THE STRUCTURE OF THE CELL WALL 



67 



intermicellar spaces arc filled with pectic compounds; in woody tissues they 

 are filled with lignin, and in cutinized walls with waxes and cutin. In walls 

 that are almost pure cellulose, such as the secondary wall of cotton fibers, it 

 is possible that the intermicellar spaces are filled chiefly with water. 



The smallest visible units of cellulose walls are delicate threadlike strands 

 or fibrils. In primary walls these fibrils form a loose anastomosing network, 

 the meshes of which are usually 

 filled with colloidal pectic com- 

 pounds. In secondary walls 

 the fibrils are often grouped 

 into coarser strands which wind 

 around the cell in a steep spiral 

 the angle of which may var^^ in 

 different layers and even in dif- 

 ferent parts of the same layer 



(Fig. 15). 



The Physical Properies of 

 Cell Walls. — The physical 

 properties of primary cell walls 

 differ from those of secondary 

 walls. These differences may 

 be ascribed to the greater abun- 

 dance of cellulose in secondary 

 walls and to differences in the 

 structural organization of the 

 cellulose in the two kinds of 

 walls. Both primary and sec- 

 ondary Walls are transparent to 

 wave lengths of the visible 

 spectrum and both are usually 

 quite permeable to most sub- 

 stances dissolved in water. 



Secondary cell walls com- 

 posed predominantly of cellu- 

 lose possess tensile strengths 

 that compare favorably with 

 those of steel. The breaking strength of a flax fiber, for example, may be 

 as great as iio kg. per mm.2 of wall area while the tensile strength of a 

 hardened spring steel ranges between 1 50 and 1 70 kg. per mm2. Although 

 the tensile strength of primary walls is considerably less than that of secondary 



B 



Fig. 14. Micellar structure of ^valls. Black 

 areas represent intermicellar spaces; white areas 

 represent the cellulose micelles. {A) cross sec- 

 tion, (B) longitudinal section. Redrawn from 

 Frey-Wyssling (1936). 



