98 H. VON MOHL ON CELLULOSE. 



wetting it again with water, and if requisite also a second time 

 with tincture of iodine. 



A similar result is obtained by cutting off a section of the 

 cuticle parallel to the surface of the leaf, and treating it in the 

 same manner with potash and iodine. The cross-sections of the 

 side-walls of the epidermal cells, composed of cuticular substance, 

 exhibit exactly the same appearance as the other thick-walled 

 cells, composed of a number of superimposed layers ; between 

 them runs an external membrane common (?) to the two adjacent 

 cells, which has frequently a yellow or greenish colour at the 

 first wetting with water, but becomes likewise blue after a 

 repetition of the operation. When the cells have separated from 

 each other, this outer membrane is torn irregularly and hangs in 

 fragments attached to one or other of the contiguous cells. 



The cuticle of other fleshy or leathery leaves, for instance of 

 Aloe margaritifera, Hoy a carnosa, Hakea pachyphylla, Hakea gib- 

 bosa, &c,, behaves exactly in the same way as that of Aloe obliqua. 



These statements place it beyond doubt that the cuticle of the 

 leaves mentioned is not a homogeneous layer of substance dif- 

 ferent from cellulose, secreted upon the surface of the epidermis, 

 but that this membrane is composed of separate segments corre- 

 sponding to the epidermal cells ; that it is composed of numerous 

 superincumbent lamellae of cellulose ; and that its chemical 

 diversity from cellulose arises from the infiltration of a substance, 

 coloured yellow by iodine, which not only resists the action of 

 sulphuric acid itself, but protects the cellulose which is saturated 

 with it from the influence of sulphuric acid and iodine. The 

 result is the demolition, in regard to the thick cuticle of thick- 

 walled epidermal cells, of the evidence advanced on chemical 

 grounds against the view I formerly advanced [Vermischte 

 Schriften, p. 260) of the structure of cuticle, namely, that it is not 

 a coating over the epidermis composed of substance excreted 

 from the latter, but owes its origin to a metamorphosis of a 

 portion of the outer w alls of the epidermal cells. 



The infiltrated substance was not totally extracted by macera- 

 tion of the epidermal cells for 24-48 hours in solution of potash, 

 for the addition of sulphuric acid to the preparation saturated 

 with iodine immediately reproduced the brown colour which is 

 caused in cuticle by these reagents before it is treated with 



