564 Journal of Agricultural Research voi. xv, no. 10 



region show disorganization of cell content and partial obliteration of the 

 walls of the individual cells. The spongy parenchyma of the lamina is 

 abnormal in places; the cells in such a region are filled with a yellow 

 deposit and the walls themselves are of a yellow color. The petiole is 

 normally developed. Fibers are present, and the phloem cells in the 

 region of the inner fibers are more or less thickened. 



Stem sections are, in general, normal. While the outer phloem shows 

 no evidence of pathological condition, the cell walls of the inner phloem 

 groups are partly destroyed and the protoplast disorganized. In no 

 instance, however, is there a thickening or discoloration of the walls of 

 these cells. 



A summary of the results of microscopic study on European leafroU 

 shows the following conditions : 



1. Plants with symptoms of typical leaf roll always show pathological 

 changes in the vascular tissue. 



2. These pathological changes are the more pronounced the earlier 

 and more intense the external symptoms appear. 



3. While the primary phloem groups are most commonly affected, 

 the cells of the pericycle and fibers as well as cortex are often found 

 diseased. 



4. The phloem groups are shrunken and completely destroyed only 

 in rare cases. Most usually the cells of the groups are separated by 

 intercellular spaces; the cells may later be crushed and the cell content 

 changed into a granular or globular, gummy deposit. There is seldom 

 a complete destruction of the individual cells in a group and never a 

 sign of lignification. 



5. The number of primary phloem groups destroyed is not always 

 very extensive, and since the development of secondary elements is very 

 pronounced the translocation of elaborated food material from the 

 leaves to the tubers is not prevented or only slightly interfered with. 



DESCRIPTION OF AMERICAN LEAFROLL ON COLORADO MATERIAL 



The cause of the falling off in yield of the Irish-potato crop in northern 

 Colorado in 191 1 was thought by local growers to be species of Fusarium 

 and Rhizoctonia, but others have believed that, though there is a varia- 

 tion in the external symptoms observed, the Colorado disease is the 

 type of leafroU described by Appel and other investigators in Europe. 



The material for study was obtained from the Government station 

 at Greeley, Colo., and included one seedling and two standard varieties: 

 Netted Gem and Pearl. These plants were under observation through- 

 out the summer, the progress of the symptoms being recorded at definite 

 intervals. 



LEAFROLL IN POTATO SEEDLING 



The mature plant was dwarfed, erect and broomlike in appearance. 

 The leaves were reduced in size and folded characteristically along the 

 midrib. The seed piece was found unexhausted in the soil. 



