623 



Caspary's measurements in freezing should be recalled here. The 

 peripheral layers contract earlier and more strongly than do the central 

 layers. This contraction with cold is stronger tangentially than radially 

 and greater in the delicate parenchyma than in the prosenchyma wood. Con- 

 sequently, with the action of 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 splitting of 

 the tissue. 



If the wood ring is thought of, first of all, as isolated, this preponder- 

 ating tangential contraction in places of least resistance would necessarily 

 lead to such clefts as would correspond to the gaping frost cracks in old 

 trunks. Therefore, inner clefts must be produced from purely mechanical 

 causes and. in fact, in the medullary rays and medullary transverse connec- 

 tions. Such are actually shown in the illustration of the oak branch, injured 

 by natural frost (Fig. 148). 



If we noM^ consider the primary wood ring in its relation to the adjoin- 

 ing bark girdle, we must refer again to the fact that the bark girdle, of 

 which the peripheral cells are larger tangentially than radially, contracts 

 more strongly tangentially and, therefore, is strongly torn in this direction 

 during the action of frost. If the frost grows less, this cracking may cease, 

 indeed, but its effects remain, for the tissue which may thus be str.etched, 

 is not absolutely elastic and does not contract to its former volume. 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, therefore, is longer 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 curves outward, i. e., it decreases its 

 constricting influence on the camhial elements of the wood cylinder. 



The cambial zone responds to this with a formation of parenchyma 

 wood, as may be seen in every wound in which the bark is raised. If the 

 bark girdle closes together again into a connected layer the cambial cylinder 

 of the branch, by growth in thickness, must again resist the constricting 

 effect of the bark and, on this account, again forms normal wood elements. 



Thus the formation of the parenchyma wood bands in young branches 

 comes under the same law of unequal contraction which, in old trunks, 

 leads to the production of gaping frost clefts. 



The Rupture of the Cuticle. 



The experiments on potted specimens of forced oaks, mentioned in the 

 previous section, proved the fact, not known until then, that, on superficially 

 browned, or still green leaves, i. e., those outwardly but little affected, a 

 repeatedly interrupted black, very fine line is formed on the under side, 

 which gives the impression of very fine particles of soot which had settled 

 on it in places. With a higher magnification, it is seen that this line consists 

 of small raised places in the outermost cuticular layer which, because of its 



