1922] HYDE—GALL ON POPULUS 193 
Here and there in the xylem of the diseased area uniseriate rays 
occur, but these are far outnumbered by the multiseriate rays. The 
ray tissue in the diseased xylem makes up approximately 30 per cent 
of the wood, while it is evident, by comparing the figures, that in 
the normal wood the ray tissue makes up a much smaller pro- 
portion. 
Bark.—The distortion of the elements of the bark of the gall are 
not so pronounced asin the xylem. In the bark the principal effect 
is found to be a decided increase in size and number of the cells of 
the several tissues. As a result of the normal reaction of the host, 
in an attempt to overcome the injurious effects of the pathogene, the 
amount of suberized tissue is increased manyfold, so that now there 
are as many as fifty rows of cork cells. The parenchyma cells of 
the primary cortex are considerably larger and more than doubled 
in number. The average of numerous counts made of the number 
_of rows of parenchyma cells between the cambium and the periderm 
in normal and diseased bark shows for the former 28, and for the 
latter 65. The phloem rays are multiseriate, often comprising six or 
seven rows of cells. These rays are sometimes bent tangentially, as 
seen in cross-section. 
The phloem tissue is greatly increased in the diseased twig. 
This increase is largely due to the multiplication of the phloem 
parenchyma cells and the subsequent growth of the cells to a size 
slightly beyond the normal. Physiologically the phloem does not 
appear to be greatly interfered with during the younger stages of the 
gall. As the gall becomes larger, considerable pressure is exerted 
upon the sieve tubes, as is indicated by the fact that they are flat- 
tened radially. 
The functional disturbance seems to be more closely connected 
with the xylem than with the phloem, as it is here that the distortion 
of elements is the greatest. This disturbance is manifested in the 
gradual dying of the twig above the hypertrophy (fig. 7), which, 
however, does not usually occur unless there are several knots 
upon the twig. In cases where only one knot is found on the twig 
there usually i is no noticeable disturbance distad to the knot. 
The writer believes the death of the twig to be due to the fact 
that sufficient water and minerals cannot get through the vessels of 
the xylem to the leaves beyond the galls. The supply of water and 
