No. 406.] GALL OF THE MONTEREY PINE. 807 
varies only in the character of the outer cell wall, which, in 
this case, is thickened and strengthened by transverse ridges. 
On the inner surface and near the base of the scales the epi- 
dermal cells are shorter and resemble those of the leaf at a 
corresponding distance from the axis. The epidermal cells of 
the scales have very little contents. 
In the galled leaves the scale epidermis lining the cavity in 
which the larvze lie consists of long, thin-walled cells, which in 
size, shape, and density of contents resemble those epidermal 
cells of the leaf that line the other part of the cavity. The 
scale epidermal cells are shorter and deeper above the larvae 
where the epidermis of the scale and of the leaf meet, and they 
gradually change into the normal form, somewhat above the 
neck of the larval cavity (Fig. 6). 
It is thus seen that, in consequence of the presence of the 
parasite, the epidermis of scale and leaf is modified in shape 
and size, in structure, in contents, and in function. 
A cross-section of the base of a galled leaf bundle, made 
from one of the same age as that represented in the longitudi- 
nal section in Fig. 6, shows that the tissue adjacent to the 
cavities containing larvae is mainly parenchymatous, Ze., the 
cells that compose it are relatively large and thin-walled, and 
are those cells which, in plants, are most sensitive to stimuli. 
The tissue of the scale base is composed of large, rounded 
cells, between which are many intercellular spaces. These cells 
have but little contents. Near the juncture of scale base and 
leaf the cells are some of them larger and some smaller, and 
there are fewer intercellular spaces. In the axil of scale and 
leaf the cells are normally much smaller than any met with in 
the scale. Finally, the tissue surrounding the rudimentary 
conducting tissue, which in galled leaves abuts on the inside 
of the cavity containing the larva, is composed of small cells 
of the kind just described as occurring in the scale base. 
The thin-walled mesophyll cells of the leaf normally are differ- 
entiated into quite another tissue; as the leaf becomes older 
these cells become oblong, thick-walled, supporting cells (scle- 
renchyma) of the mature pine leaf. The cells of the tissues 
described have but little contents. 
