116 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



following the method of Hermans on results obtained by him from 

 material supplied by the writer. This will be discussed later, however, 

 after the X-ray diagram has been presented. 



The m.e.p. of conifer tracheids 



It is not feasible to decide between these possibilities, however, or to 

 interpret the X-ray diagram even of such a simple tissue as conifer wood 

 without looking a little further into the optical properties of tracheids. 

 This is due, of course, not to any defect in the X-ray method, but as a 

 consequence of our present disability, with occasional exceptions to 

 which we shall refer again, to examine single tracheids by this means. 

 In the exceptional cases optical and X-ray analysis yield results which 

 are in complete harmony, so that there can be no doubt of the validity 

 of carrying over observations made on single cells under the polarizing 

 microscope to the interpretation of X-ray diagrams of wood sections. 

 It should, of course, be remembered that the isolation of cells for exami- 

 nation under the microscope removes lignin and other incrusting 

 substances, so that the material is presented for observation under 

 slightly different conditions in the two methods. Fortunately this 

 seldom produces any undesirable complicating features. 



In order to determine the m.e.p. it is necessary first to devise some 

 way in which single walls of the cells can be made available for obser- 

 vation. This has, in fact, been done in a way which seems remarkably 

 simple when we remember that the cells concerned are of the order of 

 20 [ji in diameter. Chips of wood are first macerated either by standing 

 for several days in cold 5% chromic acid or by alternate treatments 

 with chlorine water and hot 3 % sodium sulphite. The tissue softened 

 in this way is then shaken vigorously with glass beads until the cells are 

 completely separated. After this, a suspension of the cells in distilled 

 water is run over a series of microscope slides smeared with albumen 

 fixative and allowed to dry down on the sUde. Immersion of the slide 

 in alcohol serves to harden the fixative. A sharp microtome knife is 

 then slid over the surface of each slide, removing all loosely attached 

 cells and all large conglomerations and cutting away the upper walls of 

 many cells which are affixed firmly to the slide by their lower walls. 

 The slides are then prepared for examination under the microscope in 

 the normal way, no stain, of course, being required. With practice, it is 

 possible in this way to achieve 100 or more single walls on each sHde. 

 Following the methods described earlier (Chapter IV) the angle d 

 between the m.e.p. and the cell length is then determined for a number 

 of cells, normally 50, sufficiently large to give a representative mean. 



