166 Relation of Annual Rings to Age. 



which at once defines the rings, particularly if this denser aggre. 

 gation involves a deepening of the color peculiar to the tissue as a 

 whole. Examples of this we find very generally in the Coniferan 

 Populus, Alnus, Betula, Carpinus, Ostrya, largely in Fagus y 

 Pints, Amelanchier, Primus, etc. 



In these respects, whenever rings are developed, tropical woods 

 show the same characteristics as do those of temperate climates. 

 Thus, from an examination of wcods from Brazil, we gather the 

 following data : — 



Rings conspicuous 7 



Rings imperfectly defined 4 



Hings very obscure or none 10 



JRings slightly defined structurally 4 



Rings strongly defined by alternation of more and less dense 



tissues (ducts) 3 



Rings defined by excess of color in first or last formed cells. 6 



No structural distinctions, ducts uniformly distributed 11 



Rings due to alternating growth and rest, ducts none, no 



obvious structural difference 5 



Tissue dense throughout, rings numerous and variable 2 



Outer rings broadest 2 



Whole number of species 23 



CAUSE OP VAEIATION IN DENSITY OF STRUCTURE. 



With regard to the immediate causes operating to produce 

 variation in density of structure, the views of the earlier botanists 

 have been very largely retained up to the present time. This 

 "Variation was believed to be due primarily to alternating periods 

 °f activity and repose, as defined by the seasons ; that the sharp- 

 ness of definition and regularity of occurrence must be dependent 

 upon the sharpness with which variable climatic conditions were 

 separated and the regularity of their occurrence ; and that what- 

 ever operated to interrupt a given period of activity, or introduce a 

 new period of growth into one of rest, would strongly tend to 

 modify the number of resulting rings. So that, while one ring might 

 be the product of one season, it might, on the other hand, be the 

 product of only a portion of the entire season's activity. Thus it 

 naturally came to be considered that while in northern latitudes 

 rings were an approximately exact index of age, they could not be 

 so considered in more southern and tropical latitudes. These 



