REPKOnUCTION OF THE LICHENS. 445 



viiUiiils of Thallophytes. If this be so, the niyceliuTn and 

 the " fruit" of the Ascomycetes cannot be regarded as the 

 two distinct generations, but the fruit which has been pro- 

 duced sexually belongs to the same generation as the nny- 

 celium bearing it, and the second neutral generation begins, 

 instead of ending, with the spore. Pringsheim suggests that 

 the pycnidia represent the neutral generation of the Asco- 

 mycetes, and that probably the irregular succession of these 

 two forms indicates an incomplete, as it were, preparatory- 

 form of the alternation of generations. These suggestions 

 seem to be supported by Bauke's observations upon Plcospora 

 herharum, for here perithecia and pycnidia appear to be 

 distinct generations, and their succession is extremely ir- 

 regular. Bauke explained the similarity in the development 

 of pycnidium and perithecium on the hypothesis that the 

 pycnidium was derived by adaptation from the perithecium at 

 a time when the latter had already attained its present 

 development, and that an alternation of generations became 

 apparent at a later period. Pringsheim prefers to consider 

 that both pycnidium and perithecium — in fact all the sextial 

 and neutral fructifications of Thallophytes, are simply the 

 result of modifications taking place in particular directions 

 of a single primary fructification, the neutral sporangium. 



EXPLANATION OF PLATE XX. 



Fig. 1. — Section of thallus of Collema Microphyllum. 



a. The projecting cell of the trichogyne. 

 Fig. 2. — a. Spermatia surrounding the terminal cell of the trichogyne. 



b. Coalescence of a spermatium with the terminal cell of the 



trichogyne. 

 Fig. 3. — Young apothecium of Collema Microphi/llum. 



a. Excipulum thallodes. 



b. Excipulum proprium. 



c. Hypothecium. 



The hymenium consists principally of paraphyses, between which 

 young asci are being formed from ascogenous hyphse. 



