FORMATION OF FROST RINGS IN CONIFERS. 5 



condition. Microscopically the injury appeared as a brown line 

 between two annual rings. A microscopic examination showed that 

 the wood first formed in the spring following the injury was a com- 

 paratively narrow zone of parenchyma wood, that the normal xylem 

 was soon laid down outside of this zone, and that the remainder of 

 the growth ring was normal. The medullary rays, which had be- 

 come enlarged and spread out tangentially, could be traced into this 

 parenchyma zone. Mention is made of a yellowish brown amorphous 

 substance 'occurring in the intercellular spaces. While Mix was un- 

 able to definitely determine the exact nature of this substance, the 

 writer, from his investigation of this group of substances (10), 

 would regard the brown color as a sign of humification and the 

 brown substance itself as a huminlike compound originating as a de- 

 composition product of the cell contents of the cells killed by frost. 

 The type of frost injury which Mix described is closely related to 

 that described by Neger (7) in Picea excelsa. 



GENERAL SYMPTOMS AND MACROSCOPIC APPEARANCE. 



During the field season of 1921 the writer repeatedly observed in 

 frost localities on cut-over lands in Washington, Idaho, and Mon- 

 tana areas of coniferous reproduction on which a large percentage 

 of the young trees showed the effect of repeated late frosts, both 

 externally and internally. 



In unusually severe cases the young growth had been killed back 

 until the trees had developed an abnormally compact bushy form. 2 

 Such a growth form, which was by no means common in such native 

 trees as Thuja plicata, Tsuga heterophylla, Pseudotsuga taxifolia, 

 Larix occidentalism Picea engelmanni, Abies lasiocarpa, and Tsuga 

 mertensiana, was rarely observed in Pinus contorta, P. ponderosa, 

 and P. albicaulis. 3 It was extremely common, however, in Abies 

 grandis (PL I, A). The greater tendency of young trees of Abies 

 grandis to assume this compact bushy form after injury by late frost 

 is due to the great readiness with which this species develops com- 

 pensatory shoots. Since the recovery of any given species from frost 

 injury depends largely upon its ability to retain dormant buds which 

 give rise to such compensatory shoots, it should rank very high in 

 both Abies grandis and A. concolor. 



In the cases of less severe injury the trees did not develop any 

 particularly compact bushy form and often did not appear unusual 

 in any way, yet the same frost rings occurred in the wood, although 

 less frequently and perhaps only in the wood increment of but a 

 single year. Where such frost rings occurred, however, it could be 

 detected upon close examination in practically all cases investigated 

 by the writer that the original terminal shoot of the stem in ques- 

 tion had been killed back by frost after the initiation of its growth, 

 and that in some cases the same had happened one or more times to 

 the volunteer shoots. In this connection the writer wishes to state 

 that he has never observed in any of the coniferous species studied 



a The writer wishes to make it clear that he does not consider all cases of the broom- 

 ing of young conifers to be due to late-frost injury, since this abnormal form of growtn 

 may be induced by parasitic fungi alone. In the latter case, however, the formation 

 of frost rings does not occur within the zones of annual increment. 



8 Host names for American species follow the usage in the publications of George if. 

 Sudworth, of the United States Forest Service. 



