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, 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 
