WALL STRUCTURE IN THICK CELL WALLS 



125 



it is proportional to unity. One has, then, the arcs drawn out into a 

 continuous circle with the intensity rather greater along the meridian. 

 Looking at it another way, a set of chains running round a tracheid in 

 transverse circles or a very flat spiral, would give two very wide arcs in 

 the meridional position whose arms would meet at the equator to give 

 a complete ring. 



Fig. 44. For explanation, see text. 



e ■ 



Fig. 45. For explanation, see text. 



Thus in the diagram given by a bundle of fibrous cells with spiral 

 cellulose chains four major arcs could be expected from the planes 

 of spacing 3-9 A., and four minor arcs, two along the equator and 

 two along the meridian. For the sake of completeness it may also be 

 noted that equatorial arcs may be derived in another way. Suppose 

 the tracheids are rectangular in cross-section, with two opposite 

 faces normal to the beam (Fig. 44). Then the walls A and B will clearly 

 give four major arcs in diagonal positions and perhaps spurious 

 equatorial or meridional arcs or both. The walls C and D, however. 



