xxxvi.] POTATO DISEASE, II. PASSIVE STATE. 309 



and illustrated by us in the Gardeners' Chronicle for 8th 

 July 1876. Germinating resting-spores in a more advanced 

 state of growth are illustrated at F and G. On ger- 

 mination, the walls of the oospore break up into many 

 pieces, sometimes into fine dust. The first sign of ger- 

 mination is generally shown by the walls of the oospore 

 breaking into two hemispheres, or into three or more 

 pieces. In some instances more than one oospore exists 

 within the oogonium, and all oospores may at times pro- 

 duce zoospores, as in Cystopus. In the Gardener^ Chronicle 

 for 8th July 1876 a full description, with numerous 

 illustrations of germinating oospores, will be found. 



Our experiments were at first objected to on the ground 

 that all the species of Peronospora were, it was said, so 

 sensitive to decay that they invariably perished with the 

 death of the supporting plant. This statement is now 

 known to be erroneous, and the resting-spores of the 

 Peronosporece are at this time always sought for, and almost 

 invariably found, in material which has been more or less 

 destroyed by the mycelium of the invading parasite. 

 This decayed material is obviously the only material in 

 which ripe oospores can be expected to occur. Every 

 part of the fungus, except the oospores, generally perishes 

 with the supporting plant ; the oospores or resting-spores 

 are left alive Upon or in the ground where potato material 

 has decayed, and in this position the oospores germinate 

 in June and produce the first conidia of the season. Such 

 of the conidia as are blown from the ground or from 

 decaying potato refuse on to potato plants, or certain allied 

 plants, produce disease ; such as fall in unsuitable positions 

 perish. The progress of the disease is, therefore, necessarily 

 at first extremely slow: it only progresses with rapidity 

 after the living potato plants are thoroughly invaded. 



We have secured potato oospores direct from the ground 

 by observing water filtered through earth on which 

 diseased potato material has been allowed to decay. 



The best time for seeing the reproductive organs of 



