80 BOTANY 



by chloroiodide of zinc. In unlignified cell walls pegtose is parti- 

 cularly prominent. It is easily distinguished by the readiness with 

 which it dissolves in alkalies, after being previously acted upon by a 

 dilute acid. 



Susceptibility to certain stains, for example Congo red, is a characteristic of 

 cellulose ; while other stains, such as safranin and methylene blue, colour pectose 

 more deeply. According to Mangin, the partition wall formed in the higher 

 plants during cell division consists almost wholly of pectose ; the next developed 

 laminae, the secondary cell-wall layer, of a mixture of cellulose and pectose ; the 

 last formed, or tertiary layer, chiefly of cellulose. If the secondary layer of the 

 cell wall remain unlignified, the amount of pectose contained in it increases with 

 age and helps to strengthen the middle' lamella, or primary cell- wall layer. 



Among the substances entering into the composition of cell walls, in addition 

 to cellulose and pectose, mention must be made of callose. It is characterised by 

 its insolubility in euprammonia and solubility in soda solution, and in a cold 

 1 per cent solution of caustic potash. It is coloured a red brown by chloroiodide 

 of zinc, with aniline blue it takes an intense blue, and with corallin (rosolic acid) 

 a brilliant red. Its presence in the higher plants is limited to a few special cases ; 

 it envelops the sieve-pits and is always present in 6aloified cell-wall layers, as, for 

 example, in cystoliths (Fig. 88). According to Mangin, callose exists in the cell 

 walls of the Fungi and Lichens, generally in combination with cellulose, or more 

 rarely with pectinaceous substances. Gilson asserts, on the other hand, that the 

 cell walls of all the Fungi that he has thoroughly investigated, consist of a 

 special nitrogenous substance, which he has called mycosin, and considers that it 

 corresponds to animal chitin. This chitin, according to Gilson, takes the same 

 place in the cell walls of the Fungi as cellulose in the cell walls of the higher plants. 

 In addition to chitin, the cell walls of Fungi always contain carbohydrates. 



Where cell walls become lignified or suberised, it is particularly 

 the secondary layer that receives the wood or cork substance, while 

 the tertiary or internal layer retains its cellulose character. 



The lignification is occasioned by the deposition in the cell wall of certain 

 substances, among which are always coniferin and vanillin. It is these two 

 substances which give the so-called wood reactions, — a violet colour with phloro- 

 glucin and hydrochloric acid, a yellow colour with anilin sulphate. "With 

 chloroiodide of zinc a lignified cell wall becomes yellow, not blue. 



Suberised cell walls take a yellowish brown colour with chloroiodide of zinc ; 

 with caustic potash, a yellow. Van Wisselingh has lately disputed the presence 

 of cellulose in suberised cell walls, and regards the cork substance or suberin as 

 a fatty body, which is composed of glycerine esters and other compound esters as 

 well as of one or more other substances which are infusible, insoluble in chloroform 

 and decomposed by a solution of caustic potash. 



Cutinisation, which is similar to but not identical with suberisa- 

 tion, is usually due to the subsequent deposition of cutin in cellulose 

 cell walls. 



Van Wisselingh has shown that phellonic acid, which is always present in 

 suberin, is constantly absent in cutin. Cutin withstands better the action of 



