﻿10 BULLETIN ll.il. r. S. DEPARTMENT OF AGRICULTURE. 



derived from the now cambial formation (PI. V, />; PI. VI, B and 

 O.) An unusually striking example of this radial cleft formation 

 occurred in the frost rings observed in stems of Sequoia tr<tsh' ln </- 

 tonhiiui. where clefts were present not only in the early formed 

 portion of the growth ring hut also at the outer limit of the summer- 

 wood formation (PI. VI, 6'). In the latter case the frost rings ap- 

 peared to lie between the summer wood of one growth ring and the 

 spring wood of the next, so that there was no sharp demarcation 

 between the two annual rings except where the frost ring did not 

 extend completely around the stem. Still other stems from the 

 same material, wdiich had been injured by frost near the close of 

 the growing season and had died without subsequent growth, ex- 

 hibited the same radial clefts at the periphery of the xylem. but 

 in this case the clefts were still open and free from any occlusion 

 by parenchyma cells. Such tissue disturbances result in a very pro- 

 nounced false ring formation. 



A large part of the phenomena which come to light in frost injuries 

 to young stems, however they may vary, can be traced to simple 

 mechanical processes. Sorauer (12) has proved experimentally that 

 processes of loosening are initiated in the cell membranes by the 

 action of frost; and this explains the formation of this parenchyma 

 zone instead of the normal wood elements as the result of a weaken- 

 ing of the compressing influence exerted by the bark girdle on the 

 youngest tissue, that is, the cambium. According to Sorauer, the 

 frost, without necessarily forming ice crystals in the intercellular 

 spaces, contracts the tissue in direct proportion to the thinness of 

 the walls of the tissue. The bark suffers considerably more than 

 the w r ood, which, reached later, cools down less easily and contracts 

 less. The tangential contraction is greater than the radial. As 

 Sorauer states, this difference acts like a one-sided strain and exerts 

 itself in the direction of the circumference of the trunk, to which 

 the different layers of the bark will respond to a different degree 

 when the bark as a whole is very young. Consequently, with the 

 action of the frost there must take place everywhere within a woody 

 axis a preponderance of tangential strain over radial contraction, 

 and under certain circumstances this must increase to a radial split- 

 ting of the tissue. With an equal degree of contraction at all points 

 in the bark, the cells lying nearest the periphery and most elongated 

 in the direction of the circumference of the trunk will be the most 

 displaced. As Sorauer also states, if one considers that the outer 

 cells of the primary bark, because of the greater coarseness of their 

 Avails, are not as elastic as the underlying thinner walled ones, it is 

 clear that when the strain ceases in them the permanent stretching, 

 caused by the incomplete elasticity, will be the greatest there. After 

 the action of the frost, which continues but a short time in late frosts, 

 has stopped, the tissue that has become stretched is not sufficiently 

 elastic to contract again to its original volume, and the cells retain 

 their distended and distorted form. In this way each frost action 

 leaves behind an excessive lengthening of the peripheral tissue layers 

 in proportion to the adjacent layers which lie more toward the inside. 

 The bark body as a whole is therefore larger and either does not 

 have room enough on the wood cylinder, so that in places it is raised 

 up from it, or it at least decreases its constricting influence on the 



