INVESTIGATION OF STRUCTURE IN PLANT CELL WALLS 35 



is merely the mutual arrangement and the distance apart of the three 

 sets of planes 100, 010 and 001, and these will need the six parameters 

 mentioned above, or four in the case of the monoclinic system to 

 which cellulose belongs. Just as in the case of the plane lattice, so here 

 these parameters can, under certain conditions, be determined from the 

 positions and distances apart of other sets of planes, provided these can 

 be determined in sufficient detail. Naturally, the more numerous the 

 sets of planes available the more accurately can the unit cell be deter- 

 mined. When, however, this desirable end has been reached, this forms 

 only the beginning. All that has been learned is the form and dimensions 

 of a set of points, and for a full description of the crystal it is then 

 necessary to determine what atom or atom group is associated with one 

 such point and how the atoms are arranged round the point. In general 

 this much more difficult determination cannot be achieved by physical 

 methods alone, and in the case of the more complicated crystals cannot 

 at present be achieved at all with any great precision. So with cellulose, 

 recourse has to be made, once the unit cell dimensions have been 

 determined, to the body of chemical evidence available and to other 

 physical data, in order to set the constituent glucose residues in approxi- 

 mately their correct relative positions. 



The first problem, then, in attempting to elucidate the structure of 

 cellulose is to determine the unit cell, and a few pages will be devoted 

 to the techniques involved. These are, of course, the techniques of 

 X-ray analysis suitably modified to deal with the peculiarities of biologi- 

 cal material, and it will not be possible to present anything like a full 

 and logical account of the method. Enough may perhaps be said to 

 give the reader a general idea of the processes of thought involved. It 

 is further to be noted, however, that in many crystals the regular 

 arrangement of the atoms and molecules within them confers upon them 

 special features which can be investigated by methods other than those 

 of X-ray analysis, and one of these — anisotropy of optical properties — 

 presents in cellulose a particularly valuable tool. In some ways the 

 polarizing microscope is complementary to the X-ray spectrometer, as 

 will be seen when the results obtained by the two are later compared. 

 In addition, the polarizing microscope is merely a modified form of the 

 common tool of biologists, and therefore investigations with its aid need 

 no very special apparatus and no serious problems of maintenance; 

 though this is not to say that the method can be used by untrained 

 workers without the risk of very serious misinterpretation. Both the 

 methods of X-ray analysis and of polarization optics will therefore be 

 reviewed very briefly before passing on to the results. 



