180 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



It is equally possible that the broadening is due, of course, to random 

 imperfections in the alignment and spacing of the cellulose chains with- 

 out the necessity for the assumption of smaller specific crystalline 

 regions. ' 



Taking the value of 20 to 30 A. at its face value, however, one can 

 make some interesting calculations as to the disposition of the micelles. 

 Thus, if these are taken to be dcm. in diameter and of indefinite length, 

 spaced uniformly by a distance m cm., then in a 1-cm. cube there will 

 be a total micelle volume of nd'^IAm^. This is the relative volume of the 

 cellulose, which is known to be about 8 %. Hence 



J2/4,„2_o-08, 



and therefore m=3-3d. 



The intermicellar distance would then be 70-100 A. or, if only 60% of 

 the cellulose is crystaUine, the distance would be of the order 90-120 A. 

 The actual distances would be very variable, of course, but this little 

 calculation is very instructive in showing how widely spread the 

 cellulose matrix must be in these growing walls. It is also interesting to 

 note that, if the wall could be considered as consisting of individual 

 cellulose chains arranged strictly parallel to, and equidistant from, each 

 other, then the distance between them would be of the order of 13 A. 

 This is of the order of three molecular diameters and compares favour- 

 ably with the bimolecular water layer separating the chains as suggested 

 by Berkeley and Kerr. 



We have then here, good evidence that the cellulose chains in primary 

 walls do show some orientation and that, on the whole, the orientation 

 tends to be in a flat spiral. The same conclusion can also be drawn 

 from the growing walls of other cell types. Thus in the sporangiophore 

 of Phycomyces the chitin chains in the (apical) growth zone are oriented 

 in a spiral whose angle to the transverse is around 143° (62) (see p. 189) 

 and in cotton a somewhat similar orientation has been equally clearly 

 demonstrated (51). Orientation of this general kind has also been 

 claimed for the parenchyma of Helianthus hypocotyls, staminal hairs of 

 Tradescantia, sclerenchyma cells and collenchyma cells, and for the arms 

 of stellate pith cells (see references in 58). There are occasional excep- 

 tions — for instance the primary walls of jute and hemp fibres at a late 

 stage of growth and the outer walls of epidermal cells in oat coleoptiles 

 — but the imposing array of growing cells with the same type of trans- 

 verse orientation has led to a good deal of speculation as to the con- 

 nection between orientation and growth processes. Two problems face 



