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HARRY R. ROSEN 
half of the leaf, since such growths have been described for other galls, 
but the lack of proliferation of both the upper and lower halves of the 
leaf in the portion immediately around the proboscis, which makes 
this part appear as a narrow neck between two masses of hyperplastic 
growths. This is a development which has not yet been described for 
other galls. 
Other features which are worthy of note, at this stage of gall 
development are represented in figure 7. This figure, which is a 
highly magnified view compared with the other plate figures, shows a 
cross section of gall tissue taken from the portion below the insect. 
Starting from the top, which lined the base of the insect cavity, we 
see three layers of elongated cells, representing the upper epidermis, 
the palisade layer, and a layer of irregular mesophyll. As compared 
with corresponding normal leaf tissue, these cells show marked 
hypertrophic growth. The walls of these elongated cells, especially 
the walls running parallel to the leaf surface, are very much thickened 
in some places and entirely lacking in other places of each individual 
cell. In some cells, all that is left of the walls are narrow threads, 
which often show a reticulate arrangement and stain red with safranin. 
Such cell-wall features are as rare in galls as in any other pathological 
structures. Weidel (31) reports a dissolution of walls in the larval 
chambers of Cynipid galls and P. Magnus (19) finds a sieve-like dis- 
solution of the cell walls produced by a Urophlyctis, which he figures. 
This latter case is more comparable to figure 7 since Weidel reports 
dissolution of whole walls in his insect galls. It seems that the cells 
shown in this figure might possibly assume the characters of tracheal 
elements, similar to that found by Kiister (12) in leaf callus of Cattleya. 
In the outer cells of this callus, he found reticulate thickenings, which 
in the lower part of the cell are only narrow meshes between single 
thickened bands, while in the upper part the bands are usually flatter 
and sometimes partially interrupted. As translated by Dorrance 
(12), he says: ''This case is of special interest since, aside from tyloses, 
it is the only one known to me in which hypertrophic growth, incited 
by a wound stimulus, is combined with the formation of a special kind 
of wall thickening." In the Phylloxera gall we likewise have a thick- 
ening and a partial dissolution of hypertrophied cells, giving the ap- 
pearance of tracheal formations as seen in wound callus of leaves, de- 
scribed by Kiister. This reticulate structure is only seen in younger 
galls, due to the fact that as the gall reaches maturity the cells at the 
