1916] . COSENS & SINCLAIR—AERIFEROUS TISSUE 219 
the pith as well as in the cortex in the reproductive axes of all of the 
species of Salix and Populus that were investigated. This is espe- 
cially noteworthy in the case of Salix, because here and at the bases 
of the branches alone is pith of this type to be found. Figs. 11, 12, 
13, and 14 are photographs of the pith of sections cut from the cat- 
kins of Salix purpurea, S. humilis, and S. cordata. Figs. 11 and 12 
are of S. purpurea at different stages of development, the former 
from a young catkin, the latter from a mature one. This tissue is 
sometimes more abundant and typical in the pith of the staminate 
catkin, but a striking similarity exists in the form and distribution 
of the tissue in the two catkins of the same species. An examination 
of both was made in the case of Salix alba, S. lucida, and S. longi- 
folia. In S. rostrata, S. cordata, and S. nigra, the aeriferous tissue 
of the pith is more clearly defined than in the cortex, but is present 
in both. The reverse is true in S. lucida, while in S. discolor there 
is no greater development in the one region than the other. The 
pith in the catkin of S. petiolaris is peculiar because it consists of a 
large-meshed aeriferous tissue, and often there are but two or three 
large spaces bounded by slender lines of cells. 
Influence of environment on occurrence of aeriferous tissue 
For the purpose of ascertaining to what extent the production 
of aeriferous tissue in the Salicales is dependent on external factors, 
several experiments were conducted in the growing of plants under 
conditions in which certain components of the environment could 
be controlled. Cuttings from various species were grown in culture 
solutions, in water, some totally immersed, and in soil. The effect 
produced by the variation of the illumination and temperature was 
also noted in a number of species. The following list comprises the 
more important experiments. 
LIGHT RELATIONS.—Cuttings from Salix nigra Marsh., Populus 
nigra L., Populus balsamifera L., and others were sprouted in pots 
of earth. In the conservatory some of these were placed in a situa- 
tion to receive the maximum light, others in a more sheltered posi- 
tion, and finally several in a light-proof chamber. The only effect 
noted was in the case of the sprouts grown in the dark. In those 
experiments the stems produced were flattened, and the tissues, 
