REVIEW. 
81 
At the end of September, and in many kinds of trees much sooner, 
the sections appear as in Figure 10. 
Sections of a stem at the end of the fifth year. The envelope and layers of 
the liber are too thin to be shown by the pencil. 
Here we observe that a new concentric layer of alburnum has been 
added during the fifth summer, and also an additional layer of liber 
has been parted off, and placed close to that of the preceding year, 
and lined on the inner side with an almost imperceptible membrane 
or coating of gelatinous matter, which is the vital envelope, and from 
which the new growths of wood and liber of the next, and all suc¬ 
ceeding years will be produced. 
Judging, then, from these changes, about which there can be no 
doubt (because of them we have ocular proof,) we may conceive that 
the vital envelope is constructed of an indefinite number of distinct 
concentric layers, two of which are developed annually ; the minor 
one (A, Fig. 11,) being inflated into alburnum, and the outer one 
(B, Fig. 11) into a layer of liber. 
B A (! 1) 
g ft d c b a 
Segment of a transverse section of a tree five years old, magnified; a, growth 
of alburnum first year; b, the second; c, the third; d, the fourth; e , the fifth; 
/, five layers of liber, ideally magnified; g, epidermis and cuticle. 
The appearance of the structure of the alburnum affords confirma¬ 
tions of the reasonableness of this idea. If we examine it as soon as 
it is formed, or in any future stage of its existence, we find the lon¬ 
gitudinal fibres strongly and distinctly marked, and the minute vesi¬ 
cles of the cellular fabric between the fibres posited horizontally; 
showing that they are enlarged in the same direction, that is, ad¬ 
vanced from the centre of the tree outwards. 
G 
