794 FUNCTIONS OF THE PARTS OF THE STEM. 
temporary existence, and thus enable them to be freely exposed 
to the influences of light and air, which are essential for the 
proper performance of their functions and development. 2. To 
convey air and fluid matters upwards, outwards, downwards, 
and inwards, to the organs of respiration, assimilation, trans- 
piration, development, and secretion. And 3. To act as a 
reservoir for the so-called secretions of the plant. 
A. Special Functions of the Different Parts of the Stem.—a. The 
Medulla or Pith.—Various functions have at different times been 
ascribed to the pith. In the very young plant, and in all cases 
when newly formed, the cells of the pith are filled with a greenish 
fluid containing nutrient substances in a state of solution ; but 
as the pith increases in age it loses its colour, becomes dry, and 
is generally more or less destroyed. The pith, therefore, would 
appear to serve the temporary purpose of nourishing the parts 
which surround it when they are in a young state ; and in some 
cases it seems also to act as a reservoir of the secretions of the 
plant. 
b. The Wood or Xylem.—The wood, when in a young and 
pervious condition (alburnwm), is the main agent by which the 
crude sap is conveyed upwards to the external organs to be 
aerated and elaborated ; but whether the passage is primarily 
by the vessels or the prosenchymatous cells is disputed. (See 
page 785.) As the wood increases in age, and becomes heart- 
wood or duramen, the tissues of which it is composed become 
thickened and altered in various ways, by which they are more 
or less hardened and solidified, and in this manner the stem 
acquires strength and firmness, but the tissues are no longer 
physiologically active, and are in fact useless as carriers of sap. 
Formation of Wood.—On the outside of the young wood, 
but organically connected with it and with the liber or bast 
of Dicotyledons, is the vitally active layer of cells (secondary 
meristem) called the cambium layer, from which are annually 
formed new layers of wood («ylem) and inner bark (phloém). 
The cells of the cambium layer are filled in the spring, and at 
other seasons when growth takes place, with elaborated sap, or 
that sap which contains all the materials necessary for the 
development of new structures. Great differences of opinion 
exist amongst botanists as to the exact manner in which wood 
is formed, but they are nearly all agreed that the materials 
from which it is formed are elaborated in the leaves, that with- 
out leaves there can be no additions to it, and that in proportion 
to their amount so will be the thickness of the wood. It is 
necessary, therefore, that the process of pruning timber trees 
should be carefully conducted, and that when planted they should 
be placed at proper intervals, in order that they may be freely 
exposed to those influences which are favourable for the develop- 
ment of their foliage. 
Herbert Spencer believes that intermittent mechanical 
