350 
HARRY R. ROSEN 
stimuli caused by change in tension and pressure brought about by 
the partial collapse of the attacked mesophyll? The evidence pre- 
sented above by various investigators seems to substantiate such an 
explanation to account for the abnormal production of hairs. 
Kiister claims that in Erineum galls the stimulus causing the ab- 
normal growth of the epidermal cells ''comes from a poison which the 
gall insects produce, concerning which nothing more is known." 
This theory, as well as any other chemical theory, makes it difficult to 
explain the production of hairs in the Phylloxera gall. If we assume 
that a chemical substance is introduced into the leaf by the proboscis, 
then we must also assume that this substance, starting from the 
proboscis as a center, should diffuse or osmose equally in all directions; 
or, if on the other hand we assume that the substance given off by the 
insect is not readily diffusible in the tissues but that it initiates certain 
stimuli, which can be felt some distance from the initiating center, 
then we must also assume that changes or responses induced by these 
stimuli should appear equally distributed in all directions. But in 
the Phylloxera gall, as I have pointed out, the response is rather une- 
qual, none or few hairs are produced in the depression, but they are 
produced always with perfect regularity at the edge of the depression. 
Does not the character of the response seem to point to a mechanical 
stimulus rather than to a chemical one? The mechanical stimulus 
would appear to be in the nature of a change in tension or pressure, 
which stimulates growth just where the tension would be greatest, 
i. e., Sit the borders of the depression. Kiister carefully points out 
that such forces, besides others, are at play in callus formations. 
Cornu (9), who has made a very careful study of the root gall caused 
by this plant louse, and whose work has been either overlooked or 
disregarded by cecidiologists, likewise concludes that the sucking of 
the insect on the root, resulting in cessation of growth of the attacked 
part, produces tensions which dilate other elements not hindered in 
their growth. 
In the description of the histological development of the gall when 
it is three to four days old, we noted that the upper half of the leaf 
directly below the insect showed no proliferation, while the under half, 
on both sides of the narrow portion, proliferated very abundantly. 
The lack of growth of the upper half of the leaf, which is directly below 
the insect, is not a specific character of this gall only but, as Cosens 
(10) points out, this phenomenon is the usual occurrence in the simpler 
