74 ORGANOGRAPHY. BOOK I. 



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 sea- 

 sons, or after transplantation, or in consequence of any causes 

 that may have impeded its growth, the formation of wood is 

 so imperfect as scarcely to form a perceptible zone : yet De 

 Candolle has endeavoured to show, in a very able paper, Sur 

 la Longevite des Arbres, that the general accuracy of calcula- 

 tions is not much affected by such accidents ; occasional inter- 

 ruptions 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 temperate regions in which the sea- 

 sons are distinctly marked; in such the zones are not only 

 separated with tolerable precision, but do not vary much in 

 annual dimensions. But 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 ob- 

 server finds himself incapable of distinguishing with exactness 

 the formation of one year from that of another. In the wood 

 of Guaiacum, Phlomis fruticosa, Metrosideros polymorpha, 

 and many other Myrtaceae, for instance, the zones are ex- 

 tremely 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, they are so confounded, that there is not the 

 slightest trace of annual separation. It is also to be remarked, 

 that in Zamias we seldom find more than two or three zones 

 of wood, whatever may be the age of the individual ; and yet 

 it appears from Ecklon's observations, that a Zamia, with a 

 trunk only four or five feet high, can scarcely be less than two 

 or three hundred years old. {Lehm. Pug ill. vi.) 



With regard to judging of the age of a tree by the inspec- 

 tion of a fragment, the diameter of the stem being known, a 

 little reflection will show that this is to be done with great 

 caution, and that it is liable to excessive error. If, indeed, 

 the zones upon both sides of a tree were always of the same, 

 or nearly the same, thickness, much error would, perhaps, not 

 attend such an investigation ; but it happens that, from vari- 

 ous causes, there is often a great difference between the growth 

 of the two sides, and consequently, that a fragment taken from 



