118 
Sydney H. Vines. 
Recently this view of De Saussure’s, which has just been discussed, 
has been revived by Kraus 1 ). His Statement of it is 1) that young lea- 
ves, so soon as they are exposed to light, develope chlorophyll-grains, in 
which starch is at once formed by assimilation, and 2) that it is at Ihe 
expense of this starch that the young leaf grows. The observalions 
upon which this Statement is based consist of comparative measurements 
of the Iength and breadth of normal and of etiolated leaves in different 
stages of their growth, as well as of measurements of their histological 
elements. — Commencing soon after the first appearance of the leaves 
when they are nearly equal in size, these measurements shew that the 
growth of the normal exceeds that of the etiolated leaves, so that in a 
few r days the former are more than twice as large as the latter. The 
microscopical measurements shew, as might be expected, that the size of 
the histological elements of the normal leaves is greater than that of the 
elements of the etiolated leaves, and also that the total thickness of the 
former is greater than that of the latter. Tests for the presence of starch 
shew r that the starch, which first makes its appearance in a young normal 
leaf, is a producl of assimilation, and that it has not been conveyed from 
other parts of the plant, for 1) starch is not to be found in a very young 
leaf, 2) when it can be detected, it occurs in the chlorophyll-grains of 
the mesophyll, more especially toward the margins of the leaf, and first 
of all in the spongy parenchyma of the lower surface, and 3) the cells 
forming the sheaths of the fibrovascular bundles contain very small quan- 
tities of starch which increase as the leaves develope and expand to the 
light. 
These facts are doubtless well-established, but they simply confirm 
the a priori ideas which would naturally be formed with reference to 
the development of leaves from the bud, and they are insufficient to 
juslify the second clause of Kraus’ Statement. It may be readily adrnit- 
ted that the starch which makes its appearance in leaves shortly after 
their emergence from the bud is a product of assimilation and that it 
has not been conveyed from other parts of the plant, for it is perfectly 
natural that as soon as the young leaves are under conditions in which 
assimilation is possible, they should at once begin to perform this function, 
and that starch should therefore make its appearance in the chlorophyll- 
grains; but there is no ground for the assumption that the further growth 
of these young leaves is simply and only dependent upon the starch 
which they themselves produce. 
There are already facts on record which shew that leaves are ca- 
pnble of growth whilst they are not assimilaling. Kraus himself quotes a 
t) Ueber die Ursache der Formvernnderung etiolirter Pflanzen. (Jahrb. f. wiss. Bot. 
Bd. VII. 1869.) 
