66 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



that the m.e.p. of the upper wall is tilted at the angle 6 to cell length but 

 in the opposite direction. 



In a population of cells, therefore, a series of s can be observed for 

 individual cells and the average calculated. With care, each value of d 

 is reproducible to within ±0-5°, and with practice something hke 30-50 

 determinations can be made per hour. The results are recorded, ^s 

 usual in statistical work, as range, average and standard error. 



Fig. 31. LM and OP are the edges of a cell seen in face view so that the observer is 

 looking down through two equally thick walls. ABCD represents the trace on the 

 wall surface of the index ellipsoid of the upper wall, and EFGH that of the lower 

 wall, the m.e.p.s being assumed to lie at the same angle to the cell length. Reasons of 

 symmetry show that under these conditions the effective extinction positions 

 lie along the thick broken lines. In point of fact, two superposed plates of 

 this kind cannot be treated exactly as a single uniaxial plate; the extinction 

 positions do not show true extinction, but represent rather positions of minimum 

 intensity. Wherever, therefore, the "extinction" positions of a double wall lie 

 parallel and perpendicular to cell length then the m.e.p.s of the individual walls 

 must make the same angle to the length of the cell but on opposite sides. The or- 

 ganization of the m.e.p. is then spiral. A special case occurs when the angle is zero. 



