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takes place in the cortical cells pierced by the proboscis of the insect other 
than the reaction to the mouth parts in them by laying down an ''organic 
sheath" around these mouth parts. The living cells of the wood beneath 
the mouth parts proliferate strongly, building up a mass of hyperplastic 
parenchyma in which remnants of ducts are scattered. Multinucleate 
cells are reported present, but the author does not state in what cells this 
condition occurs. 
Petri (4), in a comprehensive study of the grape Phylloxera root gall, 
finds the following phenomena to be exhibited: A "warty deposit" of 
calcium pectate is formed around the setae of the insect after their insertion, 
by the cytoplasm of the cells in which the setae are present. On the exterior 
of this sheath Petri believed he demonstrated a layer of tannic substance 
which upon oxidation gave the characteristic brown color to the old sheaths. 
The first important changes in the surrounding tissues are the sudden cessa- 
tion of growth and the non-appearance of differentiation in all near-by cells. 
These cells show hypertrophy of their nuclei. A short distance from the 
mouth parts a "ringwall" of hyperplastic tissue springs up. 
Rosen (7) studied the grape Phylloxera leaf gall with the following results : 
The initial depression is produced through "a lessened growth of the attacked 
mesophyll." "After three to four days of insect attack, the lower half of 
the leaf tissue which surrounds the portion in which the proboscis is inserted 
has proliferated enormously. The whole thickness of the leaf in the region 
immediately around the proboscis shows no proliferation. That portion 
of the leaf which is beneath the insect does not proliferate but the upper 
half at the sides of the insect grows upwards and forms the walls of a large 
insect cavity. Upper epidermal cells and several layers of mesophyll cells 
in the portion of the gall below the insect, show peculiar thickening and 
dissolution of their walls." "The investigation establishes the fact that 
the proboscis may pass through the entire thickness of the leaf." "The 
continuous sucking action by the insect at one fixed point for fifteen days is 
believed to be the initial stimulus for gall development." 
Methods 
The material of Pachypsylla mamma and observational data concerning 
it were obtained in and near Manhattan, Kansas. That of P. asteriscus 
was acquired in northeastern Texas. 
At the critical time in the spring, hatching of the nymphs and gall initia- 
tion were observed in the field and laboratory, making possible the fixation 
of gall material in its earliest stages. All material was fixed in weak chrom- 
acetic killing fluid (Schaffner's formula), embedded in paraffin, cut 10 
microns thick and stained with Flemming's triple stain. All histological 
drawings were made with the aid of a camera lucida. 
