THE GROWTH RING 



though only nineteen centimeters in diameter. Further, within 

 each zone (Pseudotsuga) the range of variation is narrower, and 

 oftentimes the rings present a remarkable degree of uniformity. 

 So well marked are these differences that when a number of trees 

 are examined it is possible to establish an exact correspondence 

 of zones by means of the average dimensions of the component 

 growth rings. These facts point with some force to the probable 

 operation of similar, if not identical, conditions of growth over a 

 somewhat extended period, while the periodicity of growth also 

 suggests a corresponding periodicity in environmental conditions. 

 It has elsewhere been shown (52, 35) that in the case of trees 

 exhibiting four such zones the following results are obtainable : 



VARIATIONS OF ZONES AND GROWTH RINGS IN 

 PSEUDOTSUGA DOUGLASII 



