508 



LIFE-HISTORIES 



symhionts,^ and thoir co-operative mode of life, symbiosis^ 

 Plants which grow attached to some support from which 

 they derive no nutriment are termed epiphytes.- Lichens 

 are aerial epiphytes or "air-plants," 



Several aerial forms of Chlorophycese besides Pleurococcus, and 

 also a number of C3^anophyce2e including species of Chroococcus 

 and Nostoc, serve as the algal symbiont in various lichens. So little 

 has their structure been modified by the s^'mbiosis, they may almost 

 always be referred to forms found living independently. The fungal 

 symbionts, on the other hand, have become so changed in many 

 ways, that usually much uncertainty attends the effort to find their 



Fig. 336, II. — Beard-lichen, a, h, a group of eight gonidia among which a 

 hypha is branching, c, a soredium con.sisting of a single gonidium 

 surrounded by hyphse, viewed as if cut across, d, the same in which 

 the single gonidium has multiplied into several, e, a soredium which 

 contains but a single gonidium, germinating by developing below in 

 contact with the bark of a tree fine hypha-branches which serve as 

 organs of attachment. /, the same, older, showing the multiplication 

 of the gonidia, and the upward growth of the hyphte to form a thallus. 

 (Schwendener.) 



near kin among non-symbiotic fungi. They may always be classi- 

 fied, however, as either Ascomycetes or Basidiomj^cetes, and it is 

 to the former class that the great majority of lichen fungi belong. 

 Ascolichcnes are thus symbiotic Ascomijcetes. 



186. The spore-base lichens (Class Basidiollchenes) include 

 only a few tropical forms of sijmbiofic Basic! iomycete-s wliich maj' be 

 represented by the mushroom-lichen (Cora pavonia. Fig. 337). 

 This consists of one of the tougher mushrooms associated with a 

 Chro()Coccus, or with another bluish alga, and assumes quite differ- 

 ent shapes according to which alga is present and according as the 

 algal or the fungal symbiont predominates and so determines the 

 form. 



187. The lichen subdivision, lichens in general. Li- 

 chens include about 5,000 species, none of which are of 



' Sym-bi-ont, Sym-bi-o'sis < Gr. symbiosis, li^dng together; < syn, 

 together; bios, life. 



^ Ep'i-phyte < Gr. cjd, upon. 



