38 BOTANY 



reaction holds for many hemicelluloses which are also polysaccharides. 

 The cell wall nearly always contains other substances in considerable 

 amount, some of which are stained other colours than blue by chlor- 

 zinc-iodide. The PECTIC SUBSTANCES are especially important ; these 

 take a yellow colour with this reagent. It depends on this that many 

 " cellulose walls " do not give a pure blue with chlor-zinc-iodide but 

 stain violet, brownish violet, or brown. CHITIN is present in the walls 

 of most Fungi and Bacteria. This substance, formerly regarded as 

 peculiar to the animal body, replaces cellulose in the case of 

 the Fungi ( 33 ). 



The celluloses are insoluble in dilute acids and alkalies ; even concentrated 

 potash solution does not dissolve them. They are, on the other hand, soluble in 

 ammonia-oxide of copper, by concentrated sulphuric acid after conversion into 

 dextrose, and by a special enzyme (cytase) formed by plants. After treatment 

 with sulphuric or phosphoric acid a watery solution of iodine will colour them blue, 

 and a similar reaction is obtained by the simultaneous action of a concentrated 

 solution of certain salts, such as zinc-chloride or aluminium-chloride, and of iodine. 

 Accordingly chlor-zinc-iodide, on account of the blue or violet colour imparted by 

 it, is one of the most convenient tests for cellulose. The name of hemicelluloses is 

 given to a series of substances which are nearly related to the celluloses, but are 

 transformed by even dilute acids into soluble sugars other than dextrose. They 

 are often insoluble in ammonia-oxide of copper. As the celluloses are poly- 

 saccharides with large molecules produced from hexoses (C tj H 12 6 ), the pento- 

 sanes (C 5 H 8 4 ) n are corresponding condensation products of peutoses (C 5 H 10 5 ) 

 such as arabinose and xylose. The pectins are characterised by the ease with 

 which they dissolve in alkalies after previous treatment with dilute acids. In 

 contrast to cellulose, they- stain deeply with safranin and niethylene blue. The 

 pectins are complex compounds in which monohexoses, pentosane, and in addition 

 methyl alcohol behaving as an ester and calcium and magnesium behaving as salts, 

 are united to tetragalacturic acid (G^sfizsi a condensation product of galacturic 

 acid C 6 H 10 7 ) (*). 



Chitin is a polysaccharide containing nitrogen ; it contains acetyl-acetic-acid in 

 an acid-amide-like combination. 



The cell wall frequently undergoes chemical changes of various 

 kinds during the life of the cell ; sometimes layers already deposited 

 change, in other cases the newly deposited layers are different from 

 those first formed. These transformations stand in the closest relation 

 to the requirements of the plant to which the cells contribute. As 

 regards "cellulose walls," these in young cells are less elastic but 

 relatively more extensible than in older cells ; this is advantageous in 

 relation to the active growth in length of young parts. Such walls 

 offer little resistance to the diffusion of water and dissolved substances. 



Cellulose walls not infrequently become MUCILAGINOUS, their sub- 

 stance being transformed into a gelatinous or mucilaginous mass which 

 swells greatly in water. Frequently cell walls undergo LIGNIFICA- 

 TlON, SUBERISATION, or CUTINISATION. Lignification diminishes the 

 extensibility of the cells considerably, and increases their rigidity 



