THE REPRODUCTION OF LICHENS. 443 



In a second group, including the Erj-sipheae, Eurotium, and 

 others, the differentiation of paraphyses and asci is not so 

 evident, and the sexuality is less marked. In a third 

 group, including Morchella, Peziza, and others, this dif- 

 ferentiation does not present itself, and the sexuality cannot 

 be detected. 



A very striking instance of this has been described by 

 Bauke^ in Pleospora herharum, one of the commonest of 

 the Sphseriacege (Pyrenomycetes). The formation of the 

 perithecium commences with the enlargement and sub- 

 sequent division of usually several adjacent cells of a hypha, 

 which may be regarded as a female organ, a carpogonium, 

 but not an ascogonium. In this way a rounded mass of 

 cells is produced, wiiich becomes of a deep brown colour on 

 the exterior. This stage is usually reached in four or five 

 days from the commencement of development, but it is 

 three or four weeks before the formation of the " nucleus," 

 more accurately, of the paraphyses, begins. A number of 

 thin, closely-packed hyphae, grow out from certain of the 

 parenchymatous cells which lie at about the same level near 

 the base of the young perithecium. These hyphse absorb 

 not only the contents of the cells forming its central 

 portion, but their thickened cell-walls as well ; they seem 

 to perform the same function as the de icnte hyphge, 

 which grow out from the ascogonium of Penicillium. 

 The perithecium may undergo a period of inactivity, but 

 this is by no means necessary, before the asci are formed. 

 They arise as outgrowths from the basal cells of the 

 paraphyses. Preparations which show the development of 

 the young asci very clearly present no appearance which 

 could possibly indicate anything like a sexual process. 



It appears, therefore, that certain Ascomycetes are dis- 

 tinctly sexual, whereas others are distinctly asexual. 

 The absence of distinct sexual organs may be ac- 

 counted for either by considering the members of the 

 group in which they do not occur as primitive forms which 

 have not reached a sufficiently high stage of development 

 for such a differentiation, or (and this is, perhaps, the more 

 probable assumption) by regarding them as degraded forms 

 which have lost the sexuality which more primitive forms 

 still retain. 



It remains to consider briefly the significance of the 

 pycnidia. Their existence in lichens was first discovered 

 by Tulasne2, and he points out that the presence of them 



' " Zur Entwickelungsgeschichte der A.scomyceten." ' Bot. Zcit.,' 1S77 

 2 Loc. cit., p. 107. 



