CHAP. II. EXOGENOUS STEMS. 73 
tinues to live. As each new zone is formed over that of the 
previous year, the latter undergoes no alteration of structure 
when once formed: wood is not subject to distension by a 
force beneath it, as the bark is; but, whatever the first ar- 
rangement or direction of its tissue may be, such they remain 
to the end of its life. The formation of the wood is, therefore, 
the reverse of that of the bark ; the latter increasing by addi- 
tion to its inside, the former by successive deposits upon its 
outside. It is for this reason that stems of this kind are called 
Exogenous (from two Greek words, signifying to grow out- 
wardly). According to Dutrochet, each zone of wood is 
in these plants separated from its neighbour by a layer of cel- 
lular tissue, forming part of the system of the pith and bark ; 
but although this is true in certain plants, such as arborescent 
nettles and others, it is by no means a general law. 
After wood has arrived at the age of a few years, or some- 
times even sooner, it acquires a colour different from that 
which it possessed when first deposited, becoming what is 
called heart-wood, or duramen. For instance, in the beech it 
becomes light brown, in the oak deep brown, in Brazil wood 
and Guaiacum green, and in ebony black. In all these it 
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 inter- 
posed 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 
