index of 
the water content of the hinge cells. 
Returning to a consideration of the graph for transpiring 
power (dot and dash line), it will be of interest to discuss 
briefly, from our present knowledge of the water relations of 
plants, the influences that may be effective within the plant to 
bring about the increase in transpiring power to high values in 
the early morning, the decrease throughout the latter part of 
the day, and the decrease to low night values. 
For many plant species foliar transpiring power has been 
found, in numerous investigations, to depend very largely upon 
the condition of the stomata—the stomatal aperture, or diffusive 
capacity, being by far the most powerful influence taking part 
in the control of foliar transpiring power of many plant species.® 
Thus the openness of the stomata frequently has such an impor- 
tant influence upon the daily march of transpiring power that 
other influences may safely be disregarded. It has long been 
known that the stomata of most kinds of plants respond to the 
stimulus of light by opening, and that in darkness they close; 
thus stomata are usually open during the day, and more or less 
completely closed at night.? But stomata also tend to close, 
as many observations have shown, when the water content of 
the leaf tissues becomes reduced as a result of high rates of 
transpiration accompanied by inadequate absorption, especially 
when the leaves have become noticeably wilted.* Besides the 
stomatal condition, another influence that may possibly have 2 
pronounced effect upon transpiring power is the partial pressure 
of water vapor in the sub-stomatal spaces of the leaf; this should 
depend upon the state of the leaf tissues.® 
“See Trelease and Livingston. (14) 
*See Livingston and Estabrook. (8) 
*See Lloyd. (11) 
*See Livingston and Brown (7) and Trelease and Livingston. (14) 
