110 R. MARKHAM 



The chemical composition has not been investigated very intensively, 

 but most of the isolates contain 1.7-2 % of phosphorus in the form of 

 ribonucleic acid. 



The Eothamsted strain apparently gives rise to two components (Bawden 

 and Pirie, 1945a) and seen under the electron microscope spheres of 37 mju, 

 and 17 m/x are present. The infectivity of the Rothamsted strain is associated 

 with the larger particles, but the smaller particles are actively antigenic. It 

 is tempting to suggest that the smaller particles were fragments of the protein 

 of the larger forms and that the antigenic determinants were on the inside, 

 but very little is known of the comparative chemistry of the two sorts of 

 particle. That they are closely related is evidenced by the fact that both 

 types of particles are produced by single-lesion isolates (Baw^den and Pirie, 

 1950). 



The plate-shaped crystals of tobacco necrosis isolates are exceedingly well- 

 suited to observation by electron microscopy. Being so flat their surfaces are 

 readily revealed by replica techniques (Markham et al., 1948b) and this 

 opens up a new method for the examination of virus crystals. The sensitivity 

 of the method, however, is not to be compared even with that of 

 the simplest X-ray diffraction methods, although the interpretation is 

 more direct. For example, the crystal mentioned above gave the appearance 

 of being cubic close-packed, with an interparticle spacing of 240 A. The 

 crystals were, however, quite appreciably birefringent when seen under 

 polarized light, so that they could not belong to the cubic system (and were 

 also evidently not hexagonal). 



Crowfoot and Schmidt (1945) have investigated a crystal of a non- 

 infectious derivative of the Rothamsted tobacco necrosis virus. This crystal 

 was found in the ice chest and was particularly perfect, and large enough 

 in size and lattice spacing to give many reflections without having to be 

 oscillated. Unfortunately it was the only crystal of this material available, 

 and was destroyed before the work was completed. A certain amount of 

 confusion was caused by the authors trying to fit their observations to the 

 results of the electron micrographs, which were from a quite unrelated 

 specimen that crystallized in a different form. 



It is quite evident that the tobacco necrosis viruses have not been studied 

 as extensively as their interesting characteristics warrant. This is largely 

 due to the difficulties of propagation and maintenance of stocks, but it is 

 to be hoped that the work will be resumed using the more recent experi- 

 mental techniques. 



XII. The Alfalfa Mosaic Virus 



The alfalfa mosaic virus, an aphis-transmitted virus, was originally 

 isolated by Ross (1941) from infected bean {Phaseolus vulgaris) plants by 



