ON GRAPE MILDEW, 133 



From its very first appearance it was observed to be constantly 

 attended by a {angus {Oidium Tuckeri, Berk.). This fungus 

 resembles, to the naked eye, a white, mealy coat, which, accord- 

 ing to the virulence of the disease, forms a scarcely visible 

 powder, or a tolerably thick crust. In slight cases the fungus 

 arises only in certain places of the plant, and, without any defi- 

 nite rule, sometimes on the bark of the two-year-old shoots, 

 sometimes on tiie leaves, sometimes on the stalk of the bunches, 

 or on the berries themselves, but on such parts only as are 

 covered with a living cuticle, and never on the dead bark of the 

 older branches. In extreme cases the fungus covers every part 

 which has developed itself in the course of the summer so thickly 

 that at the distance of twenty paces the vine is at once known 

 to be diseased. 



A question has arisen in various quarters whether this fungus 

 is a peculiar species, and confined to the vine ; Marie, for instance 

 (Comp. Rend. xxxi. p. 312, 454), Guerin-Meneville, and Bal- 

 samo Crivelli (1. c. xxxiii. p. 295) assert that the same fungus 

 grows on many other plants, and that Oidium Tuckeri is iden- 

 tical with Old. leucoconium. Since it depends so much on the 

 peculiar views of individuals, whether two allied forms are consi- 

 dered as varieties or distinct species,* I shall not consider tliis 

 point more narrowly, j though the difference between Oidium 

 Tuckcin, with its clavate incrassated threads surmounted by only 

 one or two spores, and O. leucoconium, whose threads produce a 

 long row of spores, appears so striking-, that we must necessarily 

 allow the existence of a specific difference. The question seems 

 to me far more important whether Oidium Tuckeri is cajmble of 

 living on other plants besides the vine. If this were the case, it 

 would be probable that it might spread from diseased vines to 

 neighbouring plants. This, however, seems decidedly not to be 

 the case ;:]: at least, notwithstanding the most careful examina- 

 tion in a garden where plants of the most different families 

 stood close to highly diseased vines, I could not find a trace of 

 the fungus on any plant whatever ; it did not attack even Am- 

 pelopsis quiiiquefolia, although the boughs were intertwined 



* No great stress can be laid on the size of the spores, since in those of 

 Oidium Tuckeri, -which I figured by means of Sommering's apparatus, the 

 major axis varied from jL to ^"', the minor axis from ^ij to jig'". 



t It is very true that in fungi the reproductive organs vary greatly in 

 size ; but if the mean in every case be ascertainetl, it will be found available 

 for the distinction of species. To take a familiar example, peas may be 

 found in any undressed sample no larger than tares, and yet it may be safely 

 asserted that peas have much larger seeds than tares. — Traiisl. 



X It has been observed at Margate to extend to Indian Chrysanthemum 

 placed beneath vines which were affected to a great extent by the disease. — 

 Transl. 



VOL. vir. I' 



