DEVELOPMENT OF PHYLLOXERA VASTATRIX LEAF GALL 347 
normal leaf tissue he finds that galls contain a greater percentage of 
water, a smaller percentage of ash, a total absence of sucrose, a greater 
percentage of reducing sugars, four' times as much tannin, a smaller 
total nitrogen content but a greater percentage of soluble nitrogen. 
His results are quite similar to those obtained by Pantanelli with the 
exception of the total nitrogen content. Molliard thus points out 
that his- results as well as those of Pantanelli, and Paris and Trotter, 
indicate an increase in simple substances in galls. Furthermore 
Molliard finds an enhancement of respiration, an increase in the 
oxidases laccase and tyrosinase, and in free acid. He concludes that 
the insects inject into the plant tissue certain enzymes, which may 
explain the presence of a large amount of simple substances. 
VH. Discussion of the Stimuli Producing Galls 
In stimulated structures generally, we may find an increase in 
enzymes, as Czapek (ii) demonstrated for geotropically stimulated 
roots, where he found an increase in oxidases, especially tyrosinase. 
Von Schrenk (30) likewise found an increase in oxidizing enzymes in 
intumescences produced by sprays of various copper salts on cauli- 
flower leaves. It seems, therefore, that the presence of a large amount 
of these enzymes in galls, especially in the Phylloxera gall, does not 
necessarily mean that they have been injected into the attacked 
tissue, but on the contrary, from our knowledge of enzymes in other 
stimulated structures, there seems to be good reason to assume that 
such is not the case. 
Before going further into a discussion of stimuli producing galls, 
it will be best to discuss the early development of the Phylloxera gall. 
We noted in figure 9, which represents gall formation at the end of the 
first twenty-four hours of insect attack, that the portion of the 
leaf beneath the insect, in the vicinity of the proboscis, is constricted 
and measures 72 /x as compared to the normal width of the leaf, which 
measures 96 ju. Furthermore, it was shown that this difference in size 
between the constricted and the unconstricted, normal portion, was 
due to the decrease in size of the mesophyll of the attacked part. Be- 
sides this, we noted the beginning of abnormal hair production at the 
borders of the depression. 
How has the insect brought about the decrease in size of the at- 
tacked mesophyll and the consequent depression? Is it due to 
