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 germinatiqn, 

 and in such forms as Neottia and Mono- 

 tropa, which contain no chlorophyll 

 FIG. mi. A foliose lichen (Physcia) an d thus depend entirely upon outside 

 on tree bark; note the marginal vegetative sources for their food. It is to be ob- 

 propagation characteristic of lichens, also the gerved that generally the fungus does 



numerous fruiting structures, the apothecia. .. , 



not become thus dependent upon the 



From COULTER (Part I). 



other symbiont, but remains fat ulta- 



tive, although there is evidence of considerable dependence upon specific symb:onts 

 among the orchid fungi. 



Lichens. Structural relations. A lichen is a plant complex riade 

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

 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 FIG. "" A section through an apothecium of a lichen 

 , i , i (Anaptychia), showing the spore-bearing layer (hymetiium), 



entire developmental 



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



cycles. Also Spores portions are in large part invested by the dense ortical 

 from the fungUS ele- mycelium with its numerous groups of algae; considerably 



t ^L i- u magnified. After SACHS. 

 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 



