CHEMICAL NATURE OF THE CONSTITUENTS 25 



of water, and it is for this reason that the two units in cellobiose may be 

 referred to as glucose residues. As an obvious extension of this idea 

 of a 1:4 link, and in consequence of the fact that in cellulose only three 

 — OH groups per glucose residue are available for nitration or acetyla- 

 tion, it follows that the two outer ends of each cellobiose residue in cellu- 

 lose may be in turn joined by 1 :4 linkages to other cellobiose residues. 

 Thus the chemical evidence leads directly to the conception in cellulose 

 of long chain molecules in which the constituent links are /5-glucose 

 residues. This is almost as far, however, as direct chemical evidence 

 goes and it leaves open a number of questions which it is most imperative 

 to answer in terms of such a model of cellulose structure. How long 

 are these chains? How are they aggregated together? Are they pointing 

 in definite directions or are they arranged at random? These are 

 questions which cannot be answered at all, or only by inference, by 

 purely chemical methods and further discussion must be postponed 

 until suitable physical methods of approach have been described. 



Hemicelluloses and pectic substances 



The hemicelluloses, as has been pointed out already, form a complex 

 mixture of substances which have not yet been clearly defined either 

 theoretically or analytically, but in general they are more readily 

 hydrolyzed by acids, and more soluble in alkalis, than is cellulose. The 

 amount of hemicelluloses present in cell walls varies considerably from 

 a very small percentage in cotton hairs, for instance, to something over 

 50 % in some collenchyma cells. It is quite clear that even now, after 

 many years' continuous attention, it is impossible completely to solve 

 the question of the constitution of these substances — it is not even 

 possible to say whether the different sugars available from their 

 hydrolysis arise from the same molecular species or not — but for present 

 purposes they may be classed as two main groups, the polyuronides, 

 including the pectic substances, and the cellulosans. 



The polyuronides 



Corresponding to the polysaccharides, the polyuronides consist 

 apparently of chains of sugar residues in which the — CHgOH groups 

 are replaced by — COOH groups, i.e. of uronic acid residues. A number 

 of different compounds of this kind have been detected in plants (3) but 

 the commonest seems to be the one associated in pectic substances. 

 Here the uronic acid is probably polygalacturonic acid, in some close 

 association with galactose and arabinose, which led Ehrlich and other 

 earlier workers to assume a ring formula (see e.g. 5) consisting of four 



