AN ANATOMIST S VIEW OF VIRUS DISEASES 93 



pass in or out of the tracheary elements (see Bennett, 1940b, 1956; Esau, 

 1938). However, a xylem-limited virus may have some unique properties 

 that bring about a specialized behavior in the tracheary elements. 



Conclusion. The above discussion points out many of the areas in which 

 anatomic studies have proved useful in research on plant viruses. The effects 

 of viruses upon the cells and tissues of plants reveal — at least in part — the 

 specific biologic relation of the virus to the tissues of the host and thus help 

 to characterize and to classify the viruses. The anatomy of virus-diseased 

 plants also has a broad, general significance in that a step-by-step recognition 

 of viral effects increases our understanding of the reactions of plants to injuries. 

 In this respect the anatomical investigations of plants infected with viruses 

 are comparable with studies involving the use of growth-regulating substances 

 and of surgical procedures designed to reveal the phenomena that are causally 

 related to the development of the specific structures and forms in plants. 

 In all these studies basic phenomena of development may come to light 

 through responses of plants to effects that interfere with the normal develop- 

 ment. The idea that studies of plant responses to diseases and other injuries 

 enhances our general understanding of plant development — ^in fact, are in- 

 dispensable for such an understanding — has been well expressed long ago 

 by Goethe (see Esau, 1938) when he said: "Never can we obtain a complete 

 comprehension [of a phenomenon] unless we consider the normal and abnor- 

 mal both at the same time and contrasted with each other." 



LITERATURE CITED 



Artschwager, E., and Ruth C. Starrett. 1936. Histological and cytological 



changes in sugar-beet seedlings affected with curly top. Jour. Agric. Res. S3 : 637- 



657. 

 Bennett, C. W. 1934. Plant-tissue relations of the sugar-beet curly-top virus. 



Jour. Agric. Res. 48:665-701. 

 . 1937. Correlation between the movement of the curly top virus and translo- 

 cation of food in tobacco and sugar beet. Jour. Agric. Res. 54:479-502. 

 . 1940a. Relation of food translocation to movement of virus of tobacco 



mosaic. Jour. Agric. Res. 60:361-390. 



. 1940b. The relation of viruses to plant tissues. Bot. Rev. 6:427-473. 



. 1943. Influence of contact period on the passage of viruses from cion to 



stock in Turkish tobacco. Phytopathology 33:818-822. 

 . 1944. Studies of dodder transmission of plant viruses. Phytopathology 34: 



905-932. 

 . 1956. Biological relations of plant viruses. Ann. Rev. Plant Physiol. 7:143- 



170. 

 AND Katherine Esau. 1936. Further studies on the relation of the curly 



top virus to plant tissues. Jour. Agric. Res. 53:595-620. 

 Butler, E. G. 1930. Some aspects of morbid anatomy of plants. Ann. Appl. Biol. 



17:429-443. 



