312 H. K, SCHACHMAN AND R. C. WILLIAMS 



their cross-sectional shape and their surface structure. There is little dis- 

 agreement about the diameter of the particles of TMV, When the rods are 

 in a packed array their center-to -center distance is 152 A, a figure which is 

 found by both electron microscopy and by X-ray analysis of dried virus 

 paracrystals. However, when single particles are measured, it is found that 

 their diameters are about I80A (Williams, unpublished; Kaesberg, personal 

 commimication), although there is no evidence that the virus particles have 

 flattened upon drying (Williams, 1952), A possible explanation of the dis- 

 crepancy between the widths of packed and isolated particles is found in the 

 discussion below of the X-ray analysis of TMV structure. Information about 

 the diameter of TMV can also be derived from sedimentation studies. The 

 sedimentation coefficient of elongated particles is relatively independent of 

 length but markedly sensitive to mass per unit length and, therefore, to 

 thickness. The diameter can be calculated directly in the manner suggested 

 by Peacocke and Schachman (1954) for deoxyribonucleic acid. The resultant 

 value, 150 to I60A, is in fair agreement with the other measurements. Again 

 it should be noted that this calculation is based on ellipsoidal models. As 

 Peacocke and Schachman (1954) pointed out, this diameter corresponds to 

 the dry particle, and swelling in solution, if isotropic, would not complicate 

 this calculation of the diameter of the equivalent, anhydrous particle. 



The surface structure and the cross-sectional shape of the particles of 

 TMV have been investigated by electron microscopy of the intact virus rods 

 and of short segments of the rods seen end-on. If the shape and surface 

 structure of the intact particles are to be examined it is necessary to shadow 

 them and derive inferences from their over-all appearance. Residts secured 

 are somewhat uncertain and conflicting, owing probably to the shadowing 

 anomalies discussed previously. The most likely statement that can be made 

 is that the over-all shape of intact particles appears more like a cylindrical 

 rod than anything else, and that there is no clear indication of periodicities 

 in a generally pebbly-appearing surface. These observations appear to hold 

 for either air-dried or frozen-dried material. 



Electron microscopists occasionally see short segments of TMV in an end- 

 on orientation. Such segments frequently appear polygonal in contour, with 

 a hexagonal shajie the most common. WiUiams (1952) has concluded from 

 his observations of the segment that the most likely cross-sectional shape of 

 the intact virus is that of an hexagonal prism (like a lead pencil). There can 

 be little doubt but what the segments frequently appear hexagonal, but in 

 the light of the X-ray evidence on the structure of the intact virus (see 

 below) the relation of this appearance to the virus shape is obscure. Two 

 electron microscopic observations of virus shape are unquestioned: particles 

 of all lengths have blont ends wliich appear to be perpendicular to the axis 

 of the rod; when short rods are aggregated to form long ones, by lowering the 



