CROWN-ROT OF FRUIT TREES: HISTOLOGICAL STUDIES 507 
little effective regeneration has resulted (Figs. 23, 30, 32 and 35). 
In most of the last type of cases the bark died to the margin of the 
severe injury before mid-summer, so that they appeared like Figs. 
24 and 26. In instances hke those of Figs. 21, 30 and 31 the results 
were sometimes most striking, because often a fair beginning had been 
made on the new growth of wood when suddenly the bark died over 
large areas. In case such an injury occurred on a small branch or 
shoot, it usually died outright as shown in Fig. 39a. 
The transition from stages like those shown in Figs. 23, 32 and 
35 to those of Figs. 24, 25, 26, 41, 46, 47 and 51 seems fairly clear. 
The associated micro-organisms evidently play an important role in 
the decay or disintegration marking the later stages of these bark 
diseases, and in some types doubtless extend the injured areas by 
their vegetative activities in the places initially killed (Figs. 27, 30, 
33 and 35). Yet, this does not seem to be generally the case. The 
tree shown in Fig. 50 had lived at least fourteen years after the occur- 
rence of the injury, eventually resulting in "heart rot." It is evident 
from this figure that the fungus rotting the wood present at the time 
the initial injury occurred has not progressed far outside the last 
layers of injured wood. In fact, it appears as though it may have 
rotted only as much of the wood formed since the occurrence of the 
injury as had been discolored by the diffusion of disintegration 
products from the initially killed cells. This diffusion injury is shown 
in Figs. 39 and 42. Yet, in looking over the figures of Plate XXIV 
it becomes obvious that once a wound parasite, or even a saprophyte 
especially adapted to a particular host, gains entrance to such an 
admixture of dead and regenerated living tissues, some living portions 
may be killed as a result of the vegetative activities of the fungus. 
This record of low-temperature injuries occurring in the bark of 
fruit trees, and of their subsequent development into bark diseases, 
is of interest and value independent of the factors that give rise to the 
initial injuries. The diseases in question are thus traced so much 
nearer to their first causes. Both the macroscopic and the micro- 
scopic appearances of much of the bark affected indicate that excessive 
tensions are developed during the occurrence of the injuries. It 
remains to be determined whether or not the tension-injury hypothesis 
of Sorauer is applicable to low-temperature injury in general, in con- 
nection with the physiological disturbances induced by the occurrence 
of severe weather while some of the bark tissues are in certain stages 
