CHAPTER II 



THE GROWTH RING 



In proceeding to a study of the transverse section the first 

 feature to which attention is naturally directed is the growth 

 ring. These are either broad or narrow, variable or uniform, 

 eccentric or regularly concentric according to their relative pro- 

 portions in radial extent, the constancy of their radial volume, 

 and their equal or unequal development at all points about a com- 

 mon center. While a recognition of such features often serves 

 an important purpose in confirming data from other sources, they 

 are in reality of secondary importance and too much stress 

 must not be laid upon them ; in fact, they may, if necessary, be 

 entirely neglected in most cases. This element of doubt is due 

 to the fact that within a given transverse section of an entire 

 stem the growth rings vary very greatly among themselves as 

 the result of varying rates of growth induced by external con- 

 ditions of soil and climate. These variations are of such a nature 

 that, in general terms, the growth rings will have the greatest 

 radial dimension in a young stem or at the top or toward the 

 center of an old stem, while such dimension diminishes within 

 the same stem radially outward, so that in the peripheral portion 

 of a very old trunk the rings will be either actually or relatively 

 very narrow. This general rule is subject to many exceptions. 

 In consequence of the suggestions furnished by this structural 

 feature it is desirable to include its description in every diagnosis 

 of a species, and we may therefore consider somewhat in detail 

 its principal aspects of structure and variation. 



All of the North American Coniferales without exception, 

 both fossil and recent, and also all of the Japanese species so 

 far investigated, are characterized by well-defined growth rings. 

 These regularly recurring zones of growth, arising through 



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