1903] BRIEFER ARTICLES 389 
IS DETMER’S EXPERIMENT TO SHOW THE NEED OF 
LIGHT IN STARCH-MAKING RELIABLE? 
(WITH TWO FIGURES) 
In ns Practical Plant Physiology, Detmer’s method of showing the 
necessity of light in photosynthesis, is to pin disks of either card board or 
felt, exactly opposite, on both sides of a tropaeolum leaf.* With the 
exception of the area covered starch forms throughout the entire leaf. 
This experiment, slightly modified by using cork in place of felt or 
cardboard, is found in several school text-books. While performing 
this experiment as given in one of these books, the question arose 
whether the cork did not so hinder the diffusion of CO, that the failure 
to form starch was not entirely due to the absence of light. 
First, some students performed a number of experiments,’ but as 
they did not derive conclusive results, at the suggestion of Mr. L. 
Murbach and with aid from him, I made further experiments. 
It is not stated in most of the ordinary text-books that the small 
percentage of carbon dioxid in the air does not diffuse extensively 
through the intercellular spaces in leaves, though it may be inferred 
from experiments given in a number of standard handbooks. 
It is well known that the formation of starch is prevented when 
the leaf surface bearing stomata is smeared with vaseline, paraffin, or 
cocoa butter, and if only a portion of the leaf is so smeared starch appears 
for the most part or entirely in the unsmeared regions. This I verified 
in Primula obconica by using melted paraffin (nearly cold) for closing 
the stomata, which in this species are on the under surface only. The 
fact that starch forms when the upper epidermis is coated with paraffin 
shows that the warm paraffin has not injured the leaf. On the other 
hand, since light was admitted when the paraffin was placed on the 
under surface and no starch deposited, its absence can only be due to 
lack of carbon dioxid, and if it diffused far through the intercellular 
spaces, the formation of starch would still be possible. It is evident, 
then, that diffusion of carbon dioxid is not very extensive in the 
intercellular spaces of the leaf. 
s Pflanzenphysiologisches Praktikum 44. Moor, Eng. tr. 52. 
-6It should be noted that this method of fastening disks of any kind on the leaf 
necessitates pressing them very close to the surface. 
The most striking of these was with a perforated piece of cork held to the 
under side of the leaf by strips of cork on the upper surface. Starch formed only over 
the opening in the under cork. 
®In normal leaves it need be only from stoma to stoma, or the equivalent dis- 
tance in the intercellular spaces. 
