96 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



direction and odd layers in the other. Examination under the electron 

 microscope gives a triumphant verification of this conception. A photo- 

 graph of the outer surface of the Valonia wall obtained as described 

 earlier (p. 88) is presented in the Frontispiece. This has been discussed 

 already to some extent, but some other features may now be noticed. 

 Firstly, the appearance of crossed threads is exactly what we should 

 have expected in view of the X-ray photograph. Secondly, remembering 

 that this is a surface then it is clear that the threads are segregated 

 into layers which are extraordinarily thin. If we assume that the 

 threads are circular in section then it seems certain that each layer is 

 about as thick as the threads are wide, i.e. about 250-300 A. Taking the 

 thickness of the Valonia wall as 0-04 mm. (==40/^ =400,000 A.) and 

 assuming the wall to contain 50 % cellulose,* then this would imply the 

 presence of some 700-800 layers. It has now been found possible to 

 strip off these layers individually. Each has then only one set of micro- 

 fibrils {A2{b)). The segregation of the microfibrils of different orientation 

 into separate layers is therefore placed beyond doubt. 



It is of importance here to recall another feature of structure in 

 Valonia already mentioned briefly (p. 43) and illustrated (Fig. 18). 

 When a beam of X-rays is passed through the Valonia wall normal to 

 its surface then no reflection is observed corresponding to planes spaced 

 6-1 A. apart; these reflect most strongly when the beam Uqs parallel to 

 the wall surface. This means that the 6-1 A. planes tend to Uq parallel 

 to the wall surface. There is not complete restriction to this paraUel 

 position — in fact photographs taken with the X-ray beam running along 

 the chains show that there is considerable angular dispersion amounting 

 to about 70° on each side of the paraUel position. Nevertheless the 

 bulk of the planes are more or less parallel to the surface, and there are 

 none lying at right angles to it. It is not clear if the chains as just laid 

 down by the cytoplasm are arranged strictly with the planes corre- 

 sponding to 6T A. in the surface, this perfect arrangement being dis- 

 turbed by secondary effects. The fact that these planes carry the densest 

 array of — OHs is, however, most suggestive; for it may well be that, 

 since the chains are laid down presumably at a hydrated protein inter- 

 face, then the — OH groups would be held in the interface. This is 

 most significant in view of the electron micrograph, for not only are the 

 threads visible there arranged with beautiful paraHel regularity but also 

 must present the same "face" to the surface. Perhaps an analogy will 

 help to clarify the position. If we threw down on a table a number of 



* The cellulose content is not known; but the evidence obtained for the writer by 

 P. H. Hermans of Utrecht, Holland, shows the presence of a large proportion of non- 

 cellulosic substances. 



