626 



PLANT GROWTH AND PLANT COMMUNITIES 



Figure 5. Leaf enations pro- 

 duced on leaves of cabbage 

 by the cauliflower-mosaic vi- 

 rus. 



point of entry, and most "local lesion" necrotic reactions are due to 

 restriction in parenchyma cells which quickly become necrotic. Re- 

 sistant hosts not ordinarily infected by a given virus may sometimes be 

 infected by altering the environment so as to increase susceptibility, or 

 by virus transmission through a graft union. When this occurs, necrosis 

 is often the symptom produced. In many cases necrosis of the cell un- 

 doubtedly results from direct invasion by the virus; in others it is prob- 

 ably a secondary effect. The amount of virus synthesized in a cell that 

 becomes necrotic is often less than that in cells that do not die. 



One anatomical phenomenon associated with many virus diseases, 

 particularly mosaic diseases, is the presence of intracellular inclusion 

 bodies. These bodies are specific to virus-infected tissue but do not 

 occur in all host-virus complexes. All strains of the tobacco-mosaic 

 virus produce inclusion bodies in a number of hosts, whereas no such 

 bodies are produced in the same hosts by the cucumber-mosaic virus. 

 The inclusion bodies are somewhat ephemeral in nature, in that they 

 can be found for only a part of the time the plants show symptoms. 

 The presence of inclusion bodies in some host-virus combinations but 

 not in others is a moot question. It was early thought that their oc- 

 currence was related to virus concentration in the cell. This may well 



