ECOLOGY OF FUNGI 79 



advocation of the fungal nature of lichens, as follows: "Lichens com- 

 monly ^ow where there are free algae of the same species as those 

 parasitized by these Uchens. The spores of the lichens germinate and 

 attack the free alga; as other fungi attack their hosts. Lichens perform 

 like other fungi on culture media and may be made to produce their 

 reproductive organs on these media. Lichen spores also attack the 

 algal hosts, when the spores and the algae are introduced into cultures 

 together; and the resulting Hchen is normal and sometimes fructifies 

 in the cultures. Algal hosts extracted from hchen thaUi grow in cul- 

 tures like free algae of the same species grown on similar culture 

 media. The researches of Elenkin and Danilov prove that hchen 

 hyphae absorb food from the algal host cells, which are killed by 

 severe parasitism, or more probably by parasitism and saprophytism 

 combined. The relation of the hchen to its substratum proves that 

 higher hchens can take comparatively httle food from it and must 

 depend more than lower hchens upon the algal hosts; and this shows 

 that the parasitism of the hchen upon the algal host has become more 

 severe in the evolution of the higher hchens. Finally, the algae para- 

 sitized by hchens are in a disadvantageous position with reference to 

 carbon assimilation. 



"Lichens are like other fungi with respect to vegetative structure 

 and fruiting bodies. The bridges which connect lichens with other fungi 

 are not few, but many. Since it is thoroughly demonstrated that the 

 lichen is parasitic, or partly parasitic and partly saprophytic on the 

 alga, there is no longer even a poor excuse for a 'consortium' or an 

 'individuahsm' hypothesis. 



"The parasitism of Uchens on algae is pecuhar in that the unicellular 

 or the filamentous hosts are enclosed usually by the parasite, which 

 carry more or less food to the host. The host inside of the parasite is 

 placed in a disadvantageous position with reference to carbon assimila- 

 tion and may depend, for its carbon supply, more or less upon material 

 brought from the substratum by the parasite. Some algal individuals, 

 not yet parasitized, may be found in most hchen thaUi." 



Lichen thaUi are of three kinds: crustaceous, fohose and fruticose. 

 The arrangement of the layers of the hchen fungus and its algal host 

 varies in different hchens, but in Sticta the following are met in a ver- 

 tical section of the thallus (Fig. 26) : 



(a) Tegumentary layer. 



