36 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



X-ray analysis of plant cells 



It is taken for granted that readers are familiar with the conception 

 of radiations of all kinds, including light and X-ray radiation, as a 

 series of vibrations transverse to the direction of propagation. Classi- 

 cally, to overcome the difficulty that light will pass through a vacuum, 

 the concept was introduced of an "ether", an imponderable substance 

 which permeated all material bodies throughout space and whose con- 

 stituent particles were thrown into vibration by the passage of Ught — 

 whose vibrations were in fact the passage of light. In this sense, there- 

 fore, radiation can be considered to have two "directions", a direction 

 of propagation and a direction of vibration at right angles to it. 



Fig. 13. For explanation see text. 



Suppose, therefore, as in Fig. 13, a beam of radiation is passing from 

 A\o B and the condition of the vibration is observed instantaneously. 

 The trace of the positions of successive "particles" will be a series of 

 sine waves LMNOP, etc. A particle at L is momentarily undisplaced, 

 but is on the point of passing downwards. Particles such as M are 

 momentarily at rest but are on the point of moving downwards, etc. 

 Clearly at whatever moment we "stop" the vibrations for inspection 

 the distance LP will be the same; this is then a characteristic of the 

 vibration and is called the wavelength symbohzed by the Greek letter A. 

 There are, naturally, other ways of characterizing any beam of radiation, 

 but this one is most suited to present purposes. Light beams of different 

 colours, for instance, differ in wavelength from 0-4 /i (blue) to about 

 0-7 /x (red). X-rays differ from both by their very much shorter wave- 

 length, which is never more than a few Angstroms* at most, and it 

 is to these radiations of very short wavelength that attention will for the 

 moment be confined. 



It is further taken that the reader is acquainted with the use of line 

 gratings in optics to produce diffracted beams of visible light— the use 

 of gratings to produce a spectrum, for example. Fundamentally, the 

 same principle is applied in the use of X-rays here with the difference 



* One Angstrom (A.) is 10-^ cm. 



