Dr David Christison on the Rings of Certain Trees. 447 



On the Difficulty of ascertaining the Age of certain Species of 

 Trees in Uruguay, from the Numher of Rings. By 

 David Christison, M.D. (Plate IV.) 



(Read 13th March 1890.) 



Until comparatively recent times it seems to have been 

 accepted as a truth, alike by botanists and foresters, and 

 perhaps without much thought on the subject, that the 

 number of rings or zones of wood in an exogenous tree, in 

 any part of the world, accurately corresponds with the years 

 of its age. Even now I have met with not a few intelligent 

 botanists who still believe this. Great reliance is also 

 placed, even in the best text-books on forestry, upon the 

 rings as being indicative of the age, and consequently of the 

 important point of the rapidity of growth, in tropical trees. 

 Nevertheless, in recent years it has been shown that in parts 

 of the United States of America, which are not even sub- 

 tropical, the rings may be more numerous, or even less 

 numerous, than the years of age of trees. Sir Eobert 

 Christison also pointed out, about twelve years ago, that in 

 sections of tropical trees examined by him the rings were 

 confused, irregular, and far too numerous to correspond with 

 the age ; and, as it has been proved by experiment in 

 Germany that two rings can be produced artificially in a 

 single year, by stripping a tree of its leaves in midsummer, 

 it seems probable that in certain climates the same result 

 may be produced naturally, and that in very equable warm 

 climates, where there is no marked annual check to vegeta- 

 tion, rings might be expected to be formed very irregularly, 

 if at all. Even in temperate countries like our own, although 

 as a rule the zones of wood agree with the years of age, yet 

 I believe it has been ascertained that there are exceptions, 

 arising from temporary injury or in old age, from the layers 

 of wood ceasing to be produced at the lower part of the 

 trunk. 



As this is a field of inquiry which as yet has been but 

 little worked, I have brought before the Society the following 

 observations on some sections of trees sent to me last autumn, 

 by Mr Charles E. Hall, from his estancia of San Jorge, in 

 Central Uruguay. In the first place, however, I shall give a 



