ORGANS OF NUTRITIOK. 81 



marked. In exogenous trees, however, of warm climates it is 

 generally difficult, and frequently impossible, to ascertain their 

 age in this manner, in consequence of several disturbing causes; 

 thus, in the first place, the zones are by no means so well defined ; 

 secondly, more than one zone may be formed in a year; thirdly, 

 some trees, such as Zamias, the Cycas, &c., only produce one 

 zone as the growth of several years ; while lastly, in some, such 

 as Guaiacum, &c., the zones are not only indistinct, but very 

 irregular in their growth. 



It has been stated that the age of a tree may be calculated, if 

 its diameter is known, by the inspection of a fragment of that 

 tree. The manner of proceeding in such a case is as follows : — 

 Divide half the diameter of the tree divested of its bark by the 

 diameter of the fragment, and then having ascertained the num- 

 ber of zones in that fragment, multiply this number by the 

 quotient previously obtained. Thus : suppose the diameter of 

 the piece to be two inches, and that of half the diameter of the 

 wood twenty inches; then if there are eight zones in the frag- 

 ment, by multiplying this number by ten, the quotient resulting 

 from the division of half the diameter of the tree by that of the 

 fragment, we shall get eighty years as the supposed age. Now, 

 if the thickness of the zones was the same on both sides of the 

 tree, and the pith consequently central, such a result would be 

 perfectly accurate, but it happens from various causes as already 

 noticed, that the zones are frequently much thicker on one side 

 than on the other, and the taking therefore of a piece from 

 either side indifferently would lead to totally different results. 

 A better way therefore to calculate the age by the inspection 

 of a portion is that suggested by DeCandolle; namely, to make 

 two notches, or remove two pieces from opposite sides, and 

 then, having ascertained the number of zones in each, take 

 the mean of that number, and proceed as in the former case. 

 Thus, suppose two inches as before, removed from the two 

 opposite sides of a tree, and that in one we have eight 

 zones, and in the other twelve, we Jiave ten zones as the 

 mean of the two. If we now divide, as before, half the 

 diameter, twenty inches, by two, and multiply the quotient ten 

 which results by ten, the mean of the number of zones in the 

 two notches, we get one hundred years as the age of the plant 

 under consideration. Such a rule in many cases will no doubt 

 furnish a result tolerably con-ect, but even this is liable to 

 lead to error, from the varying thickness of the annual zones 

 produced by a tree at different periods of its age. 



Dr. Lindley believes that DeCandolle in calculating the ages 

 of different trees, was led into error by not sufficiently taking 

 into account the variations in the growth of the annual zones 

 at different periods, and their different thicknesses on the two 

 sides ; and, when we consider that some trees were estimated 



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