( 660 ) 
the mercury, so that the carbon dioxide formed could not escape 
from the leaf, but had to move sideways. 
The question now arose whether this closing of the epidermis 
was a necessary condition for the success of the experiments. From 
the nature of things this might be considered probable, for otherwise 
the respiratory carbon dioxide would follow the line of least resistance 
through the stomata and epidermis. Experiments made with this 
object in view have completely confirmed this opinion. A leaf of 
Dahlia Yuarezii was introduced into the apparatus first described, 
with its apex in the small bell-jar without caustic potash. One 
longitudinal half of the apex was uncovered, but to the other half 
a strip of black paper was fastened, by which a transverse strip of 
this half was darkened; this strip measured 17 mm. from the mer- 
cury in the direction of the tip (exp. XLII). In the same leaf it 
was therefore possible to compare: a lighted portion adjoining a 
darkened one, the epidermis of which was shut off by the mercury; 
and a lighted portion adjoining one darkened by paper, the epidermis 
of which was therefore not shut off. A starch zone was indeed 
formed along the edge of the mercury, but not along that of the 
black paper. When the experiment was repeated (exp. XLIII), with 
the portion of the leaf under the paper smeared with a mixture of 
cocoa-butter and wax (according to Sranr), the result was quite 
different. This mixture is known to close the epidermis almost 
completely to carbon dioxide, and the well-known starch zone now 
also appeared at the edge of the paper. \ 
The use of cocoa-wax finally led to some experiments which may 
be called extremely simple, and which once more confirmed the 
result obtained. 
Leaves of Aesculus Pavia (exp. XLIV) and of Juglans regia (exp. 
XLV), free from starch, were completely covered with cocoa-wax, 
in which condition, according to STAHI.’s experiments, extremely 
little or no starch is formed during exposure to light in the open air. 
The leaves were, however, exposed to the light by Mr. Zijrsrra 
after they had been partially covered with black paper or tin-foil, 
and now, as was to be expected, black borders of starch were formed 
along the edges of the paper and of the tin-foil. 
A rather pretty modification of these experiments was finally 
obtained by using variegated leaves, of which the colourless portions 
were quite white, and did not therefore contain any carotin, by 
means of which carbon dioxide might be decomposed, even in the 
absence of chlorophyll. It is further necessary that such leaves, in 
order to be suitable for the experiments in question, should possess 
