272 BOTANICAL GAZETTE [OCTOBER 
promptly as soon as they are exposed to the light; provided they 
had not been kept too long in the dark. If the leaves are kept 
in the dark in a Petri dish whose bottom is covered with water, a 
few notches may grow out, but they are not nearly as numerous as 
Fics. 40 AND 41 
those growing out in the light. Whether we are dealing here with a 
direct chemical effect of the light or with an indirect effect on the 
flow of substances in the leaf remains to be seen. It should not be 
overlooked that as soon as the leaves of Bryophyllum turn yellow 
they become less turgid and easily fall off from the stem. : 
X. Theoretical remarks 
We may now go back to the first experiments mentioned in this 
paper and try to analyze them on the basis of the old idea that 
the flow of material in the plant is responsible for phenomena of 
growth. We start from the assumption that a notch of a leaf can 
