4 
OX THE LATERAL ACCRETION OF THE 
siderable portion of it is expended in the formation of a concentric layer of 
woody substance betwixt the bark and the wood on every branch, and 
also on the stem, which layer becomes the new alburnum/’ pp. 86, 87 . 
The gravamen of this passage is neither more or less than that 
wood, with all its complicated structure of fibres, cells, and ves¬ 
sels, is formed by, or generated from, the (t proper juice” of the plant. 
On this statement a very natural question arises, viz. How can 
this be ? 
Before attempting to answer the question, or to examine the state¬ 
ment to which it refers, it may be as well first of all to describe the 
phenomena of the growth of the new alburnum, as it appears to the 
eye during the summer months. 
In early spring the new alburnum is scarcely discernible, having 
neither appreciable consistence nor apparent organisation; but as the 
season and growth advance, it becomes conspicuously visible, and per¬ 
fectly distinct as well from the liber on its exterior, as from the last 
year’s alburnum on its interior side. It separates these two members, 
the former being raised off from the latter by the incipient alburnum, 
which is of a viscous consistence, and is then called cambium by 
botanists. 
Soon after midsummer the new membrane begins to show, when 
examined by a magnifying glass, faint signs of organisation. The lon¬ 
gitudinal fibrous tissue, and sap or air vessels which lie in the same 
direction, together with the horizontal lying cells among which the 
fibrous and vascular apparatus are embedded, begin to be visible; and in 
a month or six weeks following, the new alburnum is complete as to the 
formation of its parts, and continues hardening into timber in the 
course of the autumn. Before the new layer of alburnum ceases to 
grow, its exterior surface is sloughed off, to form the new liber or inner 
bark, which ever after remains distinct. 
In order to observe this process, a smooth-barked, free-growing tree 
must be chosen. Into and through the bark of this, weekly or monthly 
incisions must be made, to expose the growing alburnum, and to watch 
its changes from a state of colourless lymph up to perfect wood ; or, by 
observing how a wound made in pruning is gradually covered by this 
same membrane, a goodidea may be formed of its progressive increase 
and change from its first to its last condition. 
This description of the summer growth of the alburnum has been 
verified by so many eminent naturalists, and, moreover, is so obviously 
evident to every one who attends to the accretion of a laying tree, or 
looks at a transverse section of a branch or stem after it is felled, must 
