THE BIOCHEMISTRY OF PLANT VIRUSES 67 



be banded. The light or dark bands, which alternate, can then be measured 

 directly, and the particle length deduced, correcting for the angle between 

 the extinction positions. The length observed was in fact about the same as 

 that deduced from electron microscopy (3000 A). (Fig. 6.) 



Other methods used have been ultracentrifuge sedimentation combined 

 with measurements of viscosity and diffusion, light scattering, and flow 

 birefringence. All of these methods, which are discussed elsewhere, are in 

 reasonable agreement, but cannot be regarded as being particularly precise. 



A fairly recent method, which would seem to combine precision of measure- 

 ment with some uncertainty of interpretation, is the measurement of the 

 rotary diffusion of the molecules. This can be measured relatively accurately 

 by following the relaxation of the molecules to a random order after they 

 have been oriented by external forces. In this work the orientation was 

 produced by an electric field, in which the molecules orientate themselves 

 to produce as small a capacity as possible. On removal of the electric field, 

 the particles relax to a random orientation, and this can be followed by 

 observing the birefringence of the solution. In this way a rotary diffusion 

 constant of 333 sec.~^ was obtained for zero virus concentration (O'Konski 

 and Haltner, 1957). Making a number of assumptions as to the cross section 

 and the hydration of the particles, a length of 3416 ± 50 A was deduced. 

 Errors involved in the assumptions would tend to reduce this estimate and, 

 of course, the values fomid are for the virus in solution. The electron micro- 

 scope figures are based on observations of virus which has been drastically 

 dried, a procedure which almost certainly results in shrinkage, and, of course, 

 the X-ray estimates of diameter have been obtained on virus which has been 

 more or less severely damaged by the intense radiation. These factors are 

 often overlooked in assessing the relative merits of techniques of this type. 



For the remainder of this chapter we shall assume a length of about 3000 A 

 or 300 m/x for the dry virus rods. 



The diameter of the virus was for a long time thought to be 150 A. This 

 value was obtained from the X-ray diffraction data obtained by Bernal and 

 Fankuchen (1941a) who found that the closest packing distance between 

 the hexagonally arranged rods had this size. Since this time, X-ray techniques 

 have advanced greatly, and instead of measuring the distances between 

 the centres of adjacent rods, it is now possible to plot the distribution of 

 density from the rod centers towards the periphery. By this means it is 

 possible to show that the particles extend radially as far as 90 A from the 

 axis; that is to say, the particle diameter is 180 A at its outermost edges, 

 or 30 A more than the distance between particle centers. This is in reasonable 

 agreement with the once widely held view that the particles were hexagonal 

 in cross section (if the particles were hexagonal in cross section with centers 

 150 A apart, the maximum width of the particles would be 173 A), but 



