164 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



the fibres of bamboo (48(c)). The methods followed were those outlined 

 in the last chapter (p. 137). X-ray diagrams soon showed conclusively 

 that bamboo species with longer fibres contained also steeper molecular 

 spirals, and mechanical separation of the fibres of one species into length 

 classes, with subsequent reparallelization in collodion, similarly yielded 

 X-ray diagrams from which the same deduction could be made. 



£.0012 



2 3^ 



Tracheid lenglh in mm 



Fig. 57. The birefringence in transverse section of the outer layers of tracheids in 



Pseudotsuga. 



O birefringence. Each point represents the average of 20 observations. 

 — • — the inclination of the cellulose chain in the outer layers to cell length 



calculated from the birefringences. 

 — © — spiral angle in central layer. 



A more exact expression of this relation can be given in terms of the 

 m.e.p. of these cells. As we have already seen (p. 140) the structure of 

 the walls of bamboo fibres is such that the m.e.p. cannot be considered 

 to give a meticulously accurate estimate of chain direction, but again 

 there is good reason to accept it as a close approximation to the chain 

 direction in the thick layers. As shown in Table XV, the m.e.p. is 

 inclined to cell length at an angle of 5-6° except in the case of Melocanna 

 bambusoide where the angle is about 10°. This is in good harmony with 

 the X-ray diagrams, and this is one of the lines of evidence which 

 indicates very strongly the close agreement between the m.e.p. of the 

 wall and the chain orientation in those layers which are dark in trans- 

 verse section between crossed Nicols. It is clear from the table that the 

 variation in m.e.p. is closely paralleled by a variation in cell length. 



