376 McLean . — Field Studies of the Carbon 
area. This also may have caused an error in this experiment, since the 
area of each half-leaf was measured just before drying it. 
Since the outer side of the leaf was dried in the morning in the case of 
the test leaf, the gain in dry weight is undoubtedly too low on this account. 
On the other hand, the shrinkage and consequent gain in dry weight per 
unit area would tend to partially counterbalance this. However, with errors 
of the magnitude of those pointed out in the foregoing, no close agreement 
is to be anticipated between the gain in dry weight per unit area and the 
amount of carbon dioxide absorbed. A rough similarity in the magnitude 
of the calculated gains in weight per sq. m. by the dry weight method and 
by the gas analysis method is clearly shown. 
The difference in weight of the outer and inner sides of a sugar-cane 
leaf also affects the comparison of the tested and the exposed leaf, for in 
the tested leaf the outer (heavier) half was dried in the morning, leaving the 
inner (lighter) half for the test. This would tend to make the observed 
increase in dry weight too small. In the case of the exposed leaf the 
reverse procedure was followed, the inner (lighter) side being dried in the 
morning, thus tending to make the result too high. For this reason the 
observed difference in increase in dry weight (o*io grm.) between the exposed 
leaf and the tested one is not significant, being due possibly to the error as 
explained above. 
SERIES 2 TO 6. — Tests of coco-nut pinnae attached to the tree. 
Five of the six apparently healthy leaves of a young coco-nut tree 
were tested during these series. These leaves were numbered i to 6 from 
the youngest to the oldest, as shown in Plate XVIII, Fig. 2, which is a 
photograph of the tree, taken on May 28, at 5 p.m., during the sixth series 
of tests with leaf No. 4. 
These tests were all performed in a similar manner to those described 
under series 1, except that the leaves used were attached to the plant. The 
width of spread of the edges of adjacent pinnae were measured, when 
possible, at each observation period, to compare the rate of photosynthesis 
to the changes in water content of the leaves. Each pinna has a cross- 
section like an inverted V, and this V becomes narrowed towards midday and 
again expands at night, due to expansion and contraction of the hinge cells 
at the angle on both sides of the midrib. This spread was measured at 
three marked points on each of two pinnae, and the changes in the average 
values computed for each test period are used as indices of the water 
content of the leaves. 1 
The observed data are presented in Table IV, and the computed carbon 
dioxide absorption values are summarized in Table V. Table IV is similar 
in data and arrangement to Table I, except that the concentration of acid 
1 Copeland, E. B. : The Coconut. Macmillan and Co., London, 1914, p. n. 
