Dec.9. i9i8 Histological Studies on Potato Leajroll 563 



inner phloem are afifected. The individual groups are normal in size; 

 there are a few cell wall thickenings, but no evidence of formation of 

 intercellular spaces. Most of the walls of the sieve tubes and companion 

 cells have become faint and in places are completely obliterated. The 

 cell content is mostly disorganized protoplasm. These diseased groups 

 take the cellulose stain, less distinct, however, than the normal cells. 



The basal region of the stem and the underground parts are normal in 

 structure; the flowering axis has not been examined. 



I^EAFROLIy IN MAGNUM BONUM VARIETY 



The tubers were obtained from the Agricultural College, Copenhagen, 

 Denmark, and planted, together with other varieties under investigation, 

 in the disease garden of the Plant Pathology Department of Cornell 

 University. The plants grew normally at first; most of them, however, 

 developed signs of disease in late summer, though a few showed symptoms 

 of leafroll very early. Material for study was taken in late July from a 

 plant which was large and well developed, but the axillary shoots of 

 which were erect and broomlike in appearance. The lower leaves were 

 normal, the intermediate and upper ones reduced in size and folded along 

 the midrib. Discoloration of the leaves was only slight. 



Microscopic examination of both stained and fresh material shows 

 that the vascular tissue of the midrib is abnormal in quantity with thick- 

 enings of the walls of the phloem adjacent the fibers (PI. 45, A). These 

 thickenings stain black with Haidenhein's hematoxylin — that is, they are 

 of cellulose. The lamina of the leaf, however, is severely diseased. The 

 partly obliterated cells are filled with a granular or globular deposit, which 

 is yellow in unstained sections and of the same nature as found in the 

 diseased cells of the phloem in the Paul Kruger variety. The petiole 

 shows also normal anatomical structures. The bundles of the petiolar 

 wings are above normal in size. Fibers in both external and internal 

 region are of unusually large number (PI. 45, B). 



Stem sections, whether taken near the distal or the basal end, show 

 normal structures. The fibers here too are very plentiful. The external 

 phloem shows a large amount of secondary sieve tubes and medullary 

 ray tissue, all of which appears normal and functional. 



LEAFROLIv IN EARI^Y ROSE VARIETY 



The tubers were obtained from the same source as were those of the 

 Magnum Bonum variety; they were planted at the same time and re- 

 ceived the same cultivation and care.. The plants grew very unevenly. 

 A few of them attained normal size, but most of them remained small, 

 showing symptoms of severe leafroll. Plate 36 shows such a plant. 

 The stem is not above 15 cm. tall; the leaves are rolling, but are not re- 

 duced in size. There is slight, indistinct discoloration of the lamina. 



The general amount and arrangement of the vascular tissue of the mid- 

 rib is normal. Fibers are wanting, and the phloem groups of the inner 



