HABITATS. . 241 



meuts made by De Bary seem to prove that the species of 

 Peronospora will only flourish on certain favoured plants, to the 

 exclusion of nil others. The non-parasitic moulds are scarcely 

 exclusive. In Oidium some species are parasitic, but probably 

 all the parasitic forms are states of E,ysiplic, the non-parasitic 

 alone being autonomous ; of these one occurs on Porrigo lupi- 

 nosa, others on putrefying oranges, pears, apples, plums, &c., 

 and one on honeycomb. Acrospeira grows in the interior of 

 sweet chestnuts, and we have seen a species growing within the 

 hard testa of the seeds of Guilandina JBondue, from India, to 

 which there was no external opening visible, and which was 

 broken with considerable difficulty. Several 3Iucedines are 

 developed on the dung of various animals, and seldom on any- 

 thing else. 



The Pln/somi/cclcs consist of two orders, Antennariei and JLTu- 

 corini, which differ from each other almost as much in habitat 

 as in external appearance. The former, if represented by Antcn- 

 naria, runs over the green and fading leaves of plants, forming 

 a dense black stratum, like a congested layer of soot; or in Zas- 

 fliidium, the common cellar fungus, runs over the walls, bottles, 

 corks, and other substances, like a thick sooty felt. In the Mu- 

 corini, as in the JLTucedincs, there is usually less restriction to 

 any special substance. Jlfucor muccdo occurs on bread, paste, 

 preserves, and various substances ; other species of JMucor seem 

 to have a preference for dung, and some for decaying fungi, but 

 rotting fruits are nearly sure to support one or other of the 

 species. The two known species of the curious genus Pilobnlus, 

 as well as IlydropJiora, are confined to dung. Sporodinia, St/zy- 

 gites, &c., flourish on rotten Agarics, where they pass through 

 their somewhat complicated existence. 



The Ascowyccfcs contain an immense number of species, and in 

 general terms we might say that they are found everywhere. The 

 Tubcrncci are subterraneous, with a preference for calcareous dis- 

 tricts. The Perisporiacei are partly parasitical and partly not. 

 The Erysiplici include those of the former which flourish at the 

 expense of the green parts of roses, hops, maples, poplars, peas, 

 nnd many other plants, both in Europe and in North America, 



