98 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



instance, in conifer tracheids where we shall find it very useful. Care 

 has, in any case, to be taken to distinguish between striations proper 

 and the other wall markings with which these can be confused. 



The organization of the wall as a whole 



Thus far we have been dealing with small pieces of wall as experi- 

 mental material and concentrating our attention on the detailed features 

 of isolated fragments. It is now a fascinating problem to try to stick 

 these pieces together again to see how the cellulose chains run in the 



Fig. 35. Investigation of a whole Valonia celL The empty dry cell B is held against 

 the camera slit A by the two fine wires C. These are arranged parallel to those parts 

 of one of the circles on the cell nearest the point being photographed. They cast 

 ■ shadows FF on the plate and therefore record the direction of the circle. 



whole cell. The problem could, of course, be solved in exactly this 

 "jigsaw" way. A whole cell could be cut up into pieces of some regular 

 shape (one such way would be to cut out sectors in the same way as 

 one peels an orange) and the striation directions could be observed 

 under a microscope. These could then be plotted on the surface of a 

 model of the original cell. In principle this would, in fact, be the 

 quickest way but the chances of error are so enormous that it seemed 

 undesirable to make any such attempt. Instead a much more laborious, 

 but much more sure, X-ray method was adopted. The labour involved 

 can be imagined when it is noted that each single photograph required 

 an exposure time of five hours and, taking three photographs each day, 

 the investigation still necessitated three years of uninterrupted work! 

 The method used was as follows, and as illustrated in Fig. 35. 



A cell of fairly regular shape was emptied through a fine capillary 



