1916] COSENS & SINCLAIR—AERIFEROUS TISSUE a5 
the most typically aeriferous type, consisting of polygonal spaces 
bounded by strands seldom more than one cell in width. The dis- 
tribution and character of this tissue can be seen in figs. 1 and 2. 
The gall Rhabdophaga strobiloides Walsh (text fig. 3) is invariably 
terminal, since the solitary larva occupies the tip of the growing 
point, and thus prevents any elongation of the stem past the infected 
area. The condensation of the 
stem axis that results is, in this 
case also, the chief gall-producing 
factor, the top-shaped mass of 
aborted leaves representing the 
normal leafage of nearly a meter 
of stem. The entire cortex of 
the receptacle of this gall is com- 
posed of aeriferous tissue. It 
commences to appear in the pith, 
immediately beneath, the com- 
pact tissues of the gall zones, and 
avranact es | gy a 
from the point of infection (fig. 4.) 
In contrast with the two pre- 
ceding species, a marked con- Fic. 5.—Galls on Salix cordata Muhl. 
densation of the stem of the host _ produced by Pontania pomum Wals 
does not enter into the produc- 
tion of the gall Phytophaga rigidae O.S. (text fig. 4). This is apparent 
from the normal spacing of the buds that persist on the exterior of the 
gall. The pith of the stem in this form is occupied by the larva of 
the producer, and only small areas of parenchyma remain outside of 
the protective sheath of the gall, but in these aeriferous tissue can 
be detected. The cortex, which is very thick, consists entirely of the 
same tissue (fig. 6). Aeriferous tissue is also developed far in excess 
of the normal in the cauline stelar gaps comprised within the gall. 
A section of such a stem gap is shown in fig. 3. 
The normal occurrence of aeriferous tissue in the Salicales will 
be discussed in detail later, but it may be stated here that, while it 
occurs in the stem pith of the three galls just described, it is not 
present in the corresponding regions of normal Salix. 
