216 BOTANICAL GAZETTE [SEPTEMBER 
The well known spherical gall produced by the sawfly Poniania 
pomum Walsh on the leaves of Salix cordata Muhl. (text fig. 5) fur- 
nishes another example of aeriferous tissue in association with a 
gall-producing stimulus. In its earliest stages the gall consists of 
a singularly compact tissue (CosENS 2), but gradually the aeriferous 
characters become evident, until almost the entire mass of the adult 
gall consists of tissue which is thus differentiated. Fig. 5 is a typical 
section from such a mature gall. 
Distribution of aeriferous tissue in normal plants 
For the purpose of determining the distribution of this tissue in 
normal plants, the following species of Salicaceae were examined: 
Salix fragilis L., S. cordata Muhl., S. discolor Muhl., S. humilis 
Marsh., S. longifolia Muhl., S. lucida Muhl., S. nigra Marsh., 
S. petiolaris Sm., S. purpurea L., S. rostrata Richards, S. seris- 
sima Fernald, Populus balsamifera L., P. deltoides Marsh., P. grandi- 
dentata Michx., P. nigra L., P. tremuloides Michx. 
CorTEx.—At first sight the cortex, both primary and secondary, 
of all these species appears to consist of a compact tissue, but on 
further examination a well defined aeriferous tissue was found in 
the primary cortex of every one. It can be seen in sections taken 
from the apex of the stem and for some distance back (fig. 10), 
but in older regions it is destroyed by compression. The amount 
formed varies somewhat in different species. Thus in Salix nigra, 
S. cordata, and S. fragilis the entire tissue forming the primary 
cortex is aeriferous; the meshes are relatively large, and the walls 
inclosing them are as a rule but a single cell in thickness. In 
S. rostrata and S. petiolaris it is especially well differentiated at 
the angles of the stem; at some levels it is confined to these regions. 
Among the poplars the largest amount of this tissue is possibly 
to be found in P. nigra and P. tremuloides. In the former it is very 
pronounced, for the cortex is an entire lattice work of it, and the 
air spaces are large and bounded by strands that vary from one to 
three cells in width. The meshes in P. tremuloides are likewise 
regular and clearly defined, but they attain their greatest dimen- 
sions at the angles of the stem. P. balsamifera is peculiar in that 
they are most abundant and characteristic immediately around the 
