194 BOTANICAL GAZETTE [OCTOBER 
minerals is shut off because the xylem elements are so twisted and 
distorted that the vessels as vertical tubes and efficient water car- 
riers have largely disappeared. The water and mineral nutrients 
are able to pass slowly one or two galls by diffusion, even though 
the vessels are greatly distorted, but when several galls are present 
in the path the movement is so interfered with that growth is re- 
tarded and sooner or later the tip part of the twig dies above the 
knot. 
Piru.—No striking effects on the central pith as a result of the 
gall formation are noticeable, but in some larger galls the pith area 
is somewhat compressed, and the individual pith cells in these cases 
have lost their characteristic cylindrical form and have taken on an 
angular appearance. 
CampiumM.—The cambium appears to be very active in the 
younger galls, as evidenced by the comparatively larger cells and 
nuclei. As the gall grows older this activity gradually grows less, 
until it is brought to an end by the death of the twig. In cases 
where death of the twig distad to the gall does not occur, the activity 
of the cambium eventually becomes almost negligible. The cam- 
bium ring as a whole becomes greatly distorted and interrupted in 
many places. The wood fibers that are bent toward the periphery 
are responsible for this interruption of the cambium. The cambium 
never entirely loses its identity (fig. 9). Many nascent cells are 
isolated from the cambium that give rise by continual division to 
isolated groups of phloem in the xylem region. These groups are 
more often formed between the spring wood and the summer w' 
of the preceding years’ growth. They are usually crescent-shaped, 
and suggest the pith flecks often found in other woods. ‘There are 
no isolated xylem groups formed in the phloem, but a great many 
wedgelike xylem elements extend through the cambium into this 
region (fig. 9). 
Summary 
1. The normal wood of Populus trichocarpa conforms closely to @ 
typical dicotyledonous, diffuse, porous wood. 
2. In western Montana and eastern Idaho a gall disease threat- 
ens to interfere with the commercial uses of this tree. 
