xxxvi.] POTATO DISEASE, II. PASSIVE STATE. 299 



stances (but not in all) the antheridium springs from the 

 same thread as the oogonium, and close to the oogonium 

 itself ; here the protoplasm again forms a smaller bladder- 

 like expansion, which soon becomes (as in the oogonium) 

 separated from the supporting thread by a septum, as at 

 B, Fig. 131. 



After fertilisation the oogonia readily fall from their 

 supporting threads, just as simple spores drop from their 

 supports, or as seeds drop out of seed-vessels. There is 

 no further need of connection with the parent stem, so. 

 the resting-spores, like seeds, are set free. 



It follows from this fact, that unless the oogonia are 

 sought for at the precisely right time, they will not be 

 seen upon the supporting threads, which soon perish. A 

 notable instance of this phenomenon occurs with the 

 Peronospora of the lettuce. In this species myriads of 

 oogonia are formed in dense conglomerated masses, Fig. 

 126 ; but they are no sooner formed than they are cut off 

 from their supporting mycelium by septa, and then the 

 mycelium perishes. Nothing is more common than to 

 see large numbers of oogonia and no mycelium. The 

 case is similar with free seeds or seed-vessels where the 

 old tender flower remains have perished. 



Every competent botanist who has sought for these 

 potato oogonia since 1875 has found them. Mr. C. B. 

 Plowright found both the smooth and spiny form in the 

 summer of 1876, as described and illustrated in the 

 Gardeners' Chronicle for 29th July 1876. At the present 

 time both oogonia and ripe oospores, not only of the 

 potato fungus, but of various other species of Peronospora, 

 may be purchased from the Kev. J. E. Vize. 



In 1875, when our observations were made, great 

 attention was directed towards the potato disease, as it 

 not only occurred with great virulence, but it appeared 

 unusually early in the season. In that year the leaves 

 of the potatoes were the first organs attacked, and old 

 observers said that no such curled and distorted leaves had 



