REPRODUCTION IN DISCOLICHENS 



167 



(2) The trichogynes, when present, travel up through the gonidial and 

 cortical regions of the thallus; Darbishire^ observes that in Physcia pulveru- 

 lenta, they may diverge to the side to secure an easier course between the 

 groups of algae. They emerge above the surface to a distance of about 1 5/i 

 or less; after an interval they collapse and disappear. Their cells, which are 

 longer and harrower than those of the ascogonium, are uninucleate and vary 

 in number according to species or to individual lichens. Baur^ thought that 

 possibly several trichogynes in succession might arise from one ascogonium. 



(3) How many carpogonia share in the development of the apothecium 

 is still a debated question. In Collema only one is 

 functional. Baur" was unable to decide if one or 

 more were fertilized in Parmelia acetab?dutn, and 

 in Usnea Nienburg^ found that, out of several, one 

 alone survived (Fig. 95). But in Anaptychia ciliaris 

 and in Lecanora subfusca Baur'' considers it proved 

 that severalshare in the formation of theapothecium. 

 In this connection it is interesting to note that, 

 according to Harper' and others, several ascogonia 

 enter into one Pyronema fruit. 



(4) The ascogonial cells, before and after ferti- 

 lization, are distinguished from the surrounding 

 hyphae by a reaction to various stains, which is dif- 

 ferent from that of the vegetative hyphae, and also 

 by the shortness and width of their cells. The whole of the apothecial primor- 

 dium is generally recognizable by the clear shining appearance of the cells. 



(5) The ascogonia do not always form a distinct spiral; frequently they 

 lie in irregular groups. Each cell is uninucleate and may ultimately produce 

 ascogenous hyphae, though in Anaptychia Baur^ noted that some of the 

 cells failed to develop. 



(6) The hyphae from the ascogonial cells spread out in a complex layer 

 at the base of the hymenium, and send up branches which form the asci, 

 either, as in most Ascomycetes, from the penultimate cell of the fertile branch, 

 or from the last cell, as in Sphyridium {Baeomyces rufusY and in Baeomyces 

 roseus. The same variation has been observed in fungi — in a species of 

 Pesiza', in which it is the end-cell of the branch that becomes the mother- 

 cell of the ascus; but this deviation from the normal is evidently of rare 

 occurrence either in lichens or fungi. 



d. Hypothecium and Paraphyses. The hypothecium is the layer of 

 hyphae that subtends the hymenium, and is formed from the complex of 



1 Darbishire 1900. ^ Baur 1901. ^ Baur 1904. •* Nienburg 1908. 



' Harper 1900. * Guilliermond 1904, p. 60. 



Fig. 95. Usnea barbaia Web. 

 Carpogonium with tricho- 

 gynexiioo (after Nien- 

 burg). 



