FORMATION" OF FROST RINGS IX CONIFERS. 11 



cambial ' elements of the wood cylinder. The cambial zone responds 

 to this with the 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 by growth 

 in thickness must again resist the constricting effect of the bark and 

 on this account again forms normal wood elements. 



In sections containing frost rings that when viewed macroscopi- 

 cally appear to be only one-sided, it can be recognized in a microscopic 

 examination that, as a rule, a lesser disturbance has occurred on the 

 other side of the stem (PL II, B). However, a disturbance of the 

 wood tissue by no means always extends entirely around the stem, 

 the same often being purely local and consisting of numerous isolated 

 groups of parenchyma elements. The frost rings occasioned by late 

 frost vary greatly in their position within the growth ring, but usu- 

 ally occur early in the spring wood, either in the immediate beginning 

 or after the formation of a few normal tracheids. On the other hand, 

 they may not be formed until late in the growth ring when the frost 

 must necessarily occur during the summer. Frost rings in the latter 

 position are comparatively rare, however. 



More than one frost ring may occur within the wood of any one 

 growth ring, depending upon whether or not the frost occurs more 

 than once after the spring growth has been initiated. Two frost 

 rings within one annual ring are fairly common, and the writer has 

 observed the occurrence of three frost rings in the spring-wood zone 

 of an annual ring in Pinus monticola (PI. V, A) and in Picea 

 engelmanni. 



Frost-ring formation may occur in the wood from the action of 

 either late or early frosts during the course of the growing season 

 or from the freezing of the cambium during the winter when the 

 tree is dormant. The frost rings, therefore, may register at any 

 point within the growth ring, the relative position of the frost ring- 

 within the growth ring signifying the time at which the injury 

 occurred. 



According to Hartig (#, p. 4) , frost rings arise through late-frost 

 injury only when the cambial activity has already commenced and 

 at least some few cells have been cut off toward the interior, if, there- 

 fore, the annual ring formation has been initiated. It has been the 

 writer's experience with late-frost injury that, while the number of 

 spring- wood tracheids that intervene between the outer limit of the 

 summer wood of the preceding annual ring and the frost ring is 

 usually fairly uniform on any radius, the frost rings sometimes ap- 

 pear to abut* directly on the summer wood of the preceding growth 

 ring, although groups of normal spring- wood tracheids usually in- 

 tervene in places. The formation of at least some normal spring- 

 wood elements would therefore appear to be a diagnostic feature of 

 late- frost injury. 



In the case of late frost occurring unusually late in the season, the 

 frost rings may register in the median or outer portion of the growth 

 ring (PL YI, B). In the case of early frosts occurring late in the 

 season, at a time when the annual accretion of wood has not matured, 

 or in the case of frost injury occurring during the dormant period of 

 the year, the resulting frost ring registers in the immediate begin- 

 ning of the next growth ring, often tending to obscure the normally 

 sharp demarcation between the two rings (PL YI, C). 



