4 BULLETIN 1131, U. S. DEPARTMENT OF AGRICULTURE. 



reducing the bark pressure, with the result that a zone of paren- 

 chyma wood was developed as the first growth in the following 

 spring instead of the normal tracheidal wood, in so far as the stem 

 had remained living and continued its annual ring formation. In 

 his textbook, which appeared later, Neger (8) briefly describes frost 

 rings and reproduces an illustration of one caused by this winter 

 frost. 



Somerville (11) describes an abnormal zone of parenchyma tissue 

 that is very closely related to frost rings, if not actually identical 

 with them. This abnormal zone occurred in the early-spring wood 

 of a large percentage of young conifers whose wood he had occa- 

 sion to examine. All of the species examined, including Larix lepto- 

 lepis, Pseudotsuga douglasii (=P. taxifolia), Tsuga albertiana 

 (=T. mertensiana) , Cedrus deodar a, Thuja plicata, and Picea 

 sitchensis, exhibited more or, less of the injury. 



Somerville describes the abnormal wood formation only for Larix 

 leptolepis. He says that the abnormal wood formed in the early 

 part of 1912 is easily distinguished by the naked eye. On a cross 

 section it appears as a narrow brown ring, while on a radial section 

 it forms a thin brown streak. A microscopic examination shows 

 that the medullary rays are seen to pursue a most irregular course 

 and to consist of much elongated and swollen cells. The rays fre- 

 quently are discontinuous with those of the previous ring. The in- 

 tervening cells, many of which have walls much thickened, instead 

 of getting smaller as one proceeds outward, have a tendency to be- 

 come larger. A radial section along the junction of the normal 

 summer wood and the abnormal spring wood of 1912 shows that 

 the abnormal zone of tissue is largely composed of irregularly 

 shaped parenchymatous cells with simple pits and rectangular trans- 

 verse walls. It will thus be seen that the foregoing description of 

 Somerville's abnormal wood formation agrees closely with Neger's 

 description of frost rings, especially since they occur following im- 

 mediately upon the summer wood of the preceding growth ring. 



Somerville, however, states that the cause would appear to be the 

 excessive heat and drought of the summer and fall of 1911, which 

 seriously affected the growth of many trees, notably the Japanese 

 larch. He says : 



This climatic condition evidently so upset the normal function of the cam- 

 bium that when the wood of 1912 came to be formed it was found to deviate 

 greatly from the usual type. 



However, from a consideration of Somerville's description and 

 illustrations of the injury, together with the fact that it occurred 

 also in the spring wood of other years, the writer is inclined to re- 

 gard this injury as the result of frost rather than of drought. This 

 view appears to gain credence when it is considered that, inasmuch 

 as the drought occurred during the summer of 1911, it would seem 

 likely that the injury to the cambium must have occurred in ample 

 time to have registered in the latter part of the 1911 ring, whereas 

 it did not register until the beginning of the growth ring of the 

 following year. 



Mix (6) describes and illustrates the formation of a zone of paren- 

 chyma wood in apple trees varying in age from 2 to 8 years, follow- 

 ing an injury to the cambium due to freezing while in the dormant 



