MOULDS. 



161 



of spinacli utterly destroyed by this fungus ; whilst 

 on another, not twenty yards apart, not a spotted 

 leaf could be found. This mould is the Peronospora 

 effiisa of botanists ; it occurs also on some species 

 of goosefoot (plate X. fig. 215), and probably on 

 knotgrass. To the naked eye it appears in pale 

 purphsh-grey patches, which, when examined mi- 

 croscopically, are found to consist of dense bundles 

 of branched threads, bearing ellipsoid acrospores, 

 the membranes of which have a violaceous tint. 

 The oogonia produced upon the mycelium vary 

 considerably in size. The oospores are of the 

 character delineated in our plate (plate X. fig. 

 214). 



Hitherto all the species of mould to which w© 

 have had occasion to refer have been found infest- 

 ing plants more or less employed as food; but 

 there remain one or two other species to which 

 we must make special reference. One of these 

 aflFects the most universal of favourites amongst 

 flowers : this is the rose mould. Attention waa 

 directed to this mould, and it was described for the 

 first time under the name of Pero7iosjpora sparsa, 

 in the columns of the Garden&t^s' Chronicle, in 1862, 

 It occurred on a quantity of potted rose-plants in a 

 conservatory. Irregular pale brownish discoloured 

 spots appeared on the upper surface of the leaves ; 

 these extended rapidly, and in a short time the leaves- 

 withered and shrivelled up, and ultimately the 

 whole plant perished. A delicate greyish mould 



M 



