EPIDERMIS. 8 J 



The cuticle diflfers from the cell-membranes in its material composition : it consists at 

 all events in the main of cutin, even though Hofmeister's ^ observation that the cuticle 

 of Orchis Morio and Hoya Carnosa, after maceration for three weeks in potash, be- 

 came ' distinctly ' blue with solution of I in K I should prove that cellulose was 

 present {though this point nuill not be conceded). The question then arises, what is the 

 origin of cutin, from what material, and where is it produced? Since it can only 

 originate primarily from the epidermal cells, and appears originally on the apparently 

 pure cellulose membrane, and later also is seen within this, in the cuticular layer, and 

 since it is never found in the protoplasm and cell sap, its origin in the cellulose membrane 

 and from cellulose is probable, though a safe proof of this cannot be drawn from the facts 

 at our disposal. 



It need not here be emphasised that the cuticle is separated from the very first as a 

 sharply defined lamella, for which a definite structure from the very first may be 

 assumed ; and that the old view, which, in cases of excretion or secretion, imagined a 

 fluid mass filtered out, and hardened sooner or later, is here as little apposite as is the 

 case with most of the other 'secretions.' Comp. Sects. 17, 19. 



As regards the origin of the cutin which impregnates the cuticular layer of the cell-walls, 

 the same holds as has been said for the cuticle. Relatively to the formation and growth of 

 this layer, it may certainly be observed in many cases (which should moreover be more 

 carefully studied) that a progressive cuticularisation of the cellulose membrane proceeds 

 gradually from without inwards ; thus in the above-cited examples of Psilotum and Sela- 

 ginella. But in the overwhelming majority of cases — on which, however, still more thorough 

 investigation is necessary — e. g. leaves of Agave Americana', species of Aloe, branches of 

 Acer striatum ^ the cuticular layers are first formed with their definitive sharp outline as 

 thin lamellae, and grow as cutin-containing layers till they attain their definitive thickness, 

 being always sharply marked off from the cellulose layer which grows with them. It 

 may be thought that their increase depends on cuticularisation which progresses in- 

 wards, but this is not evident on observation. 



The opinion of Hartig and Karsten *, according to which the cuticle is a development 

 from the mother-cell-wall of the plant, has hardly any longer a historical interest in the 

 face of what we now know, especially concerning its restoration, its entry into the 

 stomata, and concerning the internal cuticle. 



The chemical composition of the body called cuticular substance, or, after Fremy, cutin, 

 which forms the cuticle, and is contained in the cuticular layers, is not yet quite clearly 

 known. It is indeed very probable that we have not in all cases to do with the same 

 body, that for instance important differences exist between the deep-brown coloured 

 substance of the cuticularised epidermis of Ferns, and the colourless or slightly coloured 

 cuticular substance of the epidermis of Phanerogams. Investigations on the body in 

 question refer almost exclusively to the latter. On the ground of Payen's ^ works, the 

 cuticular substances were regarded as nitrogenous bodies— products of the change of 

 cellulose by combination with a nitrogenous compound. 



Fremy ^ isolated the cuticle of various leaves, parts of flowers, and fruits (Iris, Camellia, 

 apples), and found the elastic, extensible skin, which retained its original structure 



^ Pflanzenzelle, p. 257. 



' Compare Oudemans, Memoire sur les stomates, &c. ; C. Rend, de I'Acad. d' Amsterdam, 

 vol. XIV. 



' Compare Botan. Zeitg. 1871, p. 596, Taf. II. figs. 29-35. 



* Botan. Zeitg. 1848, p. 730. — Compare Hofmeister, I.e. p. 251. 



* Details in Hofmeister, /. c. p. 249, &c. 



"■ Comples Rendus, torn. 48, p. 669— Ann. Sci. Nat. 4 ser. torn. XII. p. 331.— On Payen's 

 replies to Fr^my compare Comptes Rendus, torn. 48, p. 893. Further, the revision of the discussion 

 in point in Kopp and Will, Jahresbr. ii. d. Fortschritte der Chemie fur 1859, p. 529, &c., especially 

 PP' 536, 539. 



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