200 ANNUAL ZONES ARE [BOOK i. 



Age of Exogens. 



Each zone of the vascular system of an Exogenous stem 

 being the result of a single year's growth, it should follow 

 that, to count the zones apparent in a transverse section is 

 sufficient to determine the age of the individual under exami- 

 nation ; and further, that, as there is not much difference in 

 the average depth of the zones in very old trees, a certain 

 rate of growth being ascertained to be peculiar to particular 

 species, the examination of a mere fragment of a tree, the 

 diameter of which is known, should suffice to enable the 

 botanist to judge with considerable accuracy of the age of the 

 individual to which it belonged. It is true, indeed, that the 

 zones become less and less deep as a tree advances in age ; 

 that in cold seasons, or after transplantation, or in conse- 

 quence of any causes that may have impeded its growth, the 

 formation of wood is so imperfect as scarcely to form a per- 

 ceptible zone : yet De Candolle has endeavoured to show in 

 an able paper, Sur la Longevite des Arbres, that the general 

 accuracy of calculations is not much affected by such acci- 

 dents ; occasional interruptions to growth being scarcely 

 appreciable in the average of many years. This is possibly 

 true in European trees, and in those of other cold or tempe- 

 rate regions in which the seasons are distinctly marked ; in 

 such the zones are not only separated with tolerable precision, 

 but do not vary much in annual dimensions. But it is not 

 absolutely true in Europe, as is proved by the Beet, whose root 

 forms several zones during the growth of one year; and in many 

 hot countries the difference between the growing season and 

 that of rest, if any occur, is so small, that the zones are as it 

 were confounded, and the observer finds himself incapable of 

 distinguishing with exactness the formation of one year from 

 that of another. In the wood of Gruaiacum, Phlomis fruticosa, 

 Metrosideros polymorpha, and many other Myrtleblooms 

 (Myrtaceae), for instance, the zones are extremely indistinct ; 

 in some Bauhinias they are formed with great irregularity ; 

 and in Stauntonia latifolia, some kinds of Ficus, certain 

 species of Aristolochia, as A. labiosa, and many other plants, 



