ELECTRON MICROSCOPY OF VIRUSES 69 
microscope studies have been made of mutations of two well-known 
plant viruses with rod-shaped particles, that of tobacco mosaic and 
potato virus X. The resulting photographs showed no perceptible 
difference in the appearance or length of the particles of these two 
viruses and their strains (Takahashi and Rawlins, 1946 and 1947). 
The electron microscopy of the crystallizable viruses now offers 
a direct approach to questions of how crystals are built up from their 
molecular units and studies from this aspect have been carried out on 
several plant viruses, particularly southern bean mosaic virus, tomato 
bushy stunt virus, one of the tobacco necrosis viruses, and turnip 
yellow mosaic virus. 
Micrographs of the two first-named viruses from purified solutions 
showed a tendency towards a regular arrangement of the elementary 
particles or virus molecules. Different types of field were seen in 
electron micrographs of the bushy stunt virus. In one, the elementary 
particles appeared as well-defined spheres that packed closely together 
in a regular array. In other parts of the preparation the particles were 
ill-defined in clumps and masses. 
Particles of southern bean mosaic virus are similar both in size and 
shape to those of bushy stunt virus. Regions of regular particle 
arrangement, however, are far more numerous in the bean mosaic 
preparations, and there is a pronounced tendency for the regular 
layers of particles of this virus to be stacked on top of one another in 
a three-dimensional crystalline array (Price, Williams, and Wyckoff, 
1946). 
One of the tobacco necrosis viruses crystallizes in thin, flat plates 
which are very suitable for electron micrography. Photographs of 
single virus crystals showed very plainly the molecular arrangement 
over the face of the crystal. Later attempts recorded the regular 
molecular distribution over several faces of crystals of this virus, 
thereby demonstrating their truly crystalline structure. The upper 
photograph of Plate XVI is a micrograph of a single tobacco necrosis 
virus crystal showing the close-packed molecular net on the pyramidal 
(inclined) faces and the square net on the horizontal face (Markham, 
Smith, and Wyckoff, 1947, 1948). 
The structure of turnip yellow mosaic virus crystals has been 
examined in the electron microscope and micrographs have been taken 
(by transmission) of the virus mounted on beryllium films. Some of 
these show a number of micro-crystals having the appearance of a net 
formed by hexagonal rings. Each ring has an electron-transparent 
