354 
HARRY R. ROSEN 
the form of hooked swellings and then they die. If a poison is in- 
jected by the insect which produces first a swelling, why, he 
asks, does the same substance later on stop growth? Second: 
when roots of the vine attain a diameter of more than 3 or 
4 mm., no swellings are produced, although a considerable num- 
ber of Phylloxera are often seen on such roots. They are 
grouped or aligned in the bark cracks, alongside of each other, 
and if they give off an acrid, irritating fluid, they ought, united 
in a mass, to produce considerable disturbance and a prolifera- 
tion of the elements of the cortical tissue. Their effect, however, is 
very feeble. Cornu says if the argument against this is offered, that 
the bark cells cannot respond because they are older cells, it may be 
pointed out that each year the old bark is exfoliated by means of a new 
suberized layer, coming from the embryonic tissue, the cork cambium. 
The effect produced is a hypertrophy and a coloring of certain gum 
reservoirs. Third: the galls of the stem and tendril are produced by a 
portion of the cortical tissue around the Phylloxera and not immediate- 
ly below it. He says, it is not a local excess of acrid liquid concen- 
trated at one point, which stops the formation of new tissues; 
for, in the cells more or less distant from this point, where the 
effect of this excess should be less effective, the hypertrophy 
should still manifest itself. This it does not do. Directly below the 
insect beneath a few layers of cells, is the generative zone, as Cornu 
points out, referring to the cambium layer of the stem or tendril. 
This zone does not produce any new growths. The swelling of the 
root is not produced under the insect, at the point punctured by it and 
where it produces a depression, but in the region farthest away, which 
makes up the hook form of the swellings. Fourth: if there has been 
a chemical irritant introduced, it should manifest itself by a swelling 
immediately in contact with the insect, for even after an attack of 
only several hours, there is found, around the point which has been 
occupied, an obvious depression. Cornu came to the conclusion that 
the puncturing by the proboscis together with the absorption of cell 
contents is sufficient to explain cellular segmentation and production 
of new tissue. To prove this contention and to show that irritating 
liquid had nothing to do with gall formation, Cornu injected into 
roots, stems, and leaves, 25 per cent, solution of acetic acid and 10 per 
cent, solution of sulphuric acid. Although in one or two cases he 
obtained swellings, he got no depressions, so that he is satisfied that 
