c«7e 



44 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



most of the arcs could be accounted for. This would make, for 

 instance, the planes spaced 3-9 A. apart the 101 planes (Fig. 19). 

 Verification that the angle j8 is of the order of 90° was soon forthcoming 

 in the observations made on plates of cellulose such as occur in tuni- 

 cates and in the green alga Valonia (see Fig. 18). The unit cell of cellu- 

 lose could then be imagined to be as in Fig. 19. This was soon followed 

 by a new suggestion (15) in which the axes were taken as: 



fl=8-34A. c=7-9A. i3=84° 



a scheme in which the planes of 6-1 A. spacing become the 101 planes, 

 those of 5-4 A. spacing the lOT planes and the 3-9 A. planes the 002. 

 Geometrically this scheme is equivalent to that of Sponsler (Fig. 20) 



and, it should be noted, demands a 

 point in the middle of each unit cell 

 identical with those at the corners. 

 This latter scheme of Meyer and 

 Mark's was accepted with little ques- 

 tion until comparatively recently 

 (see 16) and will be adopted here 

 since possible modifications are of 

 no moment for the discussion which 



Fig. 20. Part of the oc plane in cellu- follows. Space will not allow any 

 lose. EBGC represents the unit cell „x+^^„+ ^^ oV,^,i7 v,^«, tVi*. won'mic 

 of Sponsler. ABCD represents the attempt to show how the various 



unit cell proposed by Meyer and diff"raction arcs in the cellulose dia- 



Mark. The two are clearly identical , indexed from either of 



in a geometrical sense. ^^^"^ ^^^ ^^ maexea irom eimer or 



the two unit cells proposed, nor is it 

 the purpose of this book to make any such attempt. For the sake of 

 completeness, and for future reference, however, the indices of some of 

 the reflections are given in Plate II, Fig. 2(Z>), and it may perhaps be 

 noted that some of the indices can, in point of fact, be allocated by 

 inspection. Clearly, since all reflections on the equator arise from planes 

 parallel to the b axis, the index k must uniformly be zero, i.e. all 

 equatorial reflections have the indices /lO/. On the first layer line, where 

 the path difference between neighbouring lattice points is one wave- 

 length, the index k must be 1 (see p. 40) so that all reflections on this 

 layer line must be h\l. Similarly all reflections on the second layer line 

 must be h2l, on the third hV and so on. 



The arrangement of glucose residues within the unit cell 



The final step which remains to be taken is to place the atoms of which 

 cellulose is composed in their proper places within the unit cell. With 



