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 endeavoiu'ed 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, Pugill. 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 alwa^'s of the same, 
or nearly the same, thickness, much error Avould, 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 
