494 
J. G. GROSSENBACHER 
stems or branches involved. It is also shown that if the area of the 
bark most severely affected is large compared with the circumference 
of the stem involved, the result is more serious than when the injured 
patch is comparatively small. This holds for both large and small, 
as well as for young and old stems. If the section from which Fig. 26 
is made were photographed whole and magnified, a picture much like 
Fig. 50 would result, although the wood cylinder within the injured 
zone was not decayed or even entirely dead when cut on May 29. 
The new growth of wood shown in Fig. 26 as well as that in Fig. 25 
consisted of only a very thin layer, while the wood shown in Figs. 49 
and 50 represented several years' growth. Fundamentally, however, 
these sections are not only comparable but very similar. 
The Cause of the Initial Injuries 
The work so far has clearly shown that the initial injuries in the 
bark of trees that result in crown-rot arise during the dormant season, 
but their cause has not been definitely established. The years of 
observation and a few experiments together with the histological 
study here reported, indicate the most probable factors, and thus pave 
the way for an experimental study of the problem. In general terms 
it may be said that these initial injuries are due to a lack of adjustment 
between radial or bark-growth and the environment. 
Some Facts about Bark-Growth as Related to the Development of 
These Injuries. — In the study of forest trees it has been shown that 
growth and development proceed in a wave-like manner. The various 
functions, the size of cells, and the amount of annual growth increase 
to maxima in certain stages of a tree's life, and decrease to minima 
again at other stages. These periods or cycles are repeated at inter- 
vals more or less cliaracteristic of a species. Kapteyn^ calls attention 
to growth cycles that may be traced in the wood and extend over 
periods of 12.4 years, apparently independent of the species. Bailey 
and Shepard^ found that the length of coniferous tracheids varies 
in more or less definite cycles usually ranging from 35 to 80 years, and 
apparently differing in different species. 
It is a well-known fact, for instance, that at a certain age of a 
Kapteyn, J. C, Tree-Growth and Meteorological Factors, Rec. Trav. Bot. 
Neerland. 11: 70. 1914. 
^ Bailey, I. W., and Shepard, H. B., Sanio's Laws for the Variation in Size of 
Coniferous Tracheids, Bot. Gaz. 60: 66. 1915. 
