THE VEGETABLE CELL 



S5 



Fig 40. 



iX> 



Cells of the epidenni& of tiho upper 

 face of tlae leif of Mmia car w^a a, 

 the portion ot tlicii wallb *ic(iiniiu„ a 

 3 dlow colour witli lodme 



Fiff. 41. 



the enter wall exliibitb tliese properties ilowu to a certain ileptlij so that 

 a layer (fig 4:0, a) is thus formed, which is 

 most {strikingly distinct from the subja- 

 cent ceils, and when the latter have been 

 dissolved in sulphiu'ic acid, remains behind 

 as a continuous and apparently homogene- 

 ous membrane. Since Ad. Brongniait 

 (''Ami. des Be. Nat, jSer,,'' § i, 65) had dis- 

 covered that a continuous membrane, not 

 composed of cells, called by him cuticula, 

 might be separated by maceration from 

 the outer surface of the epidermis, it ap- 

 peared natural to suppose that the layer just spoken of, which is fre- 

 quently very thick and is coloured brown by lodme and sulphuric acid, 

 was this cuticula, and to ascribe its origin to a secretion upon the outside 

 of the epidermis-cells, a process of which Schleiden ev^n gave a detailed 

 description (" Gnmdz. der wiss. Bot.^'' 1st ed. i, 2d>B). This view, however, 

 proposed by Trevii*anus, and defended by linger, Harting, Mulder, and 

 otheis, is in great part wx'ong. The so-called cuticle consists, with the 

 exception of a layer extremely tlim in most 

 plants (fig. 41, a), of the thickened walls of 

 the cells, which are infiltrated with a sub- 

 stance coloured brown by iodine, to which 

 they owe their power of resisting the action 

 of sulphuric acid. When this substance is 

 removed by caustic potash, not only is the 

 composition out of cell-membranes evident, 

 since the separate layers of these become 

 visible, but iodine now very readily produces a 

 blue colour (^'Bot. Zeitung,'' 1847, 592). This 

 composition of the so-called cuticle, of cell- 

 membranes, is seen beyond all doubt in the epidermis of an old stem of 

 Viscum alhwn {^g. 42) ; the epidermis-cells consist here of two or three 

 geneiations enclosed one within 

 another, of which all the thick- 

 ened walls on the outer side 

 have become blended together 

 into a membrane composing the 

 cuticle (H. V. Mohl, " Ob the 

 Bpidermis of Viscum album;' 

 ~Bot Zdtmig, 1849). T call 

 these layers belonging to the 

 epidermal cells the cuticular 

 layers of tJie epider^nis, to dis- 

 tingiiish them from the mass 

 secreted on the outside of the 

 ceUs, the true cuticle, which is 

 soluble in caustic potash, in 

 most cases forms but a very 

 thin coating over the epidermal 

 cells, and only rarely, as in the 



shoots of Mphedra, and the upper surface of the leaves of Ofca% forms 



B 2 



The epideraijs of the npper si<!e 

 of the leaf of Haifa carnmatTevntetl 

 V, ith caiwtic alkali a, the cuticle 

 soparating-, 5, the swollen, lami- 

 nated cuticular layers of the epider- 

 mal cells 



Fig. 42. 



)xir^r 



Epidermis of an old stem of VUcnmi alfyum^ 



