COMPOUND CARBOHYDRATES 379 



3. "Hemicelluloses." — This term has no critical chemical significance, 

 but is used to designate a heterogeneous group of polysaccharides which are 

 of widespread occurrence as cell wall constituents in plants. Superficially 

 most hemicelluloses bear some resemblance to cellulose, but differ from it in 

 being easily hydrolyzed by dilute acids and alkalies. 



The hemicelluloses include the mixed pentosan-hexosans as classified in 

 Table 35. The term is frequently loosely employed to include also various 

 pentosans and hexosans such as xylans, galactans and mannans. Apparently 

 all of these types of compounds may be present as intimate mixtures in the 

 cell walls of plants. 



Hemicelluloses are wndely distributed in plants. They are found espe- 

 cially in seed coats, endosperms, nutshells, stony fruits, and in woody tissues. 

 The endosperms of the seeds of nasturtium, date, and onion, for example, 

 contain hemicelluloses. The hemicelluloses present in many seeds are used as 

 food by the young seedlings during germination. Compounds of this type, 

 although cell wall constituents, may therefore serve as reserve foods. The 

 hemicelluloses found in the cell walls of the woody tissues of some trees, as 

 for example the apple tree, also serve as reserve food which is digested and 

 utilized when growth of the stems is resumed in the spring. 



Compound Carbohydrates. — These are compounds which have been de- 

 rived from carbohydrates and non-carbohydrate groups, the latter, however, 

 often being close chemical relatives of the carbohydrates. 



Among the substances usually classed in this group are the pectic com- 

 pounds. These are important constituents of the cell walls of plants. Three 

 types of pectic compounds are generally recognized: pectic acid, pectin, and 

 protopectin. All of the pectic compounds are markedly h3^drophilic and form 

 sols and gels with water. 



Recent investigations indicate that pectic acid has a structure analogous 

 to that of cellulose (Bonner, 1936). Pectic acid "molecules" seem basically 

 to be built up by the condensation of long chains of galactose molecules. 

 Many of the galactose residues have, however, been converted into galacturonic 

 acid residues and some have been modified to arabinose residues. The products 

 of hydrolysis of pectic acid are galacturonic acid, galactose, and arabinose, 

 but they are not present in constant proportions. In other words the so- 

 called pectic acid is not a compound in the strict chemical sense of the word, 

 but a group of similar compounds. Pectic acid does not occur in plants in 

 the free state, but its salts, especially calcium pectate, are of widespread oc- 

 currence. This salt is the most important constituent of the middle lamella 

 and acts as a cementing material between the adjacent primary cell walls. 



Pectin apparently differs from pectic acid principally in the presence of 



