SAPROPHYTISM AND SYMBIOSIS 



803 



considerable change may have been 

 wrought in the latter through symbi- 

 osis. In fact, experiments have shown 

 that lichen fungi when grown independ- 

 ently differ in form and in chemical 

 composition from the same fungi when 

 grown in symbiosis with algae. While 

 the lichen body is that of the fungal 

 symbiont, it is generally quite unlike 

 ordinary fungus bodies, being flat, com- 

 pact, and expanded like a liverwort. 

 Since the algae make up what may be 

 called the synthetic tissue, the advantage 

 of the flat expanded surface is as evident FIG. 1113. A cross section of 



as in liverworts a stem ^ l ^ e beard lichen (Usnea 



, . , ,. T barbata) at the point of origin of a 



The autonomous features of lichens. branch (j)> the latter bdng shown 



In Spite of the proven duality of lichens, in longitudinal section; c, cortical 



there are Various things which suggest absorptive layer; a, algal layer; /, 



.... - loose internal mycelium; w, an axial 



that they possess a high degree of auton- strand of densely placed hyphae> 

 omy or unity. It is quite conceivable 

 that this autonomy might ultimately be- 

 come so complete as to make impossible 

 the separate cultivation of the two symbionts. Perhaps the most strik- 

 ing evidence of autonomy is afforded by the soredia (figs. 1114-1116), 



which are unique reproductive or- 

 gans consisting of a group of algal 

 ce ll s invested by fungal hyphae; at 

 maturity the soredial structure buds 



,, 15 off from the lichen thallus like a 



FIGS. 1114-1116. Soredia from the gemma (p. 808), forming, perhaps, 

 beard lichen (Usnea barbata): 1114, a the most efficient means of repro- 

 simple soredium consisting of an algal ductlon p OSSe ssed by lichens, since 

 cell, surrounded by a web of fungal hy- r 



phae; 1115, a soredium in which the algal the fungal spores are of value only 



cell has reproduced by division; m6, a when they happen to fall among 



germinating soredium in which the algae appropr iate algae. This is almost 

 are dividing, the hyphae forming an apex 



of growth; all figures highly magnified, the only case where two symbionts 

 From SJHWENDENER. have a common reproductive body. 1 



1 The fungal symbiont of Lollum is scattered with the seeds, the mycelia occupying 

 a definite layer ; the bacterial galls of Ardisia also are propagated by seed. 



forming the highly elastic mechan- 

 ical tissue of the lichen; highly 

 magnified. From. SACHS. 



