36 THE PLANT CELL WALL 



of such walls result from the statistical arrangement of chains 

 along the fiber axis. Of widespread occurrence, especially in second- 

 ary walls, is the spiral texture. The angular deviation of spiralled 

 chains from the fiber axis is described as their angle of ascent. 

 In hemp, the outer layer of the secondary wall ascends to the 

 left at 28° whereas the inner layer ascends at only 2°. In cotton, 

 the inner layer of the secondary wall ascends at 24°, the outer at 

 35°, with reversals in the direction of spiral. Even ramie has a 

 slight angle of ascent. In the cotton fiber and secondary walls 

 of tracheids, a lamellated annular structure may occur. In contrast 

 to the textures already examined, this so-called ring structure is 

 produced by parallel chains lying perpendicular to the fiber axis. 

 A variant on ring structure which occurs very commonly in many 

 primary walls is the tube structure. The cellulose chains exhibit 

 considerable scattering, but resemble the ring structure in their 

 general orientation. 



The full extent of cellulose organization in the cell wall is best 

 shown in wood fiber cells (Fig. 2), which possess a tubular primary 

 wall and a tripartite secondary wall containing an outer double 

 spiral, a middle steep spiral in which fibrils are arranged in con- 

 centric lamellae, and an inner flattened spiral. 



The overall relationship among the variously oriented fibrils 

 approaches orthogonality. This condition is to be found in another 

 great bio-mechanical system, bone, in which organo -mineral 

 lamellae laid down along compression and tension lines are ar- 

 ranged orthogonally. 



Within the reticulum formed by organized cellulose fibrils 

 and microfibrils is located the whole array of non-cellulosic sub- 

 stances. These extend beyond the primary wall itself as an inter- 

 cellular layer of amorphous character partially identified with the 

 term, middle lamella. In young cells, the intercellular gel is com- 

 monly rich in calcium and magnesium pectates, but may also 

 contain pentosans and dispersed cellulose fibrils. The non-cellulosic 

 polysaccharides which extend into the primary wall include pectic 

 substances and cellulosans. In the secondary wall, cellulose pre- 

 dominates, but it has been claimed that the hemicelluloses present 

 serve as binding agents for cellulose aggregates or bodies. 



