263 



Pa-onospora arenari^ (De By), which, though considered rare, seems to be 

 locally plentiful, has too succulent a host plant to let the resting spores be easily 

 discovered I tried to produce some artificially, but to no purpose. 



Other species I have endeavoured to get, but signally failed-f ailed, probably, 

 to make me persevere all the more in working to find them. 



Since the above was written I have examined Professor Farlow's paper on the 

 Enumeration of the Peronosporece of the United States. He gives desenpt.ons of 

 26 species of Peronospora with the oospores of all of them, naming their measure- 

 ments He then gives five more species, as "species whose oospores are unknown. 

 P. sordida (Berk.) and P. sparsa (Berk.) are however the only two British ones 

 enumerated in these five. 



Cstopus spinulosus (De By.) is a plant recorded, to the best of my knowledge, 

 only in Norfolk, and in the neighbourhood where I live. I imagine it ,s tolerably 

 frequent, however, judging by the abundance of it when it is found at Forden. 

 It grows on the common thistle by the sides of the high road, close to the grassy 

 banks where the grass and the road unite, but the most prolific spot of all is where 

 the stones are cast which are to be used for repairing the road. I have noticed year 

 after year how very favourable to the luxuriance and development of the Cystopm 

 this sort of place is. The conidia, when once found, are a tolerably sure clue to 

 the oospores, for the conidia retain their place as a centre, and as they ripen the 

 mycelium extends from them into the parent host on all sides. The result of this 

 is, that the nourishment of the thistle in that place is absorbed by the fungus, 

 which thus finds a suitable nidus for its oospores. A brown patch on the decaying 

 leaf of thistle is au almost sure index to the resting spores, and of course the more 

 lifeless the leaf looks the surer of success the searcher for the oospores will be. 

 The oospores are so fine, and so large, that by soaking in water for a short time 

 they can be seen with an ordinary pocket lens in situ. It took me some time to 

 find these plants in the oospore state last August, but when once discovered they 

 are easy enough to find afterwards. 



Cstopus cubuus (Str.) I have had no opportunity of examining alive. Cys- 

 topus'candidus (Lev.) seems to follow a similar life history in developing its 

 winter state that C. spimdosus does. I believe, however, that when it grows on 

 certain plants, horse-radish, for instance, the tissue of the host plant is so thin 

 that the ripening of the oospores takes place considerably on the ground, and 

 therefore is not so easy to find. You can easily see, even in a few days, what 

 ravages the fungus has made in the conidial state by the holes formed in the 

 horse-radish itself. On Shepherd's Purse the oospores are much more easily traced 

 than in the above plant, owing to the thickness of the plant on which they mature. 

 Oistopus lepigoni (De By.) has recently been sent to me by Mr. Plownght. 

 The oospores are plainly enough to be seen, but will be much better as to ripeness 

 in a short time. 



