THE BIOCHEMISTRY OF PLANT VIRUSES 



69 



This involves considerable difficulty because, among other things, it is 

 known that the absorption spectrum of substances such as nucleic acids are 

 not only dependent upon their compositions but are also dependent in some 

 way upon the geometry of the absorbing systems. 



230 250 270 290 310 330 

 (m//.) 



Fig. S. Absorption spectrum of tobacco mosaic virus (0.27 mg./ml.) in a 1-cm. layer, 

 recorded on a Cary No. 14 spectrophotometer. Note the shallowness of the trough at 

 245 m^i, and the high absorption at 310 ni/n. Compare with Fig. 1.3. 



The solutions of the virus are, of course, strongly birefringent when 

 oriented by flow (Bawden and Pirie, 1937a) and, when sufficiently concen- 

 trated, they are birefringent spontaneously. This effect can be caused in 

 suspensions of rods by pure form birefringence or, if the rods themselves 

 are anisotropic, this can contribute toward the birefrmgence also. An attempt 

 has been made to determine to what extent intrinsic birefringence is present 

 in the particles by measuring the intensity of birefringence as a function 

 of the refractive index of the suspending medium (Lauffer, 1938). Very 

 little intrinsic birefringence was fomid in the particles, but the main interest 

 of this work is that the virus apparently tolerated a high concentration of 

 anilin (Lauffer and Robinson, 1949). 



