508 



LIFE-HISTORIES 



sijijibionts/ and their co-opcra1ivo 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 ChlorophyccEe besides Pleurococcus, and 

 also a number of Cyanophycese including species of Chroococcus 

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

 has their structure been modified by the symbiosis, they may almost 

 always be referred to forms found living independently. The fungal 

 symbionts, on the other hand, have l)ecomc so changed in many 

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



MiS.'i 



m wi f» W4' 



Fig. .3.36, II. — Bpard-lichon. a, 6, a group of eight gonidia among which a 

 hi'plia i.s branching, c, a sorediuni consisting of a single gonidium 

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

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

 contains but a single gonidium, germinating by developing below in 

 contact w'ith the Ijark of a tree fine hypha-branches which serve as 

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

 of tlio gonidia, and the upward growth of the liyi>hce to form a thallus. 

 (Schwendencr.) 



near kin among non-sj'mbiotic fungi. They maj' always be classi- 

 fied, however, as either Ascomycetes or Basidiomj'cetcs, and it is 

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

 Ascolichcncs are thus symbiotic Ascomycetes. 



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

 only a few tropical forms of symbiotic liaxiiliomijeetes which itiay be 

 represented by the mushroom-lichen (Cora pavonia, Fig. 3.37). 

 This consists of one of the tougher mushrooms associated witlt a 

 Chroococcus, 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 ,syml)iont ])redominates and so determines the 

 form. 



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

 chens include about 5,UU0 species, none of which are of 



'■ Sym-lii-ont, Sym-bi-o-'sis < Or. symbiosis, living together; < syti, 

 tOKclhor; liios, life. 



^ Ep'i-phytc < Gr. ipi, upon. 



