1916] COSENS & SINCLAIR—AERIFEROUS TISSUE 217 
leaf traces, especially in close approximation to the bundles. 
Indeed, at the nodal levels the entire cortex consists of aeriferous 
tissue. It is less apparent in P. grandidentata and P. deltoides, for, 
though present, it is not as abundant nor as regularly defined as in 
the other species. Incidentally it may be stated that wherever 
this tissue occurs in the cortex of the normal plant it is much more 
abundant and better developed in corresponding parts of stem galls. 
This also applies to Salix. 
Pitu.—The pith is distinctive and important. It has been 
noted that the pith in the galls cited is occupied by a very pro- 
nounced aeriferous tissue, but this is not the case with the normal 
pith of any species of willow except in the reproductive axis. In 
every species examined it is a compact tissue without any indica- 
tion of other than small, ordinary, inconspicuous intercellular air 
spaces. The pith illustrated in figs. 8 or 10, for example (a normal 
pith), stands out in striking contrast to the pith of the gall of fig. 2. 
It is especially noteworthy that in the latter case the medullary and 
cortical tissues are identical, both in their conformation and in the 
size and shape of their cellular constituents, while in the normal, 
the cells of the pith are much larger and closely packed. The air 
spaces of the aeriferous tissue do not result as an indefinite elonga- 
tion of the cells, as in certain oak-leaf galls, but are the result of 
cell divisions which follow a definite law with regard to their 
polarity. The compact normal pith and the open abnormal pith 
appear to be fundamentally different types on this basis and on 
the basis of the peculiarities in the distribution of the latter to be 
noted. 
Fig. 7 is a photograph of a normal stem of Populus balsamifera 
taken from a new shoot, and it illustrates the difference manifest 
by Populus in comparison with Salix. In all poplars investigated 
the plan of organization of the pith is that of an aeriferous tissue, 
though lacking the regularity found in the cortex. Were it estab- 
lished, as some believe, that Populus is the more primitive genus, 
this phenomenon would be important in explaining the aeriferous 
tissue in the pith of the galls of Salix. 
NopE.—We have been taught to regard the node as a conserva- 
tive part of the stem, one in which discarded tissues may still persist, 
