THE SKELETON OF THE PLANT 41 



tissue of the flowering plants is less resistent, giving very 

 easily the reactions which have been just described. A 

 third variety is hydroly sable 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, 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 from cellulose convert it into pectine, or into 

 pectic acid, the former being soluble in water, the latter in 

 alkalies. The cell- wall can be shown to contain the two 

 constituents by the action of Schweizer's reagent, which, 

 when used with proper precautions, dissolves out the 



