246 BOTANICAL GAZETTE [APRIL 
shoots are due to the presence of glucose when the other conditions 
of a humid atmosphere and an abundance of water for absorption are 
present. 
As before stated, intumescences were not formed in complete 
darkness, but were developed in very great numbers in the shade and 
in the light, when supplied with an abundance of water and the root 
conditions were favorable for absorption. They were not formed on 
the young growing tips or on the old leaves which had ceased to 
grow. The explanation would therefore seem to be connected with 
the phenomenon of growth. It is probable that they did not form in 
total darkness because here the plants were stimulated to a rapid 
elongation of the cells in the stem and could thus take care of the 
abundant water supplied. At the same time, the glucose would not 
be so abundant as in plants growing in the light, since no assimila- 
tion takes place in darkness. For the same reason they did not form 
on the growing tips of the plants. They were formed in the sunlight 
and in the shade because elongation of the cells is not so rapid as in 
the dark and carbon assimilation is active. The plants could not 
thus take care of the water supplied fast enough, and the abundance 
of glucose in the growing leaves brought about the abnormal tur- 
gescence which caused the intumescences. That the intumescences 
were not found on the old mature leaves was probably due to the fact 
that these leaves had stopped growing and the cell walls were firmer, 
while less glucose was supplied to the cells from the tubers. When 
the tubers were healthy, intumescences formed equally well on 
plants from whole and cut potatoes. It would seem, therefore, that 
bright light is not necessary to the formation of intumescences in the 
case of the potato plant, nor does it act as a stimulus to their forma- 
tion. This stimulus comes from the increased absorption of water 
and lessened transpiration with an abundance of glucose, and when 
enough light is present to produce the conditions for normal growth 
and assimilative activity. 
To obtain a more accurate knowledge of the light intensity, in the 
various places in the greenhouse where the plants were placed, Wynn’s 
photographic exposure meter was used. This instrument is manu- 
factured for the use of photographers in measuring light intensity. 
The light values are obtained by exposing sensitized paper for a 
