CROWN-ROT OF FRUIT TREES: HISTOLOGICAL STUDIES 503 
type of injury was studied by Sorauer^^ about twenty-five years ago. 
By comparing Fig. 29, Plate XXIV (copied from Sorauer) , with Fig. 2, 
Plate XXI, it is evident that the initial injuries were very similar in the 
two cases, except that that shown in Fig. 2 is much more severe. 
Sorauer found that this premature roughening is of especially frequent 
occurrence on rapidly growing varieties of fruit trees when they are 
from six to eight years old. The same cold spell that resulted in the 
bark-roughening described by Sorauer had also caused the bark of some 
trees to rupture and of others to ''scald" or die to the wood in long 
patches. Some cambium, medullary rays, protoxylem, and pith 
tissues were killed and discolored ; in the cortex the larger patches of 
dead collenchyma cells were subsequently cut off by new phellogen. 
This type of bark and twig injury of pear trees was apparently also 
studied histologically by Miczynski.^^ He shows the distribution of 
dead and discolored tissues in a colored plate. In cases in which the 
cambium had been killed, the new cambium developed in the inner 
phloem much like that described in a former section of this paper. 
Another good but general account of bark injuries of fruit trees is 
given by Oberdieck.^^ He gives many clear details regarding numerous 
cases. 
Sun-scorch is usually confined to trees that have not yet reached 
the rough-bark age, and consist of dead and discolored bark on the 
trunk or main branches, usually (though not always) on the west or 
southwest sides. Histologically its early stages are similar to those 
giving rise to the premature bark-roughening described by Sorauer. 
In many cases of the sun-scorch type, however, only the outermost 
collenchyma cells are involved, and consequently the resulting new 
bark surface looks only slightly frayed. Numerous interesting obser- 
vations have been made on this bark disease, and in many of the 
discussions one may find pertinent suggestions. Hess,^^ for instance, 
notes that this trouble develops on smooth-barked forest trees one or 
38 Sorauer, P., Uber Frostschorf an Apfel- und Birnstammen, Zeit. Pflanzenkr 
i: 137. 1891. 
3^ Miczynski, K., Ueber das Erfrieren der Gewebe des Birnbaums, Bot. Centralbl. 
48: 228. 1891. 
4° Oberdieck, J. G. C., Beobachtungen iiber Erfrieren vieler Gewachse und 
namentlich unserer Obstbaume in kalten Wintern; nebst Erorterung der Mittel 
durch welche Frostschaden moglichst verhiitet werden kann, pp. 108. Ravensburg. 
1872. 
4^ Hess, R., Der Forstschutz. Leipzig. 1878. 
