INVESTIGATION OF STRUCTURE IN PLANT CELL WALLS 



47 



regular arrangement is not, in fact, uniformly a feature of the structure 

 is also evident from the diagram. It will be noted that in the diagrams 

 shown in Plate II, Figs. 1 and 2, the diffraction arcs lying along the 

 meridian are rather narrow radially, while along the equator all the 

 arcs are very broad radially. The beams of X-rays diffracted from 



(b) 



Fig. 23. Diagrams to illustrate the effect of the number of diffracting points on the 

 character of the diffracted beams. In both figures the radiation is conceived as 

 travelling parallel to the surface of the paper, from below upwards. The diffracting 

 points are indicated by black circles, and the waves diffracted from each such point 

 as circles drawn with these points as centres, {a) Many diffraction points. The con- 

 stituent, circular wave fronts result in the transmission of the incident beam in the 

 original direction. At the same time a wave front is reflected at a considerable angle 

 to the incident wave front. Both are emphasized in the diagram by thickened lines 

 (the phenomenon is best observed by holding the page on the level of the eyes and 

 glancing along the horizontal thickened line). If the page is now rotated about a 

 vertical line, it will be observed that tangents to a series of circles become collinear 

 when glancing along the sloping thickened line. Both wave fronts are sensibly 

 straight and the records on a photographic plate would be clearly defined spots. 

 (6) Few diffraction points. Two wave fronts are delineated as in (a). Here, however, 

 both have curved margins. The record on a photographic plate would therefore be 

 two diffused images, rather denser towards the centre and gradually decreasing in 



density towards the outside. 



