CROWN-ROT OF FRUIT TREES: HISTOLOGICAL STUDIES 497 
Environmental Factors Having a Causal Relation to the Injuries. — 
My work in northern orchards showed that bark injuries which are 
caused by the combination of immaturity of tissues and the occurrence 
of low temperature may give rise to crown-rot. In Florida the 
occurrence of a temperature only about two degrees or less below the 
freezing point of water, when certain bark tissues of citrus trees are 
immature, may result in similar injuries and give rise to equally 
destructive diseases. Severe droughts often cause similar injuries. 
When the bark of citrus trees is dormant, it will endure temperatures 
even below — io° C. and severe droughts without serious injury. 
It is still uncertain whether this bark injury is due chiefly to simple 
physical causes, such as contraction or to chemical and physiological 
changes induced in the protoplasm by low temperatures and drought, 
or to both sets of factors acting together. As a matter of fact, changes 
of both kinds take place in plants subjected to low temperature and 
untimely droughts, and we have fairly tangible evidence that both 
may be injurious. 
The photographs submitted with this paper give ample evidence 
that high tensions, and even ruptures, accompany some at least of the 
more severe bark injuries. Trunk measurements previously published 
also show the occurrence of high tensions. In some cases, however, 
no actual ruptures appear to result, and yet tissues become discolored 
before the commencement of the next vegetative period. Injurious 
low-temperature tensions of less degree than those required to rupture 
the bark are evidently of frequent occurrence, and these are apparently 
responsible for much of the bark injury afterwards resulting in disease. 
An extreme form of this effect is shown in Fig. 37a. Some of the 
milder tensions are also shown in Figs. 3, 4 and 5 of Brown's^^ recent 
paper. Sorauer^^ has given much attention to this type of injury 
^1 Crown- Rot of Fruit Trees: Field Studies, N. Y. Agr. Exp. Sta. Tech. Bull. 
23: 36. 1912. 
Sorauer, P., Experimentelle Studien iiber die mechanischen Wirkungen des 
Frostes bei Obst- und Waldbaiimen, Landw. Jahrb. 35: 469. 1906. 
, Weswegen erkranken Schattenmorrellen besonders leicht durch Monilia? 
Zeit. Pflanzenkr. 22: 285. 1912. 
, Einige Experimente zum Studium der Frostwirkungen auf die Obstbaume, 
DieNaturw. i: 1055; 1094. 1913. 
12 Brown, H. P., Growth Studies in Forest Trees. 2. Pinus Strobus, Bot. Gaz, 
59: 197. 1915. 
, Altes und Neues iiber die mechanischen Frostbeschadigungen, Zeit. 
Pflanzenkr. 24: 65, 1914. 
