SECT. I MORPHOLOGY 77 



The cell walls never consist entirely of pure cellulose, but contain 

 a considerable amount of other substances, which are not stained blue 

 by chlor-zinc-iodide. In unlignified cell walls PECTIN substances 

 are particularly prominent. They are easily distinguished by the 

 readiness with which they dissolve in alkalies, after being previously 

 acted upon by a dilute acid. 



Susceptibility to certain stains — for example, congo red— is a claaracteristic of 

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

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

 higher plants during cell division consists almost wholly of pectin substances ; 

 the next developed lamellae, the secondary cell-wall layer, of a mixture of cellulose 

 and pectin substances ; the last formed, or tertiary layer, chiefly of cellulose. 



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

 cellulose and the pectin substances, mention must be made of callose. It is 

 characterised by its insolubility in cuprammonia and solubility in soda solution, 

 and in a cold 1 per cent solution of caustic potash. It is coloured a red brown by 

 chlor-zinc-iodide, with aniline blue it takes 'an intense blue, and with coralliu 

 (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 calcified cell-wall 

 layers, as, for example, in cystoliths (Fig. 83). Chitin, a proteid substance, 

 according to Gilsox, takes the same place in the cell walls of the Fungi as 

 cellulose in the cell walls of the higher plants ("■*). 



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. 



Lignification depends on the introduction into the membrane of certain 

 substances, and lignified cell-walls are characterised by staining yellow with acid 

 aniline sulphate, violet with phloroglucin and hydrochloric acid, and red with 

 ammonia after previous treatment with a one-per-cent solution of permanganate of 

 potash. "With chlor-zinc-iodide lignified walls stain yellow, not blue. The sub- 

 stances to which these lignin-reactions are due have not yet been recognised with 

 certainty. Czapek ("^) ascribed them to an aromatic aldehyde which he named 

 hadromal, but this has been disputed and a return made to an older view which 

 ascribes the same role to vanillin ('^). 



Special aromatic substances which increase the j)o\vers of resistance of the 

 membrane infiltrate the cell walls of Bryophyta ("). A definite but not clearly 

 recognised substance impregnates the walls of various tissues in Ferns and gives them 

 the characteristic dark brown colour. It may precede the deposit of lignin ("*). 



Corky cell walls contain suberin and take a yellowish brown colour with chlor- 

 zinc-iodide ; with caustic potash, a yellow colour. Van Wisselingh C"^) has 

 disputed the presence of cellulose in suberised cell walls, and regards the cork 

 sul)stance 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 in- 

 fusible, 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 cellulo.se 

 cell walls. 



