254 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I4O 



wood with the frost injury and recovery, give evidence of the cause 

 of such intra-annuals. Other evidences connected with frost, namely, 

 resin canals, compression wood, and parenchyma cells, give almost 

 equally good support to a temperature cause. In some cases the 

 densewood, or other features, alternates tangentially with spots of 

 frost injury and recovery which are scattered around the circuit. In 

 the remainder of the cases, a single unified arc of frost injury is con- 

 tinued around the circuit by densewood, by resin canals, or by paren- 

 chyma cells. These features thus either accompany the frost effects or 

 replace them. 



Densewood cells, whether isolated, in patches, or in a continuous 

 zone, resemble ordinary densewood. Examples of densewood made 

 by low temperature are numerous. Perhaps the best photographs of 

 them are on plates i, figure 2; 30, figure i ; 32 ; and 34. In those 

 instances where frost effects and densewood cells are intermingled, no 

 doubt can arise as to the intra-annual character of the growth layer. 

 However, where the densewood is a continuous band completing the 

 circuit from end to end of an arc of frost injury, the outer margin is 

 commonly sharp, with a sharpness equal to that of an annual incre- 

 ment. The frost injury may disappear longitudinally along the branch 

 and leave an entire band of densewood as part of a bona fide growth 

 layer. Frost-induced growth layers, either partial or entire, have been 

 observed in junipers (CMJ i and 2, TTJ 2-4), in spruce (TRSp i 

 and 2), in apple (TTAp 2-3), in Arizona cypress (TTC 5-5, 34-2; 

 XSC 1-3, 2-1, 2-2, 3-1, 3-2, 6-1, 8-1, 8-2, 8-3, 9-2, 9-3, 9-5, 9-6, lo-i, 

 10-2, 1 1-2, 11-3, 13-1, 13-2), in yellow pines (TTP 21-2, 21-5, 21-6, 

 21-7, 21-9, 21-10, 23-1, 23-3, 24-10, 24-12, 24-14), and in Jeffrey pine 

 (XSJf i-i). Indeed, CMJ 2-1-b appears to have two 1938 frost in- 

 juries, the inner one accompanying and causing a sharply bordered 

 intra-annual. In all trees designated TTC, frost-connected arcs and 

 circles of densewood, either diffuse or sharp, are common features. 

 These features of densewood, combined with the many not frost-con- 

 nected, suggest strongly that the Arizona cypress is highly sensitive 

 to slight changes in habitat factors, one of which is certainly temper- 

 ature. The most pertinent evidence of the role of frost in producing 

 growth layers comes in the intimate association of densewood cells 

 with frost-effect pockets where densewood and frost injury alternate 

 around the circuit of a cross section. 



Longitudinal transitions and interrelationships illustrate even more 

 clearly that growth layers can be formed by low temperature within 

 the regular growing season. Transitions occur either inward or out- 

 ward on a branch. A continuous circle of frost injury and recovery 



