. SAPROPHYTISM AND SYMBIOSIS 



801 



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 

 as in liverworts. 



The autonomous features of lichens. 

 In spite of the proven duality of lichens, 

 there are various things which suggest 

 that they possess a high degree of auton- 

 omy or unity. It is quite conceivable 

 that this autonomy might ultimately be- 



FIG. 1113. A cross section of 

 a stem of the beard lichen (Usnea 

 barbata) at -the point of origin of a 

 branch (s), the latter being shown 

 in longitudinal section; c, cortical 

 absorptive layer; a, algal layer; /, 

 loose internal mycelium; w, an axial 

 strand of densely placed hyphae, 

 forming the highly elastic mechan- 

 ical tissue of the lichen; highly 

 magnified. From SACHS. 



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 sorcdia (figs. 1114-1116), 



which are unique reproductive or- 

 gans consisting of a group of algal 

 cells invested by fungal hyphae; at 

 maturity the soredial structure buds 

 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- 

 sirnple soredium consisting of an algal duct j on ssessed by l lchens since 

 cell, surrounded by a web of fungal hy- 

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



cell has reproduced by division; 1116, a when they happen to fall among 

 germinating soredium in which the algae appropr i ate a lgae. This is almost 

 are dividing, the hyphae forming an apex l r 



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

 From SOHWENDENER. have a common reproductive body. 1 



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

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



