AN ANATOMIST'S VIEW^ 

 OF VIRUS DISEASES 



Katherine Esau 



Anatomic studies of plants affected with viral diseases have taught us much 

 about the behavior of viruses in their host plants and, at the same time, 

 have added to our store of information on reactions of plant cells and tissues 

 to internal injuries. Interest in the anatomy of plants having disorders in- 

 duced by diseases and other agencies may be traced back to the early days of 

 anatomical research. This parallel development of the inquiries into the nor- 

 mal and the abnormal structures is only natural, for plants are constantly 

 exposed to agents and conditions that interfere with their development. In 

 fact, our usual division into normal and abnormal structures is rather arbi- 

 trary. When we say "normal" we simply mean that something is true in the 

 majority of instances. Some would object even to this broad definition of 

 normal since, actually, the peculiar changes in diseased and otherwise dis- 

 turbed plants are normal reactions to the injurious or merely modifying 

 effects. The difficulty of defining normality is further compounded by the fact 

 that whatever criteria we may choose for distinguishing between normal and 

 abnormal, be it with reference to structures or to reactions, the normal and 

 the abnormal intergrade with one another so that a clear delimitation between 

 the two is not to be found. 



We need terms and categories, however, for recording our observations 

 and for conveying their meaning to others. The important thing is not the 

 term or category itself, but that the sense in which they are used be made 

 perfectly clear. For the present purpose we may agree that normal is some- 

 thing we have chosen — somewhat arbitrarily, to be sure — to set up as a 

 norm; with reference to plants, this norm may be a structure or activity 

 usually encountered in plants that grow in conditions most appropriate for 

 them, free of diseases or other disturbances. The opposite is abnormal, that 

 is, something that deviates from that which we established as a norm. 



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