300 INTRODUCTION TO CRYPTOGAMIC BOTANY. 



controlled. Under proper conditions of temperature, the 

 acetic fermentation will take place on tlie application of yeast, 

 but not so surely or speedily as by the mycelium of the Peni- 

 cilliwm, which is known under the name of the Vinegar plant, 

 a filamentous condition instead of a vesicular. 



317. The production of yeast depends upon the extreme 

 facility with which moulds adapt themselves to peculiar cir- 

 cumstances. The proper position of such moulds is upon 

 the surface of decaying substances ; but several species are 

 capable of sustaining hfe when completely immerged. In such 

 a condition they cannot produce any real fruit, but they are 

 propagated by means of shoots from the mycelium. Sub- 

 stances, which would prove fatal to many other vegetables, as 

 solutions of arsenic, opium, and many other poisonous chemical 

 substances, do not prevent the growth of moulds. One form 

 proves an intolerable nuisance in electrotyping, being deve- 

 loped in the solution of copper used in that process, and 

 becoming itself eventually thoroughly electrotyped. Under 

 such circumstances, they have the power of separating the 

 metal or other noxious principle, while they avail themselves 

 of any nutritive matter with which it may be combined. 

 These fluid-born states of Penicillium, and other more or 

 less allied Mycelia, are often regarded as Algse, but they have 

 no affinity with those vegetable productions. 



318. One genus of moulds was long considered as peculiarly 

 destructive to living vegetable tissues, and the grape mildew, 

 peach mildew, blanc de rosier, &c., are all attributable to it ; 

 but it has already been shown that these supposed species of 

 Oidium are not true moulds, but merely states of different 

 species of Erysiphe. This is, however, not the case with that 

 class of moulds which belong to the old genus Botrytis, or to 

 Corda's genus or subgenus, whichever may be the more correct 

 term, Peronospora. These moulds run, by means of their my- 

 celium, amongst the loose tissue of the leaves, and at length 

 protrude fertile branches through the stomates. Tulasne, 

 Caspary, and others, have lately discovered that there is 

 another form of fruit, with far more complicated and larger 

 spores developed at the base of the fronds. The genus Arto- 



