256 



THE FUNGI 



First. The fungi are absolutely dependent upon the algae for 

 their organic food (such as the carbohydrates), which, of course, 

 the algae are able to manufacture in the manner characteristic 

 of green plants (photosynthesis). The relation of the fungus to 

 the alga is then in all essentials that of a parasite to its host. 



Second. The algae receive a certain 

 sort of protection in the lichen thallus. 

 Thus they have fixed positions on ex- 

 posed rocks, cliffs, trees, and other 

 objects where they could hardly grow 

 otherwise, or at least not in the same 

 luxuriance. The substance of the lichen 

 also retains moisture, so that the algal 

 cells are not so subject to drought. 



It is well known that many of the 

 lowly algae would grow in situations 

 frequented by lichens if left alone, and 

 it is evident that the lichens arise be- 

 cause fungus spores fall among the algae, 

 and germinating produce hyplue which 

 live parasitically upon them as hosts. 



The algae are then, in a sense, slaves of 

 FIG. 226. A leaf-like, or fo- the f m { Th are not killed for tha( . 



Hose, lichen (Cetraria) , J , '. 



would be oi no advantage to the fungus, 



s, sac fruits ... . i P 



which requires them to manufacture 



its organic foods. The term slavery perhaps best expresses the 

 relation of the algae to the fungi in the lichens. 



Life habits of the lichens. Lichens are found on rocks, cliffs, 

 branches and trunks of trees, and on the ground, when the latter 

 cannot support green vegetation, either because it is too bar- 

 ren, or is exposed to unfavorable climatic conditions. They are 

 most luxuriant in temperate and sub-arctic regions, especially 

 where there is much rain. They form the bulk of the vegeta- 

 tion on the tops of mountains and in the arctics, where grass 

 and other alpine seed plants cannot grow. They are abundant 



