NUTRITION: LICHENS. 



317 



which are in solution in the water of the soil. If we make a thin section of 

 the leaf-like portion of a lichen as shown in fig. 418, we shall see that it is 

 composed of a mesh of colorless threads which in certain definite portions 

 contain entangled green cells. The colorless threads are those of the fungus, 

 while the green cells are those of the alga. These green cells of the alga per- 

 form the function of chlorophyll bodies for the dual organism, while the threads 

 of the fungus provide the mineral constituents of plant food. The alga, 

 while it is not killed in the embrace of the fungus, does not reach the per- 



Fig. 418. 



Lichen (peltigera), section of thallus ; dark zone of rounded bodies made up largely of the 

 algal cells. Fungus cells above, and threads beneath and among the algal cells. 



feet state of development which it attains when not in connection with the 

 fungus. On the other hand the fungus profits more than the alga by this 

 association. It forms fruit bodies, and perfects spores in the special fruit 

 bodies, which are so very distinct in the case of so many of the species of 

 the lichens. These plants have lived for so long a time in this close associa- 

 tion that the fungi are rare'y found separate from the algse in nature, but in 

 a number of cases they have been induced to grow in artificial cultures sep- 

 arate from the alga. This fact, and also the fact that the algee are often 

 found to occur separate from the fungus in nature, is regarded by many as an 

 indication that the plant body of the lichens is composed of two distinct or- 

 ganisms, and that the fungus is parasitic on the alga. 



