2O LECTURE II. 



cork-cells contain a substance to which he gives the name of Suberm, 

 and he points out that Cutin and Suberin are closely allied if not identical 

 substances. This is shewn by their chemical composition and by their 

 properties : 



Cutin (Fremy). Suberin (von Hohnel). Cellulose. 



C. 73'66 74 44*44 



H. 1 1 '37 10 6'i7 



O. H'97 16 49'38 



both cuticle and cork give eerie acid (Doepping) (impure suberic acid, 

 C 8 Hi 4 O 4 ?) when treated for a considerable time with nitric acid, or with 

 nitric acid and potassic chlorate (Schultz's mixture) ; when heated with 

 strong solution of caustic potash a soap is formed in both cases. It was 

 thought, in accordance with de Bary's researches, that the presence of 

 wax in or upon cuticularised epidermal cells was a feature which clearly 

 distinguished cuticularised cells from corky cells, and a similar view was 

 also held with regard to the presence of silica in the cell-wall : von Hohnel 

 has, however, shewn that wax is present in the walls of the cork-cells in 

 the Willow, and silica in those of a number of plants. There is, there- 

 fore, a considerable body of evidence in favour of von HohnePs view as 

 to the identity of cutin and suberin. It is of interest to note that suberin 

 appears to be a substance intermediate between cellulose and vegetable 

 wax. 



Cells with lignified walls occur very commonly in plants, 

 not only in the wood, of which they are especially character- 

 istic, but also in the bast (usually the bast-fibres) and in the 

 ground-tissue (sclerenchyma). They can usually be readily 

 distinguished in a section on account of the yellow or brown 

 colour which is given to them by the lignin, but if the propor- 

 tion of lignin is small, the cell-wall remains uncoloured. In 

 this case the presence of lignin can be readily demonstrated 

 by treating the section with a solution of anilin chloride 

 acidified with hydrochloric acid ; this reagent produces a 

 bright golden-yellow colour in cell-walls which contain even a 

 trace of lignin. As in cuticularised cells, so also in lignified 

 cells the alteration of the cell-wall is most marked in its ex- 

 ternal layer, and least in its internal layer. The external 

 layer of the cell-wall (forming the middle lamella) resists the 

 action of strong sulphuric acid, but is dissolved by heating 

 with Schultz's mixture, whereas the internal layer dissolves 

 in the strong sulphuric acid but resists treatment with Schultz's 



