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



REVIEW OF THE LITERATURE. 



Despite the great mass of literature on the subject of frost injury, 

 there are but few descriptions of the pathological effect of the 

 injury on the structure of the wood. In fact, the effect on forest 

 growth of temperatures below the freezing point, or frost, is seldom 

 considered, except in so far as it causes injuries the external mani- 

 festations of which are readily apparent. 



The so-called frost rings, or " moon rings," as they sometimes are 

 called when extending only a part of the way around the stem, 

 occurring in young trees as a result of the action of frost, have 

 been mentioned by various European writers, but it is only rarely 

 that their structure and origin have been studied from the stand- 

 point of their pathological anatomy, and illustrations of this abnor- 

 mality are rare. 



Mayr (5, p. 36) * states that the stimulating action of a mild late 

 frost on the annual ring already in a state of cambial activity exerts 

 itself in such a way that in place of the elongated tracheids a short- 

 celled parenchyma arises. According to Mayr (J, p. 37), this 

 abnormal wood may occur either on only one side of the stem or 

 extend entirely around it, depending upon the way in which the cold 

 air strikes the plant. In either case internal healing ensues, pro- 

 ceeding from the parenchyma cells of the wood, which fill up pos- 

 sible cavities with wound parenchyma, while a new cambium is 

 developed in the bark from the bast parenchyma remaining alive. 

 In addition, Mayr states that if the frost has killed the bast together 

 with the cambial layer, then the entire plant part dies. 



Hartig (2) investigated the action of a May frost on the shoots 

 of young trees of Pinus sylvestris. He describes in detail the for- 

 mation of zones of parenchyma tissue, which constitute the so-called 

 frost ring, in the growth ring developing during the year of the 

 injury. He likewise describes the peculiar permanent distortion 

 of the injured young shoots, a circumstance occasioned by their loss 

 of turgor and consequent drooping after the freezing, followed by 

 an effort to redirect their shoots upward. In many cases, however, 

 the whorls of shoots were killed outright. 



Hartig also investigated the formation of similar frost rings in 

 young trees of Picea excelsa, Larix europaea, and Chamaecyparis 

 law-soniana, but he illustrates their formation only in Pinus syl- 

 vestris and Picea exeelsa. He states that frost-ring formation was 

 so frequent in a spruce 2 meters high that he counted 10 frost rings 

 in a section about 15 years old, so that 25 rings were to be counted 

 in a casual macroscopic examination. The frost-ring formation ex- 

 tended down into the stem parts, which were 10 to 12 years old. In 

 the larch the frost rings occurred only in its youth, as in the spruce, 

 and were found only in the youngest to the 4-year-old axes; in the 

 Lawson cypress, however, frost rings still occurred in the older axial 

 parts, and such ring formation was noted also in the interior of the 

 phloem. Hartig gives the same account later in his textbook of plant 

 diseases (3). 



1 Serial numbers (italic) in parentheses refer to " Literature cited " at the end of this 

 bulletin. 



