PLANT STRUCTURE AND COMPOSITION 183 



the chains are directed alternately with the glycosidic linkages oriented 

 in opposite directions. Within the crystalline regions the closely 

 packed cellobiose units of the cellulose chains are held together by 

 hydrogen bonding, presumably involving chiefly the — OH groups of 

 carbons 2 and 6. 



The cellulose molecules of one micelle are interwoven with those 

 of others in the amorphous region. They may be incorporated into 

 different micelles in the succeeding crystalline regions. The fabric-like 

 web which results from this pattern is responsible for the strength of 

 the cellulose fiber making up the cell wall. 



Water absorption in the cellulose fiber is confined almost entirely 

 to the amorphous regions. Swelling in hydrated cellulose is perpen- 

 dicular to the linear chains, with very little elongation of the fiber. 

 In addition, the amorphous areas are more subject to chemical and 

 enzymatic attack than the crystalline areas. 



Although the cellulose once laid down in the cell wall does not 

 appear to be utilizable by the plant itself, nature has provided a large 

 number of enzymes found in microorganisms that are capable of 

 hydrolyzing cellulose at a rapid rate. If this were not the case, the 

 surface of the earth would soon be covered with dead plant tissue, 

 such as annual plants, grasses, leaves, and trunks of trees. Such an 

 accumulation would soon lead to depletion of the COo reserves (page 

 68). Many bacteria produce extracellular enzymes capable of hy- 

 drolyzing cellulose, the reaction proceeding as follows: 



^ , , celkilase ,, , . ,, , . cellobiase 



Cellulose > cellodextrms -^ cellobiose > glucose 



Many workers feel that this process is operative in rumen fermentation 

 (Chapter 17) and leads to the utilization of quantities of cellulose in 

 the roughage in the multistomach animals. 



Hemicelluloses and Other Cell-wall Polysaccharides 



Interlaced with the cellulose web in the primary and secondary cell 

 walls are numerous other polysaccharides characterized chiefly by their 

 solubility in 17.5 per cent sodium hydroxide solution and their ready 

 hydrolysis in hot dilute acid. This group of compounds was given 

 the name hcjuiccJlulosc by early investigators because of the presumed 

 relationship to cellulose. The absence of any direct relationship has 

 been evident for some time, but the name, once applied, has remained 

 unchanged. The monosaccharide component of hemicellulosc varies 

 with the species involved and with age within the species. The hemi- 

 cellulose contents based on the dry weight of the plant residues vary 



