INTRODUCTION TO CRYPTOGAMIC BOTANY. 277 



leaves. It would be as rational to expect that plants would 

 thrive under a brown bellglass, as that vegetation should not 

 be impaired where the greater part of a plant is covered with 

 St thick dark felt. The black mildews are often accompanied or 

 preceded by a Coccus, and I believe that the first stage of 

 growth of most Antennarice is more frequently observed 

 where the leaves have been soiled with honey-dew. 



282. The white mildews, at least, are easily kept under by 

 the application, at a sufficiently early stage of growth of the 

 minute parasite, of powdered sulphur. Its mode of action, 

 probably, depends on the disengagement of sulphurous acid.* 

 The day is not far distant when these affections will be as 

 effectually subdued as the bunt in wheat. 



283. The genus Chcetomium is developed on decaying sub- 

 stances, as paper, straw, &c., or even on old plaster walls, where 

 there can be little organic matter present. The thin brittle 

 walls and mouthless perithecia distinguish it principally from 

 Sphceria. The asci in the young plant are evident enough, 

 but they are soon absorbed, leaving behind a mass of very 

 dark sporidia. As in Meliola and Erysiphe, their walls pro- 

 duce threads which are sometimes amorphous, but occasionally 

 very beautiful in structure. The species occur in various parts 

 of the world, and probably, from their ubiquitous habit, live 

 in great measure on matter derived from the atmosphere. The 

 use of the fulcra in Erysiphe and Meliola, so variable in form 

 and number, does not seem to be well ascertained. It is very 

 doubtful whether they extract anything from the matrix. 



* The crude sulphur, it should seem, combines with oxygen in a 

 nascent state, and so forms sulphurous acid, which is eminently destruc- 

 tive to mucedinous bodies, as appears from its effect iu stopping fer- 

 mentation. The more finely the sulphur is divided the better, and 

 therefore it is most efficacious when applied in such a form as to cause 

 a chemical deposit of sulphur, or, as it is called in the older chemistry, 

 a magisterium on the leaves. (See Gard. Chron. 1855, p. 403.) For 

 this purpose, a solution of sulphuret of potassium is treated with 

 an acid. Liver of sulphur, which consists of tersulphuret of pot- 

 assium, hyposulphite of potash, and sulphuret of potassium, answers 

 the same purpose. (See also Bouchardat, Trait6 de la Maladie de la 

 Vigne.) 



