NATURAL HISTORY SOCIETY OF GLASGOW. 205 



spots on the potato haulm and leaf are the result of their action. 

 If we consider that it is only when the zoospores act on the leaf 

 that these brown spots are produced, it seems evident that the 

 fungus is not the result of a diseased state of the potato, but really 

 causes that disease. Both modes of reproduction — by conidia and 

 by zoospores — are asexual, and both bodies are used up in the 

 production of the mycelium, while its delicate filaments can but 

 rarely survive the cold and wet of winter, and must usually 

 perish with the leaves and haulm of the foster plant. In other 

 species of Peronospora bodies called oospores had been known 

 for many years as the products of the contact of two sexual, 

 spore-like bodies — the antheridium m and oogonium • but it was 

 not till 1875 that anything similar was discovered in P. infestans, 

 the credit of this important accession to our knowledge being 

 whollv due to Mr. Worthinojton Smith. The antheridium not 

 only comes into contact with the oogonium, but fixes a small 

 fecundating tube — a pollinodium — into its wall, discharging through 

 it part of its protoplasm and producing fertilisation. The oogonium 

 then matures into the oospore in the same way as the ovary of 

 phanerogams becomes the perfect fruit. When this " resting spore" 

 is mature, the mycelial threads disappear and it lies free among the 

 cells of the tissue of the potato plant. These oospores are not 

 delicate and unenduring like the conidia and zoospores, but become 

 at length dense, dark bodies, covered with reticulated warts. The 

 potato plant decays during winter, but they survive and are 

 washed by the rains into the soil, where they rest for a variable 

 period, when they germinate in the damp earth, either fixing 

 themselves on potato plants, if these are near, or producing 

 zoospores, which act similarly to those previously described, 

 germinating in the same way, If potato plants are not available 

 to which they can attach themselves, in process of time they perish. 

 Farmers have often unwittingly done everything possible to 

 facilitate the attachment of these germinating oospores to their 

 host, the potato. Some of them, laudably desirous of utilizing 

 everything, have the haulms, decaying tubers, and other refuse of 

 their potato crop removed to their manure heaps, where they rot 

 and become manure. But the little oospores do not rot. They 

 lie with their vitality shut up in them till next spring, and when 

 the farmer has his manure spread thick over his potato fields, he 

 is sowing at the same time myriads of these little germs, which 



