78 



MOI-' I'UOLoCV 



matia that fuse with the trichogyne (fig. 185). 

 Perithelia are formed, and the asci bud out from 

 the auxiliary cells (fig. 186), the whole process 

 suggesting the formation of cystocarps among the 

 red algae. 



Lichens 



General character. — With a few excep- 

 tions (p. 91), lichens are Ascomycetes 

 parasitic upon certain algae, the relation 

 between the two organisms being so intimate 

 as to result in a structure resembling a single 

 organism. The dual nature of lichens was 

 announced by Schwendener in 1868, but it 

 was many years before the proof of it be- 

 Figs. 185, 186. — siigmato- came convincing. In 1889 Bonnier began 

 u yCfS i u 5, . sex ° r ,i ans j. "f. to synthesize lichens: that is, to bring to- 



branch showing anthendia dis- J ' ' ° 



charging spermatia; left branch gether "wild algae" and lichen fungi and 

 being a procarp showing tricho- tnU s produce artificial lichens. The para- 



gyne (with attached spermatia), ... . ,. . ,, , ,, , , 



. , ... sitism is peculiar in that the algae do not 



carpogonium, and auxiliary ' ° 



cells; 186, the formation of asci seem to be harmed in most cases, the cells 

 within the periihedum after i )e j ng rar ely penetrated by the fungus. The 



fertilization. — After Thaxter. , , . ,. , , . r 



algae concerned in lichen formation are for 

 the most part Cyanophyceae and Protococcales. As would be expected, 

 the algae thrive without the fungus, just as do the wild species; but the 

 lichen fungus soon perishes if it does not come into contact with the 

 appropriate algae. (See p. 91 and fig. n 17.) 



Body. — The lichen fungus usually forms a thallus body much more 

 definite and differentiated than do other mycelia, the thallus often re- 

 sembling in form that of certain liverworts. In structure, there is a 

 distinct compact cortical region 

 and a central looser region, in 

 either of which the algae may 

 occur (fig. 190). Two structural 

 types of thallus body are recog- 

 nized, dependent upon the dis- 

 tribution of the aigae: (1) homoi- 

 omerous t in which the algae are 



, , , . . , . Fin. 187. — Physcia: a flat foliose lichen 



scattered; and (2) heteromerous, in on baH c, showing numerous shallow apothe- 

 which the algae occur in layers, cia. — After Coulter. 



