CROWN-ROT OF FRUIT TREES I HISTOLOGICAL STUDIES 479 
rence of droughts or low temperatures are closely related to the 
development of crown-rot there; and similar investigations in Florida 
are revealing the prevalence of comparable adverse environmental 
conditions, preceding the occurrence of foot-rot on citrus trees. In 
the latter case it was also noticed that the leaf-surface area of a tree 
exposed to sufficient light for active photosynthesis, when compared 
to the surface area of bark on the branchless portion of the trunk, is 
often rather small in trees which appear especially subject to this 
disease; that is, it is suggested that a scarcity of elaborated food 
coursing downward in the bark of the trunk below the branches may 
also have a causal relation to the occurrence of crown-rot. 
Methods of Work. — It was found possible to cut usable sections 
from material of small diameter embedded in celloidin without the 
previous use of softening agents. The series used in this study was 
obtained chiefly in the manner described and from material collected 
and fixed in spring and summer of 1912. The citrus material was 
collected later in Florida and similarly treated. 
Flemming's strong solution and Carnoy's mixture were used almost 
exclusively for fixation. The former gave more satisfactory material 
for staining, but the latter was more largely used because of its more 
rapid penetration. Flemming's triple stain and Heidenhain's iron- 
alum haematoxylin stain were most frequently used. The triple 
stain was found to yield much quicker and more satisfactory results 
when used after a mordant such as iron-alum, but for use in making 
photographs iron-alum haematoxylin proved more desirable than 
Flemming's. The same was later found to be the case in sections 
from foot-rot material of citrus trees. 
The Early Stages of the Disease. — The first visible stages of crown- 
rot consist of discolored and often ruptured tissues variously dis- 
tributed in streaks and patches in the bark. In cross-section the 
injured patches are often arranged more or less concentrically about 
the wood cylinder, although they are usually most severe on one side 
of the stem. In the mildest forms of the disease the medullary rays 
of the inner phloem and groups of parenchyma cells about the 
sclerenchyma strands and inner cortex are affected, although at times 
only the one or the other of these tissues is involved. In more severe 
cases much of the phloem and practically all the cambium may 
be injured. 
The severity and course of the disease following these evident 
