i8o REPRODUCTION 



probable that the lichen trichogyne is a new structure evolved in response 

 to some physiological requirement— either sexual or metabolic — of the deeply 

 embedded fruit primordium. 



b. The Ascogonium. In fungi there is usually one cell forming the 

 ascogonium, a coenogamete, which after fertilization produces ascogenous 

 hyphae. There are exceptions, such as Cutting' found in Ascophanus carneus, 

 in which it is composed of several cells in open contact by the formation of 

 wide secondary pores in the cell-walls. In lichens the ascogonium is divided 

 into a varying number of uninucleate cells. Darbishire^ (in Physcia) and 

 Baur' (in Anaptychia) have described an opening between the different cells, 

 after presumed fertilization, that might perhaps constitute a coenogamete. 

 Ascogenous hyphae arise from all, or nearly all the cells, whether fertilized by 

 spermatia or not, and asci continue to be formed over a long period of time. 

 There may even be regeneration of the entire fruiting body as described in 

 Graphis elegans and in Pertiisaria, apparently without renewed fertilization. 



Spermogonia (or pycnidia) and the ascosporous fruits generally grow, on 

 the same thallus, though not unfrequently only one of the two kinds is 

 present. As the spermogonia appear first, while the apothecia or perithecia 

 are still in the initial stages, that sequence of development seems to add 

 support to the view that their function is primarily sexual; but it is equally 

 valid as a proof of their pycnidia! nature since the corresponding bodies in 

 fungi precede the more perfect ascosporous fruits in the life-cycle. 



The differences in fertility between the two kinds of thallus in Collema 

 crispiim may be recalled*. Baur considered that development of the carpo- 

 gonia was dependant on the presence of spermatia: a strong argument for 

 the necessity of fertilization by these. The conditions in Parmelia acetabulum, 

 also recorded by Baur, lend themselves less easily to any conclusion. On 

 the thallus of that species the spermogonia and carpogonia present are out 

 of all proportion to the very few apothecia that are ultimately formed. 

 Though Baur suggested that cross-fertilization might be necessary, he admits 

 that the development may be vegetative and so uninfluenced by the presence 

 or absence of spermatia. 



It is the very frequent occurrence of the trichogyne as an integral part of 

 the carpogonium that constitutes the strongest argument for fertilization by 

 spermatia. There is a possibility that such an organ may have been uni- 

 versal at one time both in fungi and in lichens, and that it has mostly 

 degenerated through loss of function in the former, as it has disappeared in 

 many instances in lichens. Again, there is but a scanty and vestigial record 

 of spermogonia in Ascomycetes. They may have died out, or they may 

 have developed into the asexual pycnidia which are associated with so many 

 species. If we take that view we may trace the same tendency in lichens, as 



" Cutting 1909. = Darbishire 1900. ^ Baur 1904. ^ See p. 161. 



