190 DIVISION II.—COURSE OF DEVELOPMENT OF FUNGI, 
the special literature. The most important and most general phenomenon of intercalary 
growth in the surface of the hymenium consists in the introduction of new asci already 
mentioned, which goes on for a long time at all points. This is the cause of the long 
continued superficial growth of many hymenia. 
Some smaller disk-shaped apothecia, those for example of Ascobolus and Pyro- 
nema, show no marginal progressive growth or only a trace of it. In this respect 
and in some others also their development approaches that of the Pyrenomycetes. 

FIG. 89. Lecauora subfusca. Median section through a young apothecium, swollen up in ammonia, somewhat 
di: i d; h i eexci from which spring the paraphyses represented by strokes run- 
ning ‘ically ds A, sh gg‘ hyphae giving rise to the asci, 7 rind, 2 medullary layer of the thallus which 
forms a rim round the excipulum. The round bodies are the algal cells contained in the thallus. Magn. 190 times. 


This is the case in a still higher degree with the ascocarps of the Hysteri- 
neae and Phacidiaceae, the structure and development of which have been but 
little examined. According to Hartig’s account of Hypoderma macrosporum and 
H. nervisequum and my own imperfect observations on some species of Rhytisma and 
Phacidium, the hymenia in these groups are formed in the interior of flat sclerotioid 
Fungus-bodies (zyloma, see p. 43), and become exposed at the time of maturity, 
when the layer of tissue over the surface of the hymenium separates from it 


FIG. go. Thelidi: is A perithecium borne on the thallus; @ group of Algae, » the part of 
the thallus in the substratum which has no Algae, 9 the perithecium in median section diagrammatically repre- 
sented and slightly magnified. 3 a group of Algae with hyphae winding round them. After Stahl. Magn. 480 
times. 
and tears asunder in the mode which has often been described in the accounts of the 
different genera. In this case there are at first only paraphyses present in the hymenia ; 
the origin of the asci which are introduced between them has yet to be exactly 
ascertained. 
Section LXI. The perithecia of the Pyrenomycetes (Fig. 90, see also 
Fig. 44), may be described as cup-shaped discomycetous sporocarps with the margin 
