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AMERICAN JOURNAL OF BOTANY 
[Vol. 9 
percent. A diminished water content in the plant as a whole has been 
studied also, by determining the difference between the rate of transpira- 
tion and that of absorption, as these rates vary independently throughout 
the day and night (Renner, 12, 13). 
The present paper describes preliminary experiments that deal with what 
may offer another method, useful with some plants, for the indirect study 
of incipient drying; this method depends upon changes in leaf position or 
leaf shape, resulting from changes in leaf water content and concomitant 
alterations in the turgidity relations of different leaf tissues. 
Leaves of the coconut {Cocos nucifera) were employed. The leaf of 
this plant is primarily to be regarded as a very large, entire or merely 
notched leaf with very regular pinnately arranged veins reaching laterally 
outward from a central rib. As it develops, the leaf blade tears, however, 
midway between each pair of adjacent veins, so that it comes to have the 
appearance of a pinnately compound leaf, the true midrib appearing like 
a rachis and the lateral veins appearing like the midribs of pinnae. The 
lateral strips, separated by the tearing just mentioned, will be called pinnae 
in this paper. 
It was noted by Copeland (2) that running ventrally for its entire 
length along each side of the midrib of the coconut pinna there is a narrow, 
colorless strip, the two strips together constituting a "hinge." Through 
the action of the hinge the two wings of the pinna may take various posi- 
tions, thus altering the general configuration of the pinna. As Copeland 
has pointed out, when the leaves are well supplied with water the hinge 
cells are distended and the two pinna wings are held nearly in the same 
plane, like the right and left halves of an ordinary leaf; but when there is 
a deficiency of water in the hinge tissue the two pinna wings revolve down- 
ward, about the pinna midrib as an axis, so that their lower faces approach 
each other. When the pinnae are on the point of beginning to curl on 
account of drying, the angle between the two wing faces is about 25 degrees 
of arc. The total actual width of an average coconut pinna is about 3.5 
cm., the wing width being half as great. Since the two v/ings remain 
approximately flat until curling begins, simply moving upward or downward 
on the midrib as an axis, the angle between them may be conveniently 
approximated by means of the distance between the two free, parallel 
edges. The behavior of the pinna hinges is illustrated by Copeland's (2) 
tables showing variations, for several days, in the distance between the 
edges of pinnae. 
Since the action of the hinges and the resulting "opening" or "closing" 
of the pinnae appeared to be related to turgidity changes, it seemed de- 
sirable to study the movements with reference to the water content of the 
pinnae and to the temperature and evaporating power of the air. The 
present paper reports a preliminary study of this kind. 
These experiments were carried out at the College of Agriculture of the 
