THE GROWTH RING 



29 



observed somewhat similar phenomena in the case of A. brasi- 

 liana. Specimens coming under my own observation, but repre- 

 senting growth which was completed in a conservatory, show that 

 in A. excelsa and A. Cunninghamii the growth rings are either 

 not determinable or at most very imperfectly defined ; while in 

 A. Bidwillii they are only faintly defined by a slightly more open 

 structure in the earliest spring wood. The same law holds true 

 for fossil species in which it meets with only partial exceptions, 

 as in A. Edvardianum. In Dammara the " growth rings are more 

 or less clearly defined," approximating in this respect to what 

 is found in the Coniferales, and between which and the typical 

 Cordaitales they may be held to represent a transitional form. 



Apart from those conditions of internal tension which arise 

 incidentally to the formation of dissimilar tissues, and which 

 induce structural alterations, growth rings may be regarded in 

 the main as a direct expression of the alternation of sharply 

 defined growth and rest periods ; and since these in turn are cor- 

 related with sharply defined seasonal changes, such as are com- 

 mon to more northern latitudes, it becomes possible to utilize 

 these facts in forming an approximate estimate of the general 

 climatic conditions under which the tree must have developed. 

 From this it is also possible to draw important inferences as 

 to the climatic conditions which must have prevailed in earlier 

 geological periods, as indicated by the presence or absence and 

 the specific character of growth rings in fossil woods, a conclu- 

 sion which gains force from the relation which the formation 

 of growth rings in Araucaria bears to climatic conditions, as 

 already stated. 



Every growth ring presents two structurally dissimilar regions 

 which correspond with different periods of activity, and it is the 

 apposition of these in successive years which principally deter- 

 mines the recognition of the growth ring. The initial growth of 

 a given year arises at the earliest possible moment at which the 

 cambium is capable of generating new tissue. The elements 

 which thus arise are applied directly to the outer face of the 

 growth ring for the preceding year. In the formation of such 



