1916] COSENS & SINCLAIR—AERIFEROUS TISSUE 221 
factors on the protoplasm of the host; or it may be considered a 
hereditary tissue that has been reinstated owing to the awakening 
of latent characteristics in the protoplasm of the infected plant, on 
account of unusual conditions associated with the presence of the 
gall producer. 
Concerning the alteration by experiment of the environment of 
the host plant, all attempts were unsuccessful either to increase the 
amount of aeriferous tissue normally present or to cause its develop- 
ment in regions where it is not found under normal conditions. 
Although the inhibition of transpiration is commonly regarded as 
an important factor in the production of this tissue, it was not 
increased to the smallest extent in the Salix sprouts developed under 
conditions of reduced transpiration. Indeed, if these experiments 
had given positive results, it would still be necessary to show that 
the conditions presented by galls are those of restricted transpira- 
tion. This is clearly not the case in Rhabdophaga triticoides Walsh, 
the gall in which the development of the tissue is the most pro- 
nounced and its characteristics the most typical. As stated pre- 
viously, the lateral buds that contain the larval producers of this 
gall are situated on a length of stem, the internodes of which are 
shortened to a very marked degree. The occupation of the buds 
by the larvae, however, has not checked the development of the 
leaves in the axils of which the buds are produced. The galls shown 
in text figs. 1 and 2 were collected after the leaves had fallen. This 
results in the leaf area of the abnormal part of the stem being 
increased far beyond the normal leafage, a condition that we can 
scarcely associate with reduced transpiration. Thus the direct 
evidence from experiments leads us to believe that environmental 
factors of light, temperature, moisture, etc., are not directly opera- 
tive in the production of aeriferous tissue. 
The anatomical structure of the normal stems of the Salicales 
strongly favors the same view by furnishing facts apparently 
explainable only on the supposition that we are here dealing 
with a true heritable tissue. In accordance with the general 
view concerning the retention of ancestral characteristics, certain 
regions of plants are regarded as more tenacious of these 
structures than others. For example, it is commonly recognized 
