22 



shade of which the spruces and finally the balsams develop, the 

 ultimate result being a return to the spruce-birch-balsam type 

 of forest. 



/. C. Hcssler. — "What is the character of the rocks on Isle 

 Royal ?" 



IV. S. Cooper. — "They belong to the Keweenawan and Cam- 

 brian." 



/. C. Hcssler. — "I have a series of photographs taken on 

 Silurian formations several hundred miles south of Isle Royal 

 which show the same topography and same general type of 

 vegetation." 



T. C. Chamherlm. — "There is no doubt a calcareous factor 

 influencing the character of the vegetation." 



/. G. Coulter. — "Mr. Cooper's interesting chart, indi- 

 cating tree successions on the basis of contrast of ages evi- 

 denced by growth rings, suggests the old question of the 

 dependability of these rings as indicative of annual incre- 

 ments. Of course even large fluctuations from the year- 

 value would not, however, invalidate Mr. Cooper's striking 

 chart, which is constructed upon a comparative rather than 

 an absolute basis. 



In tropical trees the question of the time-value of these rings 

 is more complex. The interesting fact is that these rings are 

 readily discernible in many tropical woods, although not so 

 clearly marked -as in trees of the temperate regions. In the 

 Philippine Islands, for example, such rings have been observed 

 in the wood of members of the Leguminosae and of the Diptero- 

 carpeae. In the former, a year-value suggests itself since these 

 trees do regularly lose the bulk of their foliage in monsoon- 

 forests during the dry season. In the latter case, however, 

 the time-value is more perplexing since there is no evident 

 general loss of foliage at any time of year, nor do the climatic 

 variations indicate any necessity for a stoppage or even any 

 serious pause in nutritive activities during the year, at least 

 not in the case of deep rooted old trees. However the rings 

 appear to be as well marked near the bark as near the core 

 of such trees. 



