REPORT OF THE SOCIETY 



75 



tubers are considerably shortened and the tubers are smaller, fewer, and are 

 conspicuously harder than tubers from healthy plants. This hardness of the 



tubers is maintained even for consi- 

 derable periods in storage. 



The rolling usually appears in 

 the lower leaves first but a late 

 infection maj' affect only the leaves 

 last developed. 







I 



Efifect on the Plant 



As with mosaic the leaves of 

 potatoes suffering from leaf roll do 

 not function properl^^ The causal 

 principle is translocated in the plant 

 juices and in this case it especially 

 affects that part of the vascular 

 tissues known as the phloem. In the 

 phloem it gives rise to a necrosis or 

 death of the tissue. Since it is in the 

 phloem that most of the conduction 

 of elaborated food takes place, those 

 parts awaiting supplies of food for 

 their development will receive onh^ a 

 much reduced amount. This is the 

 case with the tubers and the stolons 

 bearing the tubers. Hence those 

 tubers which do develop are small 

 and are borne on short underground 

 stems. ]Moreover the infective principle passes down the parts of the 

 phloem not killed and reaches the vascular tissues of the developing tuber. 

 In it, therefore, there may also be phloem-necrosis. When such a tuber 

 is cut across, or when it is pared to expose the vascular tissues, these are seen 

 to be browned. The general term "net -necrosis" is used to indicate such 

 an appearance, but it must be recollected that net-necrosis may be caused by 

 other conditions than those in leaf-roll. Moreover the tubers from a plant with 

 leaf-roll do not necessarily develop net-necrosis, for they maj^ not be affected 

 soon enough, nor sufficiently severely, to cause necrosis in the vascular tissues. 

 A further effect on the plant is exhibited in the next generation for tubers from 

 diseased plants, and especialh' those with net-necrosis, frequently develop very 

 w^eak, spindling sprouts. 



The yield from leaf-roll plants is materially reduced, in many cases being 

 nothing at all. 



Fig. 4 — A leaf showirg the dwj.rf, curled, 

 daik green symptoms of mosaic. There 

 are no apparent light green patches. 



