WALL STRUCTURE IN THICK CELL WALLS 95 



its own chain direction, that odd layers, say, would be bright (since 

 the section is cut parallel to the chains and therefore shows high 

 birefringence) whereas even layers (cut at right angles to the chains) 

 should be dark. This is found not to be so; the layers are always 

 uniformly bright. In some other algae, showing the same "crossed" 

 structure, the regular alternation of bright and dark is, in fact, observed; 

 but this is due to another effect altogether and we must be careful to 

 remember the state of affairs in Valonia when we come to examine these 

 other types. 



Nevertheless it is certain that the two sets of cellulose chains must be 

 segregated into layers even if these are submicroscopic; it is impossible 

 to imagine the micellar structures as being interwoven like a fabric. 

 Until the advent of the electron microscope and the shadow-casting 

 technique, it was, however, remarkably difficult to prove this. One 

 observation was made which seemed convincing, and it involves this 

 principle. If the wall does consist of innumerable layers each with its 

 own chain direction then, on mounting a piece of wall on a slide and 

 examining it under a polarizing microscope, the optical conditions are 

 somewhat as illustrated in Fig. 31 (p. 66) except that the angle between 

 the major axes of the elhpses is more nearly a right angle. It follows, 

 therefore, that the m.e.p. of the wall must he in the acute angle between 

 the two chain directions. This was found to be true, in this sense. Very 

 few pieces of wall have in fact one m.e.p. More commonly, and as 

 illustrated in Plate III, Fig. 4, the wall shows a "mosaic" of areas each 

 with its own m.e.p., but each individual m.e.p. still lies within the acute 

 angle between the cellulose chain directions. Since the chain direction 

 is uniform over a few millimetres of waU, a moment's reflection will 

 show that this fluctuation in the m.e.p. must mean that the relative 

 amounts of the two sets of chains differ from point to point in the wall 

 surface. It follows that the chains must be segregated into different 

 layers. Further, it will be clear that, if we assume that segregation is into 

 different layers, then from measurements of the phase difference shown 

 by any small piece of wall, of the wall thickness and of the interstriation 

 angle, the birefringence of each layer can be calculated (the method is too 

 complicated to be discussed here; it can be found in the references 

 quoted elsewhere (47(e)). The birefringence then calculated turns out to 

 be about 0-06 — the value for ramie fibres. This lends further support 

 to the existence of such layers. Finally, the layers must be very thin 

 since the Valonia wall shows complete extinction. 



We can therefore picture the wall provisionally as built up of very 

 many layers such that even layers, say, have chains lying in one 



