94 
ORGANOGRAPHY. 
BOOK I. 
was originally colourless, and owes its different tints to matter 
deposited gradually in all parts of the tissue ; as may be easily 
proved by throwing a piece of heart-wood into nitric acid, or 
some other solvent, when the colouring matter is discharged, 
and the tissue recovers its original colourless character. That 
part of the wood in which no colouring matter is yet depo- 
sited, and consequently that which, being last formed, is in- 
terposed between the bark and duramen, is called alburnum. 
The distinction between these is physiologically important, as 
will hereafter be explained. 
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 examination ; 
and further, that, as there is not much difference in the aver- 
age 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 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 an able paper, Sur la 
Longevite des Arbres, that the general accuracy of calculations 
is not much affected by such accidents ; occasional interrup- 
tions 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, 
