= lini 
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326 BOTANICAL GAZETTE [APRIL 
as do our data of moisture variation, from which consideration it 
may be concluded that the variation is probably not causally related 
to such accumulation of substances. While there are several 
observations in the literature which bear more or less directly upon 
this matter of the diurnal accumulation of soluble and insoluble 
bodies in the leaf, we do not as yet possess data adequate to a 
logical discussion. This should be made the subject of a special 
study in the present connection. 
Following the criterion of the time of occurrence of the minimum 
water content (2 to 5 P.M.) and of that of the maximum in relative 
transpiration, as brought out in Publ. 50, Carnegie Inst. (10 A.M. 
to 1 P.M.), it appears highly probable that there exists a causal 
relation between these two phenomena. Since the critical point 
in the graph of relative transpiration denotes, as has been pointed 
out, the entrance into operation of some internal check or hindrance 
to the loss of water vapor, and since a decrease in relative water 
content should be effective to produce such a check long before the 
water content had attained its minimum, we should expect the 
maximum in relative transpiration to occur long before the hour of 
least foliar moisture, which it appears actually to do. We may 
_ conclude tentatively, and on general lines, that, for the ordinary 
types of leaves, the retardation in water loss is manifest several 
hours before the time of minimum water content. After the 
critical point in relative transpiration, the non-stomatal retardation 
of water loss appears to be (Publ. 50) continuously active until - 
well into the night, its effects becoming mingled with those of 
- stomatal closure at or about sunset. It appears to be gradually 
removed during evening and early night, so that in the hours just 
preceding sunrise stomatal retardation seems to be alone manifest. 
If the non-stomatal effect be due to decreased water content, it 
should be removed when the leaves had regained their normal 
moisture, which condition is quite met by the observations. The 
maximum in moisture content (except in the few erratic cases 
already mentioned) occurs in the night or early morning, just as 
our hypothesis demands. 
The only direct comparison between the daily march of leaf 
moisture and that of relative transpiration which has been made 
