94 THE PLANT CELL WALL 



Among cellulose derivatives, the nitrates have had a long 

 history in fiber and photographic film technology, but they have 

 been replaced by more stable products. These esters retain their 

 importance in the form of celluloid, collodion, and gun cotton. 



Among the sugar acid polymers, alginic acid, the pectic sub- 

 stances and agar have found application in modern technology. 



Natural alginic acid is poly-l,4-anhydro-(3-D-mannuronic acid, 

 with a molecular weight of 150,000 or more. It is obtained from 

 the cell walls of Laminaria in 15-40 per cent yield. As an acid, 

 it is extracted with dilute alkali, and reprecipitated with dilute 

 acid. The free acid has not yet been developed economically, but 

 the salts have found several applications. Although the tensile 

 strength of alginic acid is low (under 10 kg/mm 2 ) the calcium salt 

 is comparable with the weaker celluloses and viscose rayons. 

 Insulating foams, fireproofing fibers, and other special fibers 

 are derived from the calcium salts. Sodium alginates are employed 

 as thickeners, emulsifying agents, sizes, pastes and descaling agents. 



Isomeric with alginic acid is pectic, or poly-l-4-a-D-galactu- 

 ronic acid. As the pectic ester, it occurs abundantly in many fruits, 

 and generally in cell walls (middle lamella). The pectins are important 

 only as gelling and thickening agents, but they could conceivably 

 be employed as are the alginates. 



Wood: Structural 



In spite of the increased modern use of metallic and synthetic 

 strutural materials, traditional substances, such as wood, retain 

 their importance. If the mechanical properties of wood are compared 

 with other substances (Table 1), its relative position can be assessed. 



The elastic (Young's) modulus is defined as 



Y _Fla_ 

 Ilk 



the ratio of tensile force (F) per unit area (a) to extensions (/) pro- 

 duced in initial length (/ ). 



This stress-strain ratio is applied within the limits of reversible 

 deformation. 



