HOW PLANTS OBTAIN FOOD. 95 



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

 fect 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 rarely found separate from the algae in nature, but in 

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



Fig. 85. 



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

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



irate from the alga. This fact, and also the fact that the algae 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. 



201. Others regard the lichens as autonomous plants, that is, the two or- 

 ganisms have by this long-continued community of existence become unified 

 into an individualized organism, which possesses a habit and mode of life 



