18 THE RESTING-SP0RE8 OP THE POTATO DISEASE. 



was necessary because the tissue of the diseased leaves was so 

 opaque and corroded, and the cell-walls were so thickened that it 

 was difficult to distinguish the threads and suspected spores from 

 the cellular tissue. I did not treat the leaves with boiling water, 

 because I wished to keep the threads and spores alive. 



From day to day I kept the diseased leaves and stems and tubers 

 wet between pieces of very wet calico, in plates under glass, and I 

 immediately noticed that the continued moisture greatly excited 

 the growth of the mycelial threads ; this to me was quite unex- 

 pected, as I had merely wished to set the spore-like bodies free. 

 So rapid was now the growth of this mycelium that after a week 

 had elapsed some decayed parts of the lamina of the leaf were 

 traversed in every direction by the spawn. Thinking the close 

 observation of this mycelium in the now thoroughly rotten and 

 decomposed leaves might end in some addition to our knowledge 

 of Peronospora infestaus, to which fungus I had no doubt from the 

 beginning that the threads belonged, I kept it under close obser- 

 vation, and in about ten days the mycelium produced a tolerably 

 abundant crop, esj^ecially in the abortive tubers of the two-sized 

 bodies I had previously seen, and measured in the fresh leaves. The 

 reason why these objects, which undoubtedly occur in and about 

 the spots, are so extremely few in number in those positions is, I 

 imagine, because they require a different set of conditions for their 

 normal growth, and these conditions are found in abundant and 

 continued moisture. 



The larger of these bodies I am disposed to consider the 

 " oospore" of the potato fungus, and the smaller bodies I look 

 upon as the " antheridia" of the same fungus, which are often 

 terminal in position. The filaments of the latter are commonly 

 much articulated, and sometimes more or less moniliform or neck- 

 lace-like. Both oospore and antheridium are very similar in nature 

 and size to those described as belonging to Peronospora alsincarum 

 and P. umhellifenim^ and this is another reason (beyond my seeing 

 undoubted P. infestans on potato leaves at the beginning of June) 

 why I am disposed to look upon these bodies as the oospore and 

 antheridium of the potato fungus. 



The larger bodies are at first transparent, thin, pale brown, fur- 

 nished with a thick dark outer wall, and filled with granules ; at 

 length a number (usually three) of vacuities or nuclei appear. The 

 smaller bodies are darker in colour, and the external coat is marked 

 with a few reticulations, possibly owing to the collapsing of the 

 outer wall. At present, I have been unable to detect any fecunda- 

 ting tube (described as belonging to the antheridium of other species 

 of Peronospora), but I have observed the two bodies in contact in 

 several instances. After fertilisation has taken place, the outer 

 coat of the oospore enlarges, and appears to be cast off. Both 

 antheridium and resting spore are so slightly articulated to the 

 threads on which they are borne that they are detached by the 

 slightest touch, but with a little care it is not really difficult to see 



