38 
ORGANOGRAPHY. 
BOOK Ir 
a part once fully formed is incapable of any subsequent change. 
Thus, pith never alters its dimensions, after the medullary 
sheath that encloses it has been once completed, and a zone of 
wood never contracts or expands after it has been deposited : 
new matter may be added to any part, but the arrangement of 
the tissue, once fixed, remains unchangeable. 
The principal argument, however, in favour of cuticle being 
compressed cellular tissue, is, that in the cuticle of many 
plants the cellular state is distinctly visible upon a section 
(Plate 1. fig. 2. a) ; that it even consists occasionally of several 
layers of bladders, as in the Oleander and many epiphytes of 
the Orchis tribe ; and that, as there is no reason to doubt 
that Nature is as uniform in the plan upon which cuticle is 
constructed as in all her other works, in those cases in which 
the cellular structure is less distinctly visible, we are never- 
theless justified by sound philosophy in recognising it ; while, 
on the other hand, it would be highly unphilosophical to sup- 
pose that the cuticle is formed in some plants upon one plan, 
and in others upon a totally different one. It may be farther 
remarked, that separable cuticle may often be traced into 
that which, being younger, is both inseparable and un- 
distinguishable from the other cellular substance with which 
it is in contact, and from which it possesses no organic 
difference. 
There is some reason to suppose that there is occasionally 
present, on the outside of the cuticle, a transparent, very deli- 
cate membrane, having no organic structure, as far as can be 
discovered with the most powerful microscopes. Some- 
thing of this kind has been noticed by Adolphe Brongniart 
in the Cabbage leaf ; an analogous structure has been re- 
marked by Henslow in the Digitalis ; and I have found it 
very conspicuous on the upper side of the leaves of Dionaea 
muscipula. It can however be found only after long ma- 
ceration of the parts ; and consequently we are uncertain 
whether to regard it as organic, which is not probable, or 
inorganic like the cuticle of man, and caused either by the 
decomposition of part of the cuticle, or by some secretion 
from it. Adolphe Brongniart has paid some attention to this 
subject {Ann, des, Sc, 2 scr. 1. 65.), and finds the pellicle by 
