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surface of the gall. But the striking feature in this huge mass of tissue 
is its compactness and the total absence of air spaces. As Cook 
points out, the palisade cells cannot be distinguished ; at the bottom of 
the cavity they are found in a more or less collapsed condition while at 
the sides they are usually not to be distinguished from those of the 
mesophyll. The vascular elements have also been changed from their 
normal arrangement. Although in total amount they are not notice- 
ably increased, they are frequently scattered and twisted into separate 
small groups. Figure 6 shows how the xylem and phloem from one 
vein are separated and scattered by wedges of parenchyma cells, 
which usually show thickened walls. Compare this scattered mass of 
vein tissue in the center of the gall with a normal vein, marked N, in 
the same figure. Sometimes the vascular elements in the gall take 
the form of cylindrical or circular masses of cells which reminds one of 
the ball-like groups of tracheids that Kiister (12) describes in callus 
tissue. What has been said for the lower epidermis can also be said 
for the upper which lines the cavity of the gall; besides that, at the 
base of the cavity the epidermal cells as well as several layers of 
mesophyll are totally or partially collapsed. 
VI. Chemical Constituents of the Phylloxera Gall 
Figure 8 is a photomicrograph of a section cut from the center of a 
mature gall. It represents a very low magnification as compared with 
the other plate figures. Some of the eggs and nymphs are still visible 
inside the cavity of the gall. This section also gives a good view of 
the so-called ''nutritive zone" of the gall, described by Cook (4). It 
is represented by the very dark mass of cells below the cavity of the 
gall. These cells are filled with tannin, crystals of various forms, 
starch grains and other substances; because of this, this portion of the 
gall stains heavily. Pantanelli (22) has made a chemical analysis of 
this Phylloxera leaf gall and finds that gall-bearing leaves contain 
more total organic nitrogen and more proteic nitrogen than normal 
leaves; the ash content is lower in lime, iron and magnesium in the 
gall-bearing leaves. He finds an abundance of starch, albumin, fat 
and phosphates in the nutritive zone. 
Molliard (20) made a chemical analysis of two leaf galls of the elm, 
both produced by plant lice, Sdiizoneura lanuginosa Hartig and Tet- 
raneura ulmi De Geer. Comparing the same weight of gall tissue with 
