6 9 



of the spore-sacs and further divide until nearly the entire apothecial 

 cavity or cup is filled ; many of them escape and are either de- 

 stroyed, or continue to exist upon the thallus. Stahl has maintained 

 that the thecial algae adhere to the spores, are ejected with them, and 

 if they fall upon a suitable place the spores germinate and the 

 hyphae enclose the algae, which then undergo a great transformation ; 

 instead of remaining pale green and small, as they were in the 

 thecium, they regain the dark green color and size of the algae of the 

 thallus. If his observations are correct it would seem to prove con- 

 clusively that the thecial algae play an important part as aids in re- 

 production. According to Reinke the spore with the adherent algae 

 may be compared to a soredium, that is, we have the union of the 

 essential elements for the formation of a new lichen, the fungal por- 

 tion being represented by the spore. Structurally, they are the re- 

 verse of the soredia, the fungal portion (spore), being enclosed 

 by the algae. As propagative organs they are less reliable than 

 soredia, since the algae do not have the protective covering required 

 to enable them to tide over unfavorable conditions for any length of 

 time, as, for example, a want of moisture. If the spore, with the ad- 

 herent algae, falls upon a dry spot, the algae would very probably die 

 because of lack of moisture before the spore could develop ; if sub- 

 sequent moisture should enable the spore to germinate there would 

 be no algae with which to enter into symbiotic association. 



As already indicated, thecial algae are of wide distribution, espe- 

 cially in immature apothecia. In most cases they die before the spores 

 mature and, therefore, can not have the biological significance that 

 they do in Dermatocarfon. It is more properly a form of contingent 

 symbiosis. The partially opened apothecial disk, no doubt, forms 

 a suitable location for the algae, which are protected by the overhang- 

 ing cortical tissue of the thallus or the exciple. As soon as this protec- 

 tion is no longer afforded, the thecial algae escape to some suitable 

 locality, or fail to maintain their existence, owing to lack of moisture. 



It is evident that the thecial algae are not always derived from 

 the algae of the thallus, as may be seen from a careful comparative 

 study. Pleurococcus seems to occur much more frequently than any 

 other form of alga, particularly in a form closely related to P. functi- 

 formis, if not identical with it. That the algae of the thallus should 

 gain access to the apothecial cup is very likely, especially in those 

 lichens with immersed apothecia (Verrucariaceae). Careful culture 

 experiments are necessary to solve some of the undecided problems. 



