196 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



cell, but as part of the v/hole growing organism: that the so-called wall, 

 in fact, represents nothing more than the outer layers of the protoplasm 

 within which cellulose is deposited. This is not the first time that this 

 possibility has been mooted but it is the first time that it has been 

 suggested on purely crystallographic grounds. 



The elegant electron-micrographs recently obtained by Miihlethaler 

 and Frey-Wyssling(41) can lead to precisely the same conclusion. In 

 these photographs, threads appear which look precisely similar to those 

 previously found in the author's laboratory in the Valonia wall, and, 

 in particular, are again rather uniformly about 250 or 300 A, wide. 

 Whether these can be strictly identical, in view of the difference in 

 sharpness of the arcs in the corresponding X-ray diagrams, is a point 

 which has already been discussed (p. 90). The threads, however, are 

 arranged in much more random fashion than are those in Valonia; 

 but they are also in parts intertwined. It is not therefore possible 

 to imagine them as being oriented on a flat protoplasm-wall interface, 

 and the simplest hypothesis would be that they are spun out of a cellu- 

 lose-cytoplasm complex. Somewhat the same intertwined condition has 

 now been shown in Valonia, so that the idea of orientation at an inter- 

 face may have to be discarded altogether {A2{d)). 



The mechanism of orientation and the growth process 



This structural complexity in a growing wall suggests at once that the 

 two problems discussed in this chapter — that of the mechanism of 

 orientation and that of the maintenance of this orientation during 

 growth — are but two aspects of one and the same problem and that 

 therefore if the one is solved, so is the other simultaneously. Just as, 

 therefore, we are unable to accept purely mechanical explanations of the 

 onset of transverse orientation so also we cannot accept mechanical 

 explanations of its maintenance. Thus the ingenious idea so often 

 figured by Frey-Wyssling (33(Z7)), in which the cell wall is idealized into 

 a network of fibrils with diamond-shaped interstices (Fig. 67(a)) and 

 growth is made possible by a "loosening" of the points of crossing 

 (Fig. 67(6)) which are otherwise fixed, is indeed hardly acceptable as it 

 stands. If the length AC has increased to A 'C, then surely the distance 

 A 'N' should be proportionately greater than /lA^— which is impossible 

 without a change in the orientation ofAB. 



It seems much more feasible to seek for an explanation at a more 

 fundamental molecular level. Up to about fourteen years ago, when the 

 discovery of the "crossed fibrillar "structure was made, orientation was 

 considered to obtain as the result of a pseudo-crystallization of glucose 



