WALL STRUCTURE IN THICK CELL WALLS 97 



pencils of circular section, then they would lie quite irregularly; they 

 would point in any direction and the line of lettering in any individual 

 might or might not be visible. We could then rearrange them so that 

 they lay quite parallel to each other and with all the lines of lettering 

 visible. This would be a very special arrangement; but it is precisely 

 this condition which is manifested in Valonia. If the pencils were, 

 however, elliptical in section (as in some pencils used by carpenters) 

 with the lines of lettering on the broader face, then the lettering would 

 always lie either on the upper or the lower side; correspondingly, if the 

 threads visible in the electron microscope are in fact flat ribbons then 

 the regular arrangement of the 6-1 A. planes could receive ready and 

 obvious explanation. It is now known (p. 89) that the microfibrils 

 are indeed flat ribbons, the width/thickness ratio averaging about 2 but 

 ranging up to 7. 



Striation direction and chain orientation 



In passing, it should perhaps be mentioned that the availability of 

 comparatively enormous pieces of cell wall whose chain directions can 

 thus unequivocally be defined made it possible to test the idea, already 

 widely held though supported only by qualitative evidence, that the 

 markings on cell walls known as striations reflect the directions of the 

 underlying cellulose framework. In Valonia two sets of striations can 

 be observed, though the ease with which they can be seen varies con- 

 siderably even over small areas of waU, and this in itself makes it highly 

 probable that striation direction and cellulose chain direction are one 

 and the same thing. A quantitative test has, however, been carried out 

 in this way. A piece of wafl was mounted over a hole in aluminium 

 foil, and a fine hair was cemented to it so that the hair lay parallel to 

 one set of striations. This was then mounted over the slit of an X-ray 

 spectrometer and a copper wire placed over the photographic plate 

 was set parallel to the hair. This gave a reference fine (the hair) on the 

 specimen and a parallel one (the shadow of the wire) on the plate. It 

 was then a comparatively easy matter to make the necessary observa- 

 tions, and it was found that striation direction and chain direction did 

 coincide within the limits of observational error (42a). 



This is, in many ways, particularly fortunate. If we can carry over 

 this correspondence to other cefls, and especially the minute cells of the 

 higher plants, then it provides a ready method of determining cellulose 

 chain direction even in the smallest pieces of wall provided these show 

 striations. It is not, of course, self-evident that this correspondence will 

 always obtain, but a similar correspondence has been found, for 

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