RUDIMENTS OF THE NATURAL SYSTEM OF BOTANY, 
179 
as every criterion of this kind should be rendered worthy of trust, it is of 
advantage to the student to know that, in the trunks of very aged trees, this 
is not to be traced. Such knowledge will very naturally lead the intelligent 
investigator to the young branches, in all of which he will not fail to detect an 
internal layer of pith. 
Some botanists of considerable eminence have wholly denied the contraction of 
pith ; but the universal observation of practical gardeners is opposed to this con- 
clusion : and, from close scrutiny, we unhesitatingly affirm that, within four or 
six years, the pith of a vigorous elder shoot will decrease to half its original size. 
We are the more tenacious of this position, because it involves one of the funda- 
mental characters of Exogens ; and, unless the solidification of pith be admitted, 
its presence as a mark of distinction falls to the ground ; as every one must have 
witnessed trees in which the pith was entirely obliterated. 
Immediately surrounding the pith of Exogenous plants, is that portion which is 
known by the name of wood. It is the structure of this member that brings them 
beneath the class Vasculares^ while the manner in which it increases alone determines 
it Exogenous. Wood is formed of various kinds of longitudinal vessels and fibres, 
intersected by horizontally disposed cells. It differs from pith, not only in figure 
and position, but also in its capacity for induration and enlargement. Pith, indeed, 
may be regarded merely as a provision of Nature for the support and development 
of the first layer of wood ; and being by this drained of its fluids and vitality, it 
is ever afterwards virtually dead, and quite useless to the vegetable. Wood, 
on the contrary, yearly gives birth to, nourishes, and matures an additional layer 
of the same nature and consistence as itself ; which new deposite performs the 
same office in each succeeding year, perhaps by exudations from its surface, which 
the action of the atmospheric elements solidifies and hardens. 
Numberless theories respecting the formation of wood in Exogens have, at 
various times, been propounded by botanists. In support of all these, both argu- 
ments and facts have been brought forward. It still remains a doubtful and dis- 
puted point ; and is one of those intricacies, to unravel which, great ingenuity, 
labour, and time are required. Whether each new deposite of wood possesses the 
power of exuding a fluid, which, though at first apparently homogeneous, is ulti- 
mately converted into the different forms of cell and vessel, of which the vegetable 
body is constructed ; whether this fluid is imbibed from the soil, and, by being 
subjected to the action of influences peculiar to each tribe of plants, is thus 
transformed and assimilated to the plant's own nature ; or whether, further, the 
leaves are the grand laboratories in which fluids extracted from the earth are 
changed in quality and substance, and thence dispersed as secretions and deposites 
throughout the vegetable frame, we cannot pretend to decide. 
Based on the above three hypotheses, as well as on others we deem it unneces- 
sary to adduce, philosophers and theorists have been prodigal of their suggestions 
as to the process by which wood is elaborated. During the spring of the present 
