CHEMICAL COMPOSITION OF CELL-WALLS 47 



hand, it seems not altogether improbable that the intact membrane 

 may in certain cases consist of similar particles. Whether, however, 

 the dermatosomes are formed from correspondingly minute protoplasmic 

 particles or " plasomes," as Wiesner maintains, or whether, on the 

 contrary, they arise by differentiation within a primarily amorphous 

 membrane must remain undecided for the present. Similarly nothing 

 definite can be predicated concerning the material which is dissolved 

 away during the process of carbonisation, and which presumably serves 

 as a cement for holding the dermatosomes together. 



The principal chemical constituents 20 of typical vegetable cell- 

 membranes are carbohydrates of the cellulose group. Cuprammonia 

 (Schweizer's reagent) is the only liquid in which cellulose is soluble 

 without decomposition. Cellulose is rapidly dissolved, and at the 

 same time decomposed, by concentrated sulphuric acid. The classical 

 colour reactions of cellulose walls are, first, the bright blue coloration pro- 

 duced by iodine in presence of concentrated sulphuric acid and, secondly, 

 the appearance of a colour varying between blue and pinkish violet on 

 treatment with chlor-zinc-iodine (Schulze's solution). The so-called 

 hemi- celluloses form a large proportion of the thickening layers in 

 thick-walled storage tissues, especially in the case of endosperms ; 

 these compounds are more easily hydrolysed than the genuine cellu- 

 loses, and are hence readily mobilised and absorbed during germination. 

 A somewhat similar " reserve-cellulose " is amyloid, a body which is 

 coloured blue by iodine alone [i.e. without previous treatment with an 

 acid]. A number of other organic compounds are widely distributed 

 in cell-walls. The so-called pectic substances, which are closely allied to, 

 if not identical with, the he mi-celluloses, are remarkable on account of 

 their mucilaginous or gelatinous consistency ; by treatment with dilute 

 acids they become readily soluble in alkalies. Chitin is an important 

 constituent of the cell-walls of Fungi. Lignified cell-walls contain 

 several characteristic substances, such as xylan and the liejnie acids, 

 which latter compounds are perhaps chemically related to the still 

 undetermined principal component of lignified membranes ; most 

 conspicuous of all, though present in relatively small quantities, are 

 those bodies which are responsible for such characteristic colour 

 reactions of lignified walls as the yellow coloration with aniline 

 sulphate or chloride and the cherry -red tint produced by phloroglucin 

 in presence of hydrochloric acid. Singer among others ascribes the 

 reactions in question to the presence of vanillin, while Czapek regards 

 the aromatic aldehyde hadromal, isolated by him from lignified tissues, 

 as the cause of the aforesaid colorations. Suberised and cutinised (or cuti- 

 cularised) cell-walls are impregnated with substances of a fatty nature ; 

 walls of this kind are insoluble in sulphuric acid, and are turned yellow 



