46 VEGETABLE PHYSIOLOGY 



with which they can be made to undergo hydrolysis, and to 

 yield some variety of sugar by such treatment. The cellu- 

 loses of cotton fibres are perhaps the most resistent of all, 

 and may be taken as representatives of the most refractory 

 group. The cellulose found in the main mass of the funda- 

 mental tissue of the flowering plants is less resistent, giving 

 very easily the reactions which have been just described. 

 A third variety is hydrolysable with still greater readiness. 

 It is to a certain extent soluble in alkalies, and is easily 

 decomposed by acids with formation of other carbohydrates 

 of low molecular weight. Such cellulose is represented in 

 the cell-walls of most seeds. 



It is probable that cellulose is chemically combined 

 with a certain amount of water, and that the degree of such 

 hydration differs in the different varieties described. 



Though, as already stated, the cell-wall is commonly 



said to be composed of cellulose, the latter material is always 



associated with other constituents. Among the latter we 



find various members of another group known as pectoses, 



which differ in many ways from cellulose. This group 



includes two series of bodies which vary among themselves 



as to the degree of their solubility in water. One of these 



series comprises bodies of a neutral reaction, while those 



of the other are feeble acids. In each series there are 



probably several members, which show among them every 



stage of physical condition between absolute insolubility 



and complete solubility in water, the intermediate bodies 



exhibiting gelatinous stages, characterised by the power of 



absorbing water in a greater or less degree. 



Of the neutral series the two extremes are known as 

 pectose and pectine. The former is insoluble in water j and 

 is closely associated with cellulose in the substance of most 

 cell-walls ; the latter is soluble in water and forms a jelly 

 with more or less facility. Pectose has not yet been obtained 

 pure, in consequence of its close association with cellulose 

 and the readiness with which it undergoes change in the 

 process of extracting it. The reagents which separate it 



