MYXOGASTRES 529 



description and figure is the oldest, that enables any one 

 to be certain as to the fungus he had in view. 



The action of the parasite on a potato tuber is very varied. 

 Sometimes a considerable number of small, more or less 

 circular, superficial scabs are formed, as in the accompanying 

 illustration. In other instances more or less extensive 



' 



FIG. 158. Julus pulchellus. A millipede that 

 frequently damages potato tubers ; mag. 



cavities are formed, which become lined with a dense mass 

 of snuff-coloured powder, consisting of spore-balls. These 

 cavities are sometimes quite large, and are frequently aug- 

 mented in size by Julus pulchellus, which is often present 

 in great numbers, so that eventually the potato becomes 

 hollowed out. In other cases the Sorosporium causes the 

 formation of large, projecting, more or less truncate out- 

 growths, which might at first sight be mistaken for 'black 

 scab.' 



The plasmodium of Sorosporium may be seen in the cells 

 of the potato just below the surface of the scab, and gradually 

 encroaches on the sound portion of the potato by passing 

 from one cell to another, the older or most superficial portions 

 of the plasmodium becoming gradually transformed into 

 spore-balls. 



The plasmodium appears to be only active during the 

 period when the tuber is growing, and passes into a resting 

 condition when the tuber is dormant during the winter. In 

 the spring, when the potato commences to sprout, the 

 plasmodium again becomes active and migrates from the old 

 tuber or ' set ' into the new tubers formed during the process 

 of growth. By such means the parasite passes from one 

 generation of potato tubers to another without leaving the 

 host. Many of the spore-balls produced by a diseased crop 

 pass into the soil, and it has been shown that land containing 



2 L 



