xxxv.] POTATO DISEASE, I. ITS ACTIVE STATE. 291 



apparently sound, but really diseased tubers : " It 

 should seem certain, then, that the mycelium or elements 

 of the fungus must have pre-existed in the tuber, and, 

 as it uniformly springs from the decayed spots, that it 

 has itself caused the decay. But here a difficulty arises 

 from the great obscurity, or, as some say, the total absence 

 of mycelium in an early stage of the disease. I have 

 satisfied myself, however, of its existence in some cases, 

 but not uniformly." Again, at p. 28, he writes : " On 

 examining the cuticular cells of a young tuber, with a 

 view to ascertain the changes which occur in the process 

 of greening, I found evident traces of mycelium within 

 them." In the same paper Mr. Berkeley refers to the 

 perennial mycelium of corn mildew. In the Outlines of 

 British Fungology, p. 42, Mr. Berkeley writes : " Spawn 

 . . . may exist for years without producing fruit . . . 

 whether it runs through soil or decaying substances, or 

 amongst living tissues, whether without or within their 

 walls." Professor de Bary was no doubt unaware of 

 these published observations when he wrote for the 

 Royal Agricultural Society, vol. xx. p. 265, 1876, that he 

 was perhaps the first to point out the presence of perennial 

 mycelium in the potato in 1863. No vegetable growth 

 is more common and well known than perennial my- 

 celium ; indeed, with very few exceptions, the spawn 

 belonging to all fungi must at times be perennial. Al- 

 though perennial mycelium has a far stronger hold on 

 life than have simple spores or conidia, it must not be 

 imagined that perennial mycelium always survives after 

 an unusual amount of heat, cold, or moisture. In the 

 same way as mushroom spawn often dies in the " bricks * 

 of the nurseryman, so the perennial mycelium perishes in 

 the invaded tuber of the potato. In examining diseased 

 potatoes in the winter and spring it is common to find 

 the mycelium dead, and if such diseased potatoes are 

 planted a perfectly sound crop will be the result. In the 

 Gardeners' Chronicle for 24th January 1874, we have re- 



