320 Report OF THE BoTANICAL DEPARTMENT OF THB 
In the petioles the vascular bundles, the collenchyma celis and 
the inner cortex were browned. The branches were injured 
more on one side than on the others. The pith was white ex- 
cept Some scattered starch-bearing cells, which had browned 
contents. The bast cells of the cortex were unaffected, while 
the cells immediately surrounding them were much browned. 
Only a few small clefts were found between collenchyma and 
parenchyma cells. It seems, in fact, that when any clefts result 
they generally occur where different types of tissue join. 
The tension differences caused by low temperatures between 
two or more types of adjoining tissues are said to be the cause 
not only of cleft formation in the softer, young tissues but also 
of the splitting of large tree trunks. In case of young twigs, 
the bark contracts so much more than the wood cylinder that 
it may either crack open when the temperature is very low, 
or mechanically injure tissues in the cambial region, when the 
cold is less severe. The cortical-tissue cells are distended tan- 
gentially to a greater or less degree and owing to their imper- 
fect elasticity often remain at least partially distended after 
moderate temperatures return. The net result may have been 
sufficient to make the bark too large for the wood cylinder and 
result in tangential clefts, or at least to lessen the bark pres- 
sure enough to cause abnormal thickening at certain places, 
both of which may often be observed on frost-injured fruit 
trees. At a branching node or at the junction of two seasons’ 
growth on a twig, the same forces may result in the thickening 
of the medullary rays. 
OTHER POSSIBLE CONSEQUENCES OF FROST-INJURIES. 
The various cankers.—Frost-injuries seem, therefore, to 
occur most readily at crotches and other branching points of 
limbs and twigs, as has also been observed by Morse and 
others. In view of Sorauer’s observations on the effects of 
frost-injury, under control conditions, and the fact that the 
same varieties are reported susceptible to canker as are easily 
frost-injured and that cankers are usually said to spread 
during winter and early spring makes it appear plausible that 
Spheropsis, Numularia, Gleosporium, Cytospora and Bacillus 
