DISK LICHENS 215 



some species have more or less elongated branching 

 stems. 



341. Lichens propagate by the escape of some of the 

 algal cells, with attached fungal filaments by means of 

 eruptive areas ("soredia") on the plant body. When 

 one of these comes to rest upon a favorable substratum 

 it grows directly into a lichen plant body like the original. 

 Asexual spores appear to be wanting. 



342. The sexual organs as far as known remind one 

 of those of the Red Algae. The oogone, which is a spiral 

 coil of cells, sends up a slender triohogyne to the surface 

 of the plant body. FertiUzation takes place by means of 

 minute non-ciliated sperms which are 

 produced in countless numbers in nearby 

 cavities (spermogones) in the plant body. 

 The sperms come in contact with the 

 projecting trichogyne (doubtless aided 

 by water) and fertilize the oogone, the ofetii^ofcou^i. 

 result of which is the rapid upward 



growth of filaments, the enlarged terminal cells of which 

 become asci. Mingled with the asci are long sterile cells 

 (paraphyses) for the protection of the asci and ascospores 

 in the hymenial layer, which forms a more or less disk- 

 shaped, or cup-shaped fruit. Such open fruits are known 

 as "apothecia," in contrast with the closed fruits ("peri- 

 thecia") of many of the fungi to be taken up later. 



343. The ascospores germinate by sending out one or 

 more tubes which develop directly into the ordinary fila- 

 ments of the lichen-body. Experiments have shown that 

 these filaments will not grow for any great length of time 

 unless they come into contact with green algae of the 

 proper species, to which they become attached, growing 

 rapidly and surrounding them. On the other hand, in 

 the moist tissues thus formed the green algae find protec- 



