18 THE PLANT CELL WALL 



The celluloses and pectic substances exist, as linear polymers 

 which are variable in molecular size, but uniform with respect to 

 monomer type and linkage. The properties of the pectic substances 

 vary widely according to the degree of methylation and of acid 

 groups and their interaction with polyvalent cations. Such differences 

 are minor, however, relative to their uniform identity as polygalac- 

 turonic acids. In turning, now, to the polyuronide hemicelluloses, 

 we are confronted with polydispersity, branching, and monomer 

 heterogeneity all at once. 



The acidic hemicellulose fraction which is isolated in 4 per cent 

 NaOH is not homogeneous physically or chemically. When the 

 extract is acidified (acetic acid), the hemicellulosic fraction A precip- 

 itates. This fraction consists largely of linear polymers with few 

 end groups. When the remaining solution is mixed with ethanol or 

 acetone, fraction B, which contains branched polymers, precipi- 

 tates. The still soluble component is fraction C. Each of these 

 fractions may in fact be multicomponent as shown by subfraction- 

 ation by stepwise addition of acid and alcohol. Other successful 

 fractional precipitation methods based upon complexes or organic 

 salts have been noted. 



Hydrolysis of the acid hemicellulose yields a mixture of pentose 

 and uronic acids. Of particular note are the aldobiuronic acids, 

 disaccharides whose linkage is resistant to hydrolysis. The funda- 

 mental monomers are xylose, arabinose, glucuronic acid, O-methyl- 

 glucuronic acid galacturonic acid, and occasionally rhamnose. 



The stability of the aldobiuronic acids points to their importance 

 as recurrent structural elements of the polymers. The constitutional 

 relationships among these acidic disaccharides vary considerably with 

 the source (Table 4). This tabulation includes for contrast, some hyd- 

 rolysis products more correctly associated with mucilages and gums. 



The uronic acids always occur as end groups or single unit side 

 chains. As side groups, they are commonly attached to a D-xylose 

 backbone chain at position C-2, sometimes at C-3. 



A more complete picture of specific hemicelluloses can be given 

 by examination of some which have been isolated. Thus, a homoge- 

 neous maize seed coat hemicellulose soluble in lime water gives 

 on hydrolysis: 



