238 BOTANICAL GAZETTE [APRIL 
are described, he finds in every case, except one which developed 
under forced conditions in the greenhouse, that the intumescences 
were formed in the fall or winter, when the plants were in their rest 
period or approaching it. On account of the weak illumination 
during this time, the plants could not assimilate new building mate- 
rial and thus answered the stimulus of an over-abundant absorption 
of water by the formation of intumescences. 
ATKINSON (1) explains the tomato oedema in a similar way. He 
says: “With the comparatively small amount of light carbon assimi- 
lation is lessened, so that the plant, under the forced conditions of 
growth, when it needs large quantities of carbohydrates, has to do 
with a really less quantity than is supplied in the open, when condi- 
tions for rapid growth are not so favorable and those for assimilation 
are improved.” 
KUsteR (5) produced artificial intumescences on poplar leaves 
both in the dark and in the light, but found that when formed in intense 
light they occurred on the surface of the leaf which lay next the 
water. He suggested that in this case they did not form on the 
upper side on account of a more rapid transpiration, due to the strong 
light 
PRILLIEUX (8) observing the disease on pinks and Noack (7) on 
grapes found that intumescences were produced only in weak light. 
On the other hand, Dae (3) found that intumescences were not 
easily produced on Hibiscus vitifolius in poor light; were not formed 
under blue or green glass; but develop readily under red, yellow, or 
whitewashed glass and in bright light. 
Vrata and PacotrTet (21) also strongly emphasize the fact that 
intumescences are only found in quantity on grape leaves directly 
under the glass, during periods of most brilliant illumination. These 
they regard as an effort of the plant to protect the leaf from “chloro- 
vaporization and excessive transpiration” by the formation of a 
false palisade layer. 
According to Von ScHRENK (9), intumescences are formed in the 
Missouri Botanical Garden only under these last-named conditions. 
It is a matter of considerable interest, therefore, to determine the 
conditions under which intumescences are formed in the potato plant. 
On March 12, 1906, thirty-six pots of potatoes were planted in 
