CROWN-ROT OF FRUIT TREES: HISTOLOGICAL STUDIES 48 1 
hampered by defective water conduction outward from the wood; 
and unless the weather conditions are most favorable, so that the cells 
proliferating from the bark are able to re-establish living connections 
by fusing with the wood cylinder, these proliferating cells die, and 
the result is the death of the entire bark. Although such patches of 
dead bark are produced in various ways and result from injuries of 
varying degree, we term the wounds crown-rot if they occur on the 
bases of trunks or on roots near the ground, and canker if they occur 
on stems and branches above ground. 
This histological investigation permits some inferences to be drawn 
that support most of the important contentions advanced in my 
former papers; as regards other contentions, however, the evidence 
is not so convincing. My preparations, as will appear later, indicate 
that both excessive tensions and certain degrees of immaturity of bark 
tissues have a causal relation to the development of the initial injuries 
that give rise to this bark disease. They also substantiate the results 
obtained by the cultural tests; no fungi are usually in evidence until 
the middle of May or even later. 
The Development of the Disease 
The first visible stage of crown-rot, as well as that of some other 
bark diseases, consists in a discoloration and collapse, or even in a 
rupture, of groups of tissues mainly of the inner bark. This stage is 
usually found only in late winter and spring, and is generally not 
evident to the ordinary observer unless the outer bark is conspicuously 
cleft. From late spring to mid-summer, however, most of the severe 
cases attract attention by the oozing of "sap" or gum and by the 
eventual discoloration of the outer bark. Such affected bark is most 
commonly found on the trunk near the ground, in crotches, and at 
the bases of small young branches arising from the large limbs of 
heavily pruned trees. When at or near the ground, dead bark rots 
quickly; above ground it usually dries and eventually scales off from 
the wood. 
The initially affected tissues are variously distributed in streaks 
and patches, which in cross section usually appear in more or less 
nearly concentric circles about the wood cylinder. In cases of slight 
injury, the medullary rays of the inner phloem, groups of parenchyma 
around the sclerenchyma strands or patches of cortical parenchyma 
are affected. In more severe cases, much of the phloem and all the 
