GROWTH ASPECTS OF PLANT VIRUS INFECTIONS 627 



be true, but inclusion bodies occur with some viruses which are not 

 considered to occur in high concentration. 



Inclusion bodies vary considerably in size, shape, and in the tis- 

 sues in which they occur (Esau, 1960). Those of the tobacco-mosaic 

 virus are of two distinct types. One type is a striate body which may be 

 flat and plate-like or long and needle-like. It varies in size and shape, 

 some bodies being irregular in outline and others perfect hexagonals. 

 As they turn over in the protoplasmic stream they show diJSerent facets 

 and are regarded as ti-ue crystals. They are generally in the cytoplasm 

 but may be intranuclear. The other type consists of amorphous, amoe- 

 boid bodies called "X-bodies." These also vary in size and shape, some 

 being as great as 30 microns and some as little as five microns in diame- 

 ter. They are generally rounded to oval and have the appearance of 

 dense cytoplasm. They have been shown to change their shape as they 

 are carried about in the protoplasmic stream of the cell. 



It is now generally thought that the crystalline bodies of the 

 tobacco-mosaic virus represent almost pure virus ( Steere and Wil- 

 liams, 1953; Wehrmeyer, 1957). The X-bodies are less accurately iden- 

 tified but are probably an insoluble complex of virus and cellular con- 

 stituents of the host. They sometimes contain crystalline bodies, which 

 seem to be identical to the crystalline bodies found in the cytoplasm. 

 The X-bodies are highly infectious and contain too much virus to be 

 explained as of non-viral origin. 



Virus synthesis and its effects on host physiology 



The pronounced morphological and histological abnormalities as- 

 sociated with virus diseases are undoubtedly traceable to disturbed 

 growth processes of the host. One would think that the causes and 

 nature of such profound changes would be easily detectable, but this 

 is not so. In fact, the changes in host metabolism due to virus infections 

 have not been easily established. 



In the infection process the virus nucleic acid enters the cell and 

 there presumably serves as a template for its own reproduction and 

 simultaneously alters the host metabolism so that virus protein is pro- 

 duced for incorporation along with the nucleic acid into virus nucleo- 

 protein. The fact that nucleic acid alone is infectious, and that infective 

 nucleoprotein can be reconstituted from the nucleic acid and protein 

 components, would indicate that the protein moiety may not enter the 

 host, and that the combination of the two in the cell to complete the 

 formation of new virus is simply an assembly process. 



Proteins. Since viruses are nucleoproteins, the fact that their syn- 

 thesis markedly aflFects the normal protein and nitrogen concentration 



