36 CONSTITUENTS OF THE LICHEN THALLUS 



separating the cells which, from yellow, change to a green colour and 

 become rounded off (Fig. 13). The mature lichen, a white thallus dotted 

 with black fruits, contrasts strikingly with the yellow membranous alga. 

 Lichen formation usually begins near the edge of the leaf and the margin of 

 the thallus itself is marked by a green zone showing where the fungus has 

 recently come into contact with the alga. 



More recently Hans Fitting 1 has described " Mycoidea parasitica" as it 

 occurs on evergreen leaves in Java. The alga, a species of CepJialeuros, 

 though at first an epiphyte, becomes partially parasitic at maturity. It pene- 

 trates below the cuticle to the outer epidermal cells and may even reach 

 the tissue below. When it is joined by the lichen fungus, both constituents 

 grow together to form the lichen. Fitting adds that the leaf is evidently but 

 little injured. In this lichen the alga in the grip of the fungus loses its 

 independence and may be killed off: it is an instance of something like 

 intermittent parasitism. 



J. RECENT VIEWS ON SYMBIOSIS AND PARASITISM 



No hyphal penetration of the bright-green algal cell by means of 

 haustoria had been observed by the earlier workers, Bornet 2 , Bonnier 3 and 

 others, though they followed Schwendener 4 in regarding the relationship as 

 one of host and parasite. Lindau, also, after long study accepted parasitism 

 as the only adequate explanation of the associated growth, though he never 

 found the fungus actually preying on the alga. 



In recent years interest in the subject has been revived by the researches 

 of Elenkin 8 , a Russian botanist who claims to have established a case for 

 parasitism or rather "endosaprophytism." He has demonstrated by means 

 of staining reagents the presence in the thallus of large numbers of dead 

 algal cells. A few empty membranes are to be found in the cortex and in 

 the gonidial zone, but the larger proportion occur below the gonidial zone 

 and partly in the medulla. He describes the lower layer as a "necral" or 

 "hyponecral" zone, and he considers that the hyphaedraw their nourishment 

 chiefly from dead algal material. The fungus must therefore be regarded in 

 this case as a saprophyte rather than a parasite. The algae, he considers, 

 may have perished from want of sufficient light and air or they may have 

 been destroyed by an enzyme produced by the fungus. The latter he thinks 

 is the more probable, as dead cells are frequently present among the living 

 algae of the gonidial zone. To the action of the enzyme he also attributes 

 the angular deformed appearance of many gonidia and the paler colour and 

 gradual disintegration of their contents which are finally used up as endo- 

 saprophytic nourishment by the fungus. Dead algal cells were more easily 



1 Fitting 1910. 2 Bornet 1873. 3 Bonnier i88g 2 . * Schwendener 1867. 



5 Elenkin loo^and 1904^ I9<J4 2 . 



