808 THE AMERICAN NATURALIST. [Vor. XXXIV. 
The parenchymatous tissue of the scale and leaf which is in 
contact with the epidermis of the cavity containing larvae, for 
two or three cells from the cavity, follows the shape, size, and 
appearance of the epidermal cells (Fig. 6). Beyond these layers 
of parenchyma the cells become larger than in normal corre- 
sponding tissue, and gradually merge into the tissues of the 
scale on the one hand, and of the leaf on the other. These 
cells for six or eight layers around the cavity are filled with 
granular matter that increases in density toward the cavity. 
The parenchymatous tissues which surround the parasite are 
affected in a negative manner as well. They are prevented 
from being differentiated and from developing as do their fel- 
lows under normal conditions. The scales in the normal leaf 
bundle turn brown early and die. They form the cup which 
surrounds the bases of the three leaves, or “needles,” and 
which persists during the period of the normal leaf's activity. 
In the scales of the galled bundle the parenchymatous tissues 
remain active until early autumn, or during the activity of the 
parasite. Those cells of the leaf inside the region of the cavi- 
ties containing larva, as has been stated, become in normal 
leaves thick-walled supporting tissue. In galled leaves they 
become, when fully developed, as large as the underlying 
parenchyma, and their walls in thickness reach a condition 
intermediate between the thick-walled supporting tissue, Or 
sclerenchyma, and the parenchyma. 
It is to the change in form and size of the cells which sur- 
round the cavity containing larvae that the hypertrophy is due. 
In July the condition of the galled leaf bundles of the Mon- 
terey pine differs only in degree from that just described as 
their condition in April. The parenchymatous cells are some- 
what larger, the epidermis of the cavity containing larvaé, and 
all of the cells in the adjacent tissues of the scale and leaf, are 
active in providing food for the growing larvae. 
III. 
We come now to the question, What is the immediate cause 
of the hypertrophy? And to this only an imperfect, and perhaps 
