THALLOPHYTA. 677 



About 50 species of Exoascaceae have been distinguished. 



Perisporiacece. Here are included all forms in which the asci are inclosed in 

 fruit-like bodies, i.e. the Mildews, Moulds, and Truffles. 



The Mildews are chiefly leaf-parasites, and spread their mycelium over the 

 surface of the foliage and send their suckers (or haustoria) into the epidermal cells 

 (cf. fig. 32 2 , vol. i. p. 165). In due time they produce their ripe ascus-fruits like 

 tiny black grains scattered over the surface of the leaf. Each of these fruits con- 

 sists of a shell-like investment inclosing one or more asci, each of which contains 

 8 spores. Sphcerotheca is the simplest form, there being but a single ascus in its 

 fruit. pannosa is the Rose-mildew, and S. Castagnei the Hop-mildew, a very 

 destructive parasite in Hop-growing districts. Erysiphe has several asci in its 

 fruits, and includes the well-known E. Tuckeri, the true Mildew of the Vine (to 

 be distinguished from Peronospora viticola, figured on p. 670, which is the false 

 Mildew). A tropical genus of leaf-parasites allied to our Mildews is Meliola, which 

 is widely distributed. 



The Moulds include several exceedingly common saprophytes which make their 

 appearance on the most various sorts of organic matter. The Blue Moulds, which 

 occur on jam, bread, leather, &c., are probably the best known and most commonly 

 recognized of all the smaller Fungi. These forms spread their mycelia over any 

 suitable substratum, and penetrate it with their hyphas. Their usual fructification 

 is not the ascus-fruit, but clusters of conidia, borne on erect hyphae, which stand out 

 from the mycelium. Two common Moulds are represented in fig. 193, p. 18. Asper- 

 gillus niger (figs. 193 4 and 193 6 ) bears its conidia in spherical tufts on enlarged 

 aerial hyphae. The swollen end of an aerial hypha is densely set with cylindrical 

 cells, from which the conidia are abstricted one after another. Penicillium crusta- 

 ceum (figs. 193 8 and 193 9 ) is very similar, but here the conidia are borne on a 

 hypha which branches near its extremity like a compound umbel. Another form, 

 Eurotium, is shown in fig. 385 7 , p. 679. The ascus-fruits of these Moulds are not 

 very conspicuous, nor are they always very plentifully developed. They arise on 

 the mycelium after the conidial stage is over, and when ripe are about the size of 

 small shot. They commence by the entwining of certain hyphal branches (Peni- 

 cillium, fig. 193 6 , p. 18; Eurotium, fig. 385 9 , p. 679) which have been regarded as 

 representing male and female organs (cf. p. 60). That fertilization takes place is 

 strenuously denied by many modern mycologists, and the sexual nature of the 

 entwining hyphae is not universally recognized. Be this as it may, the result of 

 the process in question (which also takes place in the Mildews) is the formation 

 of a sinuous hypha, which becomes embedded in a dense cortical sheath which 

 grows up from the mycelium close by the place of origin of the entwined hyphae. 

 This is the young fruit-body; that of Eurotium is shown in section in fig. 385 10 . 

 From the central hypha numerous asci, each containing 8 spores, are ultimately 

 developed (figs. 193 7 and 385 n ). The ripe ascus-fruit, which frequently takes 

 several months to mature, consists of a hard outer shell containing numerous asci 

 {one of these fruits is shown in fig. 385 7 , in the right-hand bottom corner); it is 



