76 ORGANOGRAPHY. 



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 very different results. A better way to calculate 

 the age of a tree by the inspection of a fragment is that first 

 employed by DeCandoUe ; namely, to make two notches, or 

 remove two pieces from opposite sides, and then, having ascer- 

 tained 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 have 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 correct, but even this will frequently 

 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 from these causes — that is, 

 by not sufficiently taking into account the variations in the growth 

 of the annual zones at different periods of their age, and their 

 varying thickness on the two sides ; and, when we consider that 

 some trees were estimated by him to be more than 5000 years of 

 age, we cannotbutbelieve with Dr. Lindley, that such calculations 

 give an exaggerated result. However erroneous they may have 

 been, still there can be no doubt but that exogenous trees do live 

 to a great age ; in fact, when we consider that the new zones of 

 wood are developed out of the cambium cells which are placed 

 on the outside of the previous zones, and that it is in these new 

 layers that all the active functions of the plant are carried on, 

 there can be, under ordinary circumstances, no real limit to their 

 age. Mohl believes that there is a limit to the age of all trees, 

 arising from the increasing difficulty of conveying the proper 

 amount of nourishment to the growing point, as the stem elon- 

 gates from year to year. Thus, in some Coniferpe, as, for instance, 

 Pinus Lambcrtii, which reaches the height of more than 200 

 feet, he believes the maximum height which the sap was capable 

 of rising to nourish the upper part of the plant was attained, 

 and the terminal shoot being then less perfectly nourished, became 

 every year more or less stunted, and the tree ultimately died from 

 want of a proper supply of nourishment. We cannot however 

 attach much importance to this opinion, because it is now known 

 that a Coniferous tree exists in California ( Wellmgtonia gigantea), 

 which has reached the height of 450 feet, and is still in full 

 vigour. 



