88 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



On both accounts, therefore, interpretation of features of biological 

 interest needs the greatest possible care. 



Among the substances used by the pioneers of this instrument as 

 objects of interest cellulose, too often in the form of dissected filter 

 paper, was frequently used and innumerable photographs are on record. 

 Unfortunately such dissected fragments produced little of interest in 

 the botanical sense. The early work suffered, naturally enough, from a 

 spate of enthusiasm without the experience necessary to ask, as it were, 

 the right question in the right way. Recently, however, two investiga- 

 tions have been reported which seem to lead toward a clarification of 

 the micellar aggregate position (41 (a) and (b)). In the first of these to 

 appear, Preston et al, realizing that an experimental object was needed 

 of which the structure was already known in some detail, and such that 

 the treatment necessary would not lead to any distortion of structure, 

 used the wall of the large vesicles of the marine alga Valonia ventricosa 

 (see p. 92). Pieces of wall (which could be several millimetres across) 

 were fixed to a glass slide and dried in a desiccator. The wall is, of course, 

 much too thick to be examined directly and so a surface replica was 

 made for observation in the microscope. Following a fairly recent 

 advance in technique, the material was first "shadowed" with chromium. 

 This is done by placing the glass slide, with the wall attached, into a high 

 vacuum in a vessel which contains also a heated filament carrying a 

 bead of metalhc chromium. The mutual arrangement of the chromium 

 and the slide is such that the atoms of the metal strike the surface of the 

 slide at a small angle. In this way, any contours on the specimen are 

 accentuated; elevations in the surface (Fig. 33) become covered heavily 

 with particles of metal on the side nearer the filament, whereas the sur- 

 face beyond is free of deposit. After the deposit has reached a suitable 

 thickness the slide is removed and sufficient formvar or collodion 

 solution poured over it that, when dried, the film can readily be re- 

 moved from the specimen and yet be thin enough to allow the passage 

 of electrons. The film takes with it the metallic deposit, so that the 

 replica carries a faithful copy of the surface plus the metal.* It should 

 be noticed that positions of thick metal deposits are opaque to elec- 

 trons and therefore register in the photograph as white areas; places 

 with no metallic deposit, i.e. the shadows, are registered as dark areas, 

 and the reproduction of the specimen illustrated in the Frontispiece is 

 such that this condition is maintained. 



Detailed consideration of this beautiful photograph will be postponed 



* It is now clear, however, that the specimen considered here is not a true surface 

 replica; it contains at least two lamellae stripped from the wall. 



