1922] HYDE—GALL ON POPULUS 195 
3. Only uniseriate rays are found in the normal wood of this tree, 
while in the diseased wood the rays are considerably broadened, 
often being several cells wide tangentially. 
4. The average increase in the diameter of the stem, due to 
gall formation of several galls’measured is 110 per cent, of xylem 
55 per cent, and of phloem 600 per cent. 
5. The xylem elements are greatly distorted, the vessels are 
flattened radially, and the wood fibers are often bent at right angles, 
due to crowding as a result of the great increase in number and size 
of the cells of the medullary ray. 
6. In the bark the greatest effect noticeable is the increase in 
size and number of the parenchyma cells. 
7. The phellogen is stimulated to unusual activity, and con- — 
sequently the suberized tissue is considerably increased. 
8. The distortion of the vascular elements, because of the inter- 
ference with the transpiration stream, often results in the twigs 
dying above the galls. 
g. The central pith is not greatly altered in the diseased stem. 
10. The cambium is sometimes altered by distortion, but never 
completely loses its identity. 
11. In addition to the evidence of constant association of 
Macrophoma tumefaciens with the lesions, the histological examina- 
tion supports the idea that this fungus is the cause of the disease. 
12. Infection experiments on pathogenicity are as yet lacking. 
Acknowledgment is made to Professor W. W. RowLEE, under 
whose direction this study was made, and to Professor D. Reppick 
for many suggestions in the preparation of the manuscript. 
CoRNELL UNIVERSITY 
EXPLANATION OF PLATE VI 
All photomicrographs show magnification of 85 diameters. 
Fic. 1,—Transverse section of normal wood, showing one annual ring 
complete. 
Fic. 2.—Transverse section of diseased wood, showing distorted multi- 
seriate medullary ray and radially flattened vessels. 
Fic. 3.—Tangential section of normal wood, showing uniseriate medullary 
rays and normal fibers. 
