INCIPIENT DRYING AND WILTING AS INDICATED 
BY MOVEMENTS OF COCONUT PINNAE i 
Sam F. Trelease 
(Received for publication September 12, 1921) 
The amount of water in the plant at any time depends upon the relation 
between the rates of absorption and of transpiration that have recently been 
in operation. During the day most plants lose water more rapidly than they 
absorb it; in the night, on the other hand, plants usually absorb water more 
rapidly than they transpire it. Thus it is generally true that the water 
content of the plant is high in the night, especially after midnight, while 
it is lower during the day, especially in the afternoon; an incipient drying, 
or saturation deficit, of plant tissues generally appears early in the day and 
becomes progressively greater as the day advances. Incipient drying 
is accompanied by d-ecreasing turgor in some or all of the leaf tissues and 
frequently results in actual wilting (Livingston and Brown, 7; Livingston, 
6). This diurnal deficit appears to be one of the most important features in 
the complex relation of the plant to its water supply; and it has an im- 
portant bearing on the general problems of drought resistance and irriga- 
tion of cultivated crops. 
A method employed by Livingston and Brown (7) in studying the 
changes that occur in the relative water content of leaves throughout the 
day and night, is to gather a large number of similar leaves from hour to 
hour and to determine the moisture content of each lot as percentage of 
the dry or green weight. As pointed out by these workers, this method 
fails to take into account the small diurnal increase in materials other than 
water within the tissues; if the water content per leaf remained constant, 
such accumulation would of course lower the percentage of water on the 
basis of weight; but they regard the percentage changes shown by their 
data as mainly due to lowered water content per leaf, brought about when 
the ratio of the rate of water loss to that of water supply became less than 
unity. Similar results for the twigs of a desert plant were obtained by 
Edith B. Shreve (14). 
Other workers (Lloyd, 10; Miller, 11) have employed circular leaf 
samples of known area, instead of entire leaves, and have calculated the 
water percentage on the basis of leaf area. The use of such samples makes 
it possible to study changes in weight that may be due to the accumulation 
of material other than water, but it does not permit a study of alterations 
due to changes in leaf area; according to Thoday (16), during wilting some 
kinds of leaves may shrink two or three or in some cases as much as six 
^ Botanical publication from the Johns Hopkins University, no. 71. 
253 
