174 The Philippine Journal of Science 1922 
pired by coconut is given off from the lower surfaces. Thus, 
considering both surfaces, the transpiring power would have 
values approximately half as great as those shown in Table 1; 
the maximum value would therefore be about 0.28, and the mi- 
nimum, 0.18. 
It should be emphasized that this paper deals with fluctuations 
in transpiring power, not in transpiration. Transpiring power 
represents only the group of internal conditions influencing the 
transpiration rate, which of course is also greatly influenced by 
environmental conditions. As numerous studies have shown, 
the actual amount of water transpired in a certain period of 
time from a given area of leaf depends upon two sets of condi- 
tions: (a) the transporting power of the leaf surface (controlled 
by the anatomical structure of the leaf, the number, distribution, 
and openness of the stomata, the way in which the leaf is exposed, 
the degree of saturation of the leaf with water, etc.), and (0) 
the effective external conditions surrounding the leaf (controlled 
principally by the evaporating power of the air—temperature of 
the air, moisture content of the air, movement of the air—and 
the intensity of the absorbed sunshine). Since, as is illustrated 
by the actual values in Table 1 and as has been shown by 3 
number of investigations, the relative change in the most in- 
fluential of the external conditions (the evaporating power of 
the air) is much greater from hour to hour or from night to 
day than is the relative change in transpiring power, it of 
course follows that changes in the actual rate of transpiration 
are determined very largely for a particular plant species by 
changes in the external conditions. Accordingly, for the whole 
night, as was found by Copeland,(5) the total transpiration 
from a coconut leaf may not be as much as one-tenth as 
great as for one hour of full sunshine. All studies thus far 
reported agree in showing that from night to day the transpiring 
power usually increases but slightly; in the present tests it in- 
creased in actual value from 0.26 to 0.56—that is, it became 
only about twice as great during the day as it was at night; the 
evaporating power of the air, however, in the day frequently 
becomes many times as great as it is at night. Numerous inves- 
tigations with many kinds of plants have shown that the actual 
transpiration rate during the daylight hours may often be more 
than thirty times as great as during the hours of darkness. The 
transpiration rate is of course proportional to the product of 
the transpiring power of the plant and the evaporating power 
of the air, and the high rates of transpiration during the day 
