94 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



The X-ray diagram 



The first X-ray determinations of structure were made in 1930 by 

 Sponsler during his determination of the unit cell of cellulose, but it was 

 not until some years later that the particular organization of the cellu- 

 lose in this plant was fully investigated (42(a)). Then it was found that 

 if a single piece of cell wall is cut from a cell, allowed to dry flat after a 

 preliminary washing with A'^/20 HCl to remove incrusting carbonates, 

 and mounted in an X-ray spectrometer with the beam at right angles to 

 the surface, a photograph such as that illustrated in Plate IV, Fig. 1, 

 was obtained. Comparison of this photograph with that of wood cells 

 (Plate I, Fig. 2) or hemp fibres (Plate II, Fig. 3) reveals immediately a 

 very striking fact. In the photographs of the fibres there occur, on the 

 equator, two arcs corresponding to planes of 3-9 A. spacing and the fine 

 joining them (the equator, carrying also arcs corresponding to 5-4 and 

 6-1 A.) is perpendicular to the direction of the cellulose chains in the 

 specimen. In the Valonia photograph, on the other hand, two such rows 

 of arcs occur (the arc corresponding to 6-1 A. is missing, as mentioned 

 earlier, p. 43), and discussed further below) so that the diagram from 

 a single piece of wall is equivalent to that of two sets of ramie fibres 

 crossed at about 90° to each other. In other words, the wall has not 

 one but two sets of cellulose chains making an angle somewhat less 

 than 90° with each other, and this opens up a problem of very con- 

 siderable magnitude. 



The first point to be solved concerns the spatial relation, within such 

 a piece of wall, of the two sets of cellulose chains and here observation 

 under the microscope gives a clue. Examination of a cross-section of 

 the wall shows many superposed and rather distinct layers in the wall, 

 and it might therefore be expected that the direction of the cellulose 

 chains would alternate from layer to layer. This expectation is not, 

 however, fulfilled. These layers can be stripped off individually from 

 the wall and examined separately. Thus, if a piece of wall is dried on to 

 a glass plate and a strip of sticky tape pressed on to it, then removal 

 of the tape carries with it a thin layer of wall which can be washed off. 

 The thinnest layer which can be stripped in this way and can be mounted 

 in an X-ray spectrometer still gives the crossed photograph typical of 

 Valonia. Each microscopically visible layer in the wall therefore is still 

 heterogeneous in cellulose chain direction. This is shown to be true 

 also by examination of cross-sections of the wall between crossed 

 Nicols. If a section is cut at right angles to one set of cellulose chains, 

 then we would expect, assuming that each layer did in fact possess 



