220 BOTANICAL GAZETTE [SEPTEMBER 
which were greatly compressed, contained many parenchyma cells 
of particularly large size, but no aeriferous tissue. 
TEMPERATURE RELATIONS.—Cuttings were sprouted as in the 
preceding series, but the experiments were conducted at several 
different temperatures. With the exception of an increase in the 
rate of growth, tissue effects were not produced. 
INHIBITION OF TRANSPIRATION.—Cuttings from various species 
of Salix and Populus were weighted down in tall cylinders containing 
water and allowed to sprout when completely submerged. Some of 
these cuttings were placed in bright sunlight and others in more 
shaded situations. Sections were taken from the newly produced 
shoots at different levels, as the growing point, the base of the 
sprout, near its origin from the main stem, and also from shoots 
that had just emerged from the water. In this series the most 
marked effect found was the flattening of the stems and the com- 
pression of the tissues proportional to the depth at which they were 
grown. In no case was there any increase in the amount of aerifer- 
ous tissue produced; indeed, in the sprouts produced on cuttings 
from Salix nigra Marsh., the tissue was entirely lacking. 
CULTURE SOLUTIONS.—Cuttings were sprouted in various nutri- 
ent solutions, including all of the formulae recommended by 
MacDoveat (5), and the results were checked by shoots produced 
in ordinary water. In the sprouts produced on Salix cuttings in 
nutrient solutions aeriferous tissue was not developed in the pith. 
ile it was invariably present in the pith of the Populus shoots, 
it was not increased in amount beyond the normal production. 
The results of all the experiments seem to justify the conclusion 
that the stimulus to the development of aeriferous tissue in the 
Salicales is not involved in light, in temperature relations, in the 
nutrient conditions imposed by the conditions of our experiments, 
or in checking of transpiration. 
Discussion 
As already stated, two theories present themselves in explana- 
tion of the unexpected appearance of aeriferous tissue in certain 
regions of insect galls where it is normally absent. The tissue may 
be regarded as a direct effect of the action of certain environmental 
