482 
J. G. GROSSENBACHER 
cambium may be involved or the phloem and cambium injuries may 
be accompanied by injuries in the cortex. The severity and course 
of the disease following such initial injuries depend upon the size and 
number of the affected patches, and upon the location of the most 
severely affected portions of bark. 
The Initial Injuries. — Some of the common types of initial injuries 
that subsequently give rise to bark diseases are shown on Plate XXI, 
The group of figures shown on this plate does not, however, include 
one of the kinds most frequently noticed: these somewhat concen- 
trically arranged injuries often occur with conspicuous radial clefts, 
as discussed in my former papers, and as indicated in Figs. 43 and 48, 
Plate VI. In many instances, however, the concentric injuries are 
not accompanied by radial ruptures, and sometimes radial clefts 
occur when other types of injuries are so slight as not to hinder 
subsequent normal bark growth. The sections shown on Plate XXI 
are all made from apple and pear material collected before growth 
started in spring (April 17, 1912, at Madison, Wisconsin). 
Fig. I shows a condition that is frequently found in injured bark. 
Dead tissues are usually evidenced in these photographs by the occur- 
rence of especially dark streaks or patches, by collapsed cells or by 
both. Sometimes ruptures are much more prominent than discolora- 
tions, as shown in Fig. 3, which shows a very common type of rupture 
or separation in the inner phloem. In Fig. i may be seen a conspicuous 
combination of the collapse of discolored tissues with ruptures in the 
inner phloem and cambium. At the left of the section shown in this 
figure the initial injury is confined chiefly to the cambium; on the 
right the principal injury occurs in the phloem, only scattered cells 
in the cambium being affected. A few groups of injured cells may 
also be seen about the sclerenchyma strands as well as farther out 
in the cortex. Fig. 2 shows a case from pear tissues in which the 
initial injury is most pronounced in the inner cortex and outer phloem, 
with only small groups of affected cells in the outer cortex and inner 
phloem. Fig. 3 is from apple. It shows a marked injury of medullary 
rays in the phloem, and a rupture of the phloem. 
The other figures on Plate XXI show, on a larger scale, small 
areas in typically injured bark. In Fig. 4 occurs a mixture of streaks 
and patches of dead and living tissues present in the outer wood and 
the inner bark of apple. In the center of this figure a large ray and 
much of the surrounding tissue is dead and collapsed (appearing 
