MYXOGASTRES 531 



such spore-balls yields a diseased crop, proving that the 

 spore-balls retain their vitality in the land for at least one 

 season, and are capable of infecting young potatoes. 



The aggregations of spores, or spore-balls, each of which is 

 the outcome of the conversion of the mass of plasmo- 

 dium contained in a single cell of the potato tuber into 

 a mass of spores, are irregularly globose or elliptical in 

 shape, and vary from 40 to 50 p in diameter. The com- 

 ponent spores are globose, and vary from 3-5-4 ^ in 

 diameter. 



The origin of the spore-balls is briefly as follows : The 

 plasmodium first appears in the cells of the potato under 

 the form of minute amoeboid bodies, capable of indepen- 

 dent movement. These amoeboid bodies soon coalesce 

 to form a compact mass, or plasmodium. This plas- 

 modium, after remaining in a- vegetative condition for some 

 time, during which the contents of the host-cell, starch, 

 protoplasm, and nucleus, have been used as food, becomes 

 frothy and vacuolate. Finally the plasmodium becomes dis- 

 tended into a thin film by the formation of a large central 

 vacuole, the thin film or wall becoming pierced here and 

 there by irregular holes. When this stage is reached the 

 outer wall of plasmodium becomes simultaneously converted 

 into a layer of cells or spores, thus the mature spore-ball 

 consists of a single layer of closely compacted spores, broken 

 here and there by irregularly shaped holes, surrounding a 

 central cavity. 



This disease is rampant in the west of Ireland, not un- 

 common in Scotland and the north of England, but is 

 rare in the south. It is also common in several continental 

 countries. 



As the parasite hibernates in the tubers, it is important 

 that those showing signs of infection should not be used for 

 1 sets.' I know it has been stated that if diseased tubers are 

 treated with formalin, the parasite is killed, and such tubers 

 may be planted with safety. This statement, although made 

 by myself some years ago, I am now very much inclined to 

 doubt. The superficial plasmodium may be killed by such 

 treatment, but the deeper-lying portion would certainly escape 

 injury. 



Infected land should be dressed with quicklime, preferably 

 in the spring, when the spore-balls are germinating in the soil. 

 As Spongospora is not known to attack any other kind of 



