38 ORGANOGRAPHY. BOOK I. 



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 cellulai* tissue, is, that in the cuticle of many 

 plants the cellular state is distinctly visible upon a section 

 (Plate I. 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 Dionsea 

 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 ser. I. 65.), and finds the pellicle by 



