
-1905.] On the Physiological Processes of Green Leaves. 59 
which it is liable are accumulative in the result, and affect that result 
absolutely. Besides the ordinary errors to which operations of measuring and 
weighing are liable, and which may be so far reduced as to be almost 
negligible, there are three possible sources of error which may have a very 
serious influence on the result. In the first place it is by no means certain 
that the cell-wells and contents of the leaf-cells after insolation are in the 
same condition as regards their constitutional water as they were before 
insolation. Weare not referring here to such metabolic changes as those due 
to the hydrolysis of starch or cane-sugar or the reverse changes, for although 
such transformations undoubtedly result in the fixation. or liberation of 
water, and theoretically must have an effect on the final dry weight of the 
leaf, yet it can be shown that this effect must be relatively small. We refer 
rather to possible changes in the power of retention of water at 100° C. by the 
colloidal elements of the cell-contents. A very small change in this power 
of water retention might make a very appreciable difference in the final 
estimate by the Sachs’ method of the apparent amount of matter assimilated ; 
but the two principal sources of error in the Sachs’ method, compared with 
which all others sink into comparative insignificance, are no doubt (1) differ- 
ences due to want of perfect symmetry in the venation and the thickness of 
the lamina, and (2) alterations of area in corresponding parts of the leaf, one 
of which is measured at once, and the other only after being placed in 
the light for some hours, during which time, although the leaf may apparently 
remain unaltered, it is put under a different state of tension. 
We have made a number of experiments on both these points. In the first 
place, in investigating the degree of symmetry which exists on the two sides of 
a leaf, only those leaves were taken which appeared to the eye to be quite 
symmetrical. The two sides of the leaf were then separated carefully from 
the mid-rib, printed on photographic paper, and the area of each approxi- 
mately equal side carefully taken with the planimeter. We may here 
mention that errors of planimeter measurement fall well within 0°1 per cent. 
The halves of the leaves were then dried under exactly the same conditions 
in a slow current of dry air until constant in weight. 
Another series of experiments was then made in order to estimate the 
possible difference in area, due to shrinkage or the reverse, which a leaf 
undergoes on insolation. A leaf of Catalpa bignonioides was divided down 
the middle, and the half to which the petiole was still attached was printed 
on photographic paper, and the area determined with the planimeter. These 
half-leaves were then placed in a glazed assimilation-case and exposed 
to sunlight under a canvas screen, air being drawn through the case just 
