1897] NORMAL AND DISEASED ORGANS OF ABIES BALSAMEA 337 
does not vary greatly from that of the normal. The width of 
the wood cylinder in one year shoots above the tumor is about 
equal to that of the normal. The second, third, and fourth rings, 
however, are wider than the normal. The tracheids of the dis- 
eased wood are thicker walled. The annual rings of the affected 
branches are always more irregular in their width, and are 
often ten times as wide on one side as on the other. This may 
occur in one ring only, the following or preceding rings being 
equally thickened throughout. This unequal radial width of 
some rings is due to disturbances in the direction of the growth 
of the shoot that year. On account of the increased growth of 
the wood and bark, as well as the increased number of branches, 
the weight of the witch broom increases with its age, and more 
rapidly than the normal branches. The growth of the healthy 
part of the witch broom branch below the tumor is also less than 
in the normal ones. This part of the branch is, therefore, less. 
able to bear the weight of the heavier affected part, and the 
witch broom becomes suspended, changing its position or falling 
on one side. The affected branches, which are all negatively 
§eotropic, assume during the next season their erect growth by 
curving upward. On the convex side thus formed there occurs 
an increased growth, not only in the width of the annual ring, 
but the tracheids are thicker walled, rounded, and are separated 
by intercellular spaces, characteristic of all regulatory tissue 
formed in the wood of coniferous trees as a result of resistance 
to mechanical forces. Although the annual rings in the 
affected branches are as wide and often wider than the normal, 
they still contain fewer tracheids per square unit of surface 
area, This is due to the fact that in the diseased branches more 
medullary rays are formed, often twice the number that are 
formed i in the normal wood. 
Resin canals, which are absent in the normal wood of A. éal- 
’ _ Samea, are always present in the wood of the tumor, and are 
ae a always present in the wood of older diseased branches 
* Haptic, R.: Den anatomischen Bau des Rothholzes. Forst.-naturw. Zeitsch- 
1163.1 1896. 
