8oo 



ECOLOGY 



clearly demonstrated that some root fungi are prochemotropic with respect to certain 

 substances that are within or about roots. Probably fungus contact with roots 



originally was casual, and the first 

 mycosymbiosis doubtless was faculta- 

 tive ; later, it may be supposed, came 

 obligate mycosymbiosis, reaching its 

 culmination in the orchids and ericads, 

 and especially in those species that re- 

 quire fungus contact for germination, 

 and in such forms as NeoUia and Mono- 

 tropa, which contain no chlorophyll 

 and thus depend entirely upon outside 

 sources for their food. It is to be ob- 

 served that generally the fungus does 

 not become thus dependent upon the 

 other symbiont, but remains faculta- 

 tive, although there is evidence of considerable dependence upon specific symbionts 

 among the orchid fungi. 



Fig. iiii. — A foliose lichen (^Physcia) 

 on tree bark; note the marginal vegetative 

 propagation characteristic of lichens, also the 

 numerous fruiting structures, the apothecia. 

 — From Coulter (Part I). 



Lichens. — Structural relations. — A lichen is a plant complex made 

 up of a fungus body in which algae are enclosed. Formerly lichens were 

 supposed to be in- 

 dividual plants, and 

 the green cells, now 

 known to be algae, 

 were called gonidia 

 (figs. 1111-1113). 

 The dual nature of 

 lichens was discov- 

 ered by making sep- 

 arate cultures of the 

 constituent algae 

 and fungi through 

 entire developmental 

 cycles. Also spores 

 from the fungus ele- 

 ment of the lichen 

 were sown among algae that had been growing separately in nature, 

 and the developing fungus mycelium enclosed the latter, forming 

 a lichen of the usual kind. Commonly the algal symbionts are 

 well-known forms, such as Pleurococcus and Nostoc, but the fungi 

 are most diverse and generally unlike other fungi, suggesting that 



Fig. 1 1 12. — A section through an apothecium of a lichen 

 (Anaptychia), showing the spore-bearing layer (hymenium), 

 beneath which is the loose mycelium of the lichen body; these 

 portions are in large part invested by the dense cortical 

 mycelium with its numerous groups of algae; considerably 

 magnified. — After Sachs. 



