CARBOHYDRATES 459 



tural organization, it has been considered under these headings 

 (page 334). 



Cellulose. — Cellulose chemists recognize not one substance 

 which is cellulose but a group of substances, the celluloses. 

 Modern research has produced an alpha cellulose which is as 

 near a chemical entity as any cellulose heretofore attained, but 

 while this may be regarded as a definite cellulose, there are 

 others. The naturally occurring celluloses are of three groups — 

 the true, the compound, and the hemi- or reserve celluloses. 

 Among the first, that of the cotton fiber is the purest, being 

 90 per cent true cellulose. Compound celluloses are true cel- 

 luloses impregnated with other substances. The hemicelluloses 

 are incompletely developed forms of cellulose and other carbo- 

 hydrate materials such as araban, xylan, etc. In spite of this 

 apparent variety, it does not appear that the celluloses of the 

 various seed-bearing plants are actually different chemical sub- 

 stances; that is to say, while physical differences {e.g., fiber 

 length) exist, and chemical differences in the constitution of the 

 cellulose of the original wood may exist, the residues, termed 

 cellulose, obtained from different woods are probably identical 

 in chemical structure. 



Protoplasm, as it builds the plant-cell wall, simultaneously or 

 subsequently secretes substances that occur either as distinct 

 layers alternating with the cellulose or, more usually, as an 

 impregnation of it. Such substances are lignin, suberin, pectin, 

 and cutin. Old wood is lignified cellulose, and cork is suberized 

 cellulose. Pectin may form distinct layers in the cell wall 

 alternating with cellulose, or it may be separately deposited. 

 In general, pectin compounds impregnate the wall, forming so- 

 called pectocelluloses. Cutin is often a surface deposit and 

 occurs as the waxy coating on glossy leaves and fruits. To be 

 superficially deposited, it must pass through the cellulose wall 

 and in so doing adds to the chemical complex that we call natural 

 cellulose. 



The hemi- or reserve celluloses constitute an interesting group 

 which differs structurally from the fibrous celluloses. They are 

 more readily hydrolyzed than the true celluloses and break down 

 into sugars (galactose and pentose) of which they are regarded 

 as the anhydrides and from which they receive their names 

 (galactosans and pentosans). 



