AN anatomist's VIEW OF VIRUS DISEASES 83 



gent paths of cellular differentiation in the xylem and the phloem, viruses may 

 profoundly disturb such gradients. 



An interesting problem is presented by the pathologic accumulation of 



HEALTHY 



Abnormal- 

 sieve elements 



Fig. 7-10. Effect of the curly top disease upon the tobacco leaf. — Fig. 7. Cross 

 section of a petiole with the vascular bundle indicated by the stipphng. The 

 rectangular area locates the position of sections hke those depicted in fig. 9 and 

 10. — Fig. 8. Leaf, with the horizontal line indicating the position of the cross section 

 of the petiole shown in fig. 7. — Fig. 9. Part of a cross section of a vascular bundle 

 from a healthy plant. Six sieve elements are mature. Above in the figure, one xylem 

 element is discernible. — Fig. 10. Part of a cross section of a vascular bundle from a 

 diseased plant. In addition to a few small normal sieve elements many large abnor- 

 mal sieve elements are present. In this figure and in fig. 9 the vascular-bundle cells, 

 except the mature sieve elements, are stippled; the adjacent parenchyma cells are 

 left blank. The densely stippled cells in fig. 10 were those with various signs of 

 disorganization in their protoplasts. (Fig. 9 a7id 10 adapted from Hilgardia Vol. 11, 

 No. 8, 1938, a7id Vol. 13, No. 8, 1941, respectively.) 



starch in leaves of plants affected by various viral diseases. These may be 

 mosaic diseases, diseases induced by phloem-limited viruses, or by those asso- 

 ciated with the xylem. It is likely that the excessive accumulation of starch 

 is a secondary effect that may be related to degenerative changes in tissues 



