Dec. 9, 191S Histological Studies on Potato Leaf roll 569 



such a degree as to cause a collapse and folding in of the walls of the 

 phloem cells. Often, it appears, the formation of intercellular spaces, 

 though initial, is only an accompanying phenomenon; the pathological 

 changes in the cells of the phloem themselves proceeding so rapidly as to 

 cause a rapid cessation in function and final death. The diseased cells 

 become filled with a granular or globular deposit, probably metamor- 

 phosed protoplasm ; the cell walls themselves also undergo changes. The 

 deposit is resistant to acids, becomes bright yellow on treatment with 

 alkalis, and gives reaction for cutin; in no instance, however, could the 

 presence of lignin be shown, a fact already claimed by Schander and 

 Tiesenhausen (7), but disputed by Quanjer. It is of interest to note 

 that usually not all cells in a group become affected and even in very 

 extreme cases there are normal, functional cells within a diseased group. 

 The cells which thus remain normal are either parenchyma cells or sieve 

 tubes. 



Accompanying the destruction of the phloem groups and the forma- 

 tion of intercellular spaces are changes in the parenchyma cells of the 

 pericycle and perimedullary zone consisting of a radial stretching of these 

 elements and sometimes in an increase in the actual number. Occasion- 

 ally the pericycle increases to such an extent as to form a band of tissue 

 between the outer fibers and the primary phloem. 



An unusual type of phloem necrosis is sometimes observed in plants 

 affected wdth leafroll. The cells of the phloem are neither shrunken nor 

 chemically changed. The walls, hov.'ever, are very attenuate, in places 

 completely obliterated, or sometimes slightly swollen (PI. 39, B). The 

 protoplasm is abundant and usually disorganized. Whether this con- 

 dition is a step in the progress of the disease and is later followed by 

 cutinization or other chemical changes, the writer can not state. It does 

 not appear to be such though, since this condition is often found in fully 

 mature plants. 



The formation of intercellular spaces is not always the first disease 

 phenomenon. In the internal region, near the fibers, a thickening of the 

 walls of both phloem and pith cells occurs. No chemical changes have 

 been demonstrated to accompany these swellings, and the walls remain 

 pure cellulose. Plate 38, D, shows the beginning of cell wall thickening 

 in a primary phloem group. Although this thickening is sometimes very 

 extensive, the cells themselves do not seem to be harmed thereby. 



Necrosis, as we have seen, is restricted to certain areas {"difjeren- 

 zierete Nekrose" of Kiister; j, p. 309), notwithstanding the close anasto- 

 mosing of the individual groups of the phloem and the direct connection 

 of external and internal phloem through the leaf gaps. If a specific 

 virus is the cause of potato leafroll, one would expect all phloem groups 

 to be indiscriminately aff'ected. Very often, however, only certain 

 groups are diseased, and if the attack is severe the pathological condi- 

 tion may still be restricted either to the internal phloem or the external 

 88094°— 19 4 



