THE GROWTH RING 25 



alternating periods of physiological rest and activity, correspond 

 very closely with annual periods, whence it is possible to utilize 

 them in determining questions of age which may be ascertained 

 in northern latitudes within an error not exceeding and usually 

 much less than one per cent (53, 162). This law has been 

 applied with success to the determination of the probable age 

 of certain blazes found in beech trees in Canada, tracing them 

 to the year 1721 and their probable origin at the hands of some 

 of the early Franciscan missionaries (54, 356, and 55, 500); 

 while Campbell has similarly applied it to a determination of the 

 age of certain Sequoias and the years in which forest fires or 

 other injuries were inflicted (9, 335). The chief sources of 

 error in such estimates lie in the difficulty of clearly recognizing 

 under all conditions the relation of a particular growth ring to 

 a certain year, since it is a well-known fact that under peculiar 

 conditions more than one ring may be formed in one season. 

 Thus Penhallow has shown that while in northern latitudes there 

 is an essential constancy in the relation of rings to annual periods, 

 this constancy diminishes toward the equator with a tendency 

 to the obliteration of the growth rings, whence it follows that at 

 some intermediate point in latitude there will be more rings 

 formed within a given period than there are years of age (53, 

 162). This is supported by the observed fact that in the 

 state of New York the common red maple (Acer rubrum) has 

 been known to form an average of three rings for each year of 

 growth, while in Florida at least forty rings have been found in 

 trees less than thirty years old. In such cases experience will 

 soon indicate to the observer that the demarcation between 

 rings of successive years is much more pronounced than bet\veen 

 those of the same year, directly corresponding to the duration 

 and intensity of the rest periods in each case, and from this 

 it is possible to reduce the error from such a source to a mini- 

 mum. Thus De Bary has shown that such double growth rings 

 as are referred to are the direct result of some disturbance of 

 the normal course of growth for the season, such as may arise 

 through the operation of frost, drought, insects, etc. (13, 514)- 



