THE PLANT-CELL. 19 



the cells have lost their protoplasmic contents ; but it must be 

 borne in mind that their formation can only take place in a 

 living plant. 



The epidermal cells of leaves afford good examples of 

 cuticularisation. If a section of a coriaceous leaf be treated 

 with iodine and sulphuric acid, it will be found that the thick- 

 ened external walls of the epidermal cells present a series of 

 layers which have assumed a blue colour, the colour being 

 most intense in the most internal layer, and becoming gradu- 

 ally less evident towards the free surface, the external layers 

 shewing it scarcely at all. From this it appears that the pro- 

 portion of suberin to cellulose gradually increases from the 

 internal to the external layers. The cuticularised external 

 layers of the walls of adjacent cells may be easily peeled off 

 over considerable areas of the epidermis as a continuous 

 membrane, interrupted only by the stomata; to this the name 

 of cuticle is given. Cork also consists of cells the walls of 

 which have undergone cuticularisation ; but here the whole 

 extent of the cell-wall is affected, and not merely a part of it 

 as in the epidermal cells. In the case of cork cells which are 

 in contact with other cells on all sides, the most external 

 layer of the cell-wall is lignified to form the middle lamella. 

 The effect of cuticularisation is to make the cell-walls more 

 resistent both chemically and physically ; thus the cuticle and 

 the walls of old cork-cells are not affected by treatment with 

 mineral acids, nor are they soluble in ammoniacal solution of 

 cupric oxide ; their double refraction is well marked, and, 

 instead of being negative like that of unaltered cell-walls, it is 

 positive; they are more elastic than ordinary cell-walls; their 

 capacity for taking up water is very small, so small, in fact, 

 that they may be regarded as almost impermeable to water. 

 This impermeability to water doubtless depends to some 

 extent upon the presence of wax or of resinous substances 

 which prevent direct contact between the membrane and the 

 fluid. 



Fre'my isolated the cuticle of various leaves; after he had removed 

 foreign bodies as far as possible, a substance remained to which he gave 

 the name of Cutin. Von Hohnel has recently found that the walls of 



2 2 



