DEVELOPMENT OF PHYLLOXERA VASTATRIX LEAF GALL 353 
These substances, Magnus believes, are given off by the insect, or they 
may be the results of a material exchange between the living cells of 
parasite and host. They may produce osmotic disturbances, which 
will affect the nutritive processes of the plant tissue involved, and so 
give rise to gall production. This is quite hypothetical and as Magnus 
himself says, no direct evidence has been brought forth in support of 
the theory. 
It is beyond the scope of this paper to go into all the theories put 
forth to account for gall production. Kiister, in his articles and books, 
and Magnus (18) give thorough, up-to-date accounts of these dis- 
cussions. Most of these are centered around a "chemical" theory, in 
which it is supposed that the producer injects some kind of chemical 
which serves as the stimulus for gall production. These theories are 
invoked to account for galls produced not only by one special class or 
order of insect gall producers, but for all gall producers, nematodes, 
mites, insects, and fungi. Kiister, who attempts to classify cecidia 
on a structural basis, is an exception. He (16) thinks that different 
kind of stimuli may produce ''organoid" galls from those which pro- 
duce "histoid" galls. He is a firm believer in the "chemical" theory 
to account, at least, for his "histoid" galls. He pushes this theory a 
step further and says that certain kinds of chemicals, "chemomorphs," 
produce certain kinds of galls. 
If any such chemical substances are injected into the vine leaf by 
the Phylloxera insect, it seems to me that their effect would be almost 
negligible as compared with the effect of a continuous sucking action 
for fifteen days at one fixed point, as far as initial stimuli produced by 
the insects are concerned. Here it should be pointed out that I have 
interested myself in the initial stimulus only. Undoubtedly the final 
stimuli for growth, in galls as well as for ordinary normal growth, are 
chemical stimuli; but, as Kiister says, between the initial cause and 
the final effect there probably intervenes a "chain of stimuli." Thus 
wounding may release certain chemical stimuli which will bring forth 
callus formation [see Haberlandt (14) ], but the initial stimulus is the 
wound. 
Cornu (8 and 9), whose thorough, painstaking study of the root 
gall of Phylloxera vastatrix strikes one as being authoritative, concludes 
that the insect does not inject any poisons or other chemicals which 
are the stimuli for gall production. He gives the following reasons 
for this conclusion. First: the attacked rootlets first are made to take 
