40 
ALBAN STEWART 
Galls of Papaipema nitela on Ambrosia trifida 
The stems of Ambrosia trifida are often parasitized by the common 
stem borer Papaipema nitela Gn., causing the formation of galls. The 
galls consist of slight swellings which are often fusiform in shape. The 
general features of such a gall are shown in the diagrammatic drawing, 
text-figure i A, which represents a gall formed by both Protomyces and 
apaipema. This being the case, no other drawing was necessary except 
for special features which are shown in figures 2 and ii. 
The structural abnormalities caused by this insect are less striking than 
those caused by Protomyces. There is a greater radial growth of the xylem 
portions of the bundles than in the normal stem, leading to the production 
of a more or less compact woody mass of tissue as shown in the lower part 
of text-figure i A. In this radial growth the bundles lose their individuality 
as such while they remain separate a short distance above and below the 
gall. Text figure i B is an outline drawing of one of the stem bundles taken 
just below a gall. The rays are broader, as shown in figure 2, and in some 
instances there is a great reduction in the number of vessels. There is also 
a considerable production of parenchyma where normally there are only 
fibers. 
An interesting feature occurs in many of the galls which is shown in 
figure II. In many instances it was found that the ray cells had prolifer- 
ated, forming callus-like masses which extend into the larval chamber. 
These masses consist of thin-walled parenchyma cells which are often very 
much elongated. Among the large cells there are still other cells which are 
very much smaller, and some of these are cambiform and appear to have 
been recently capable of division. A band of these cambium-like cells 
often extends across the base of the parenchyma mass and probably is 
capable of renewing the mass. The cells both large and small are poor in 
protoplasmic and other contents. 
Cosens (i, figs. 17 and 21) shows somewhat similar tissue-masses ex- 
tending into the larval chamber of two other lepidopterous galls, Stagmato- 
phora ceanothiella Cosens on stems of Ceanothus americanus, and Eucosma 
scudderiana Clements on stems of Solidago canadensis. In the case of the 
Eucosma gall, at least, he regards these growths as nourishing tissue for 
the larva, for he states (p. 312): "The gall mass in this case is produced 
from the vascular bundles and the intervening parenchymatous strands. 
When the larva first enters the stem it first eats out the pith. After the 
exhaustion of this source of nourishment, its food is supplied by the radial 
thickening of the bundles into the gall cavity." 
In the gall considered in the present article the cells are very poor in 
contents which make up the masses, a condition hardly to be expected if 
they were designed especially as food for the larva. Furthermore, the 
larva is able to, and does, eat out the pith for a considerable distance be- 
