FUNGI, THE LICHENS 299 



The lichens. 



447. Nature of lichens. The lichens are curious structures 

 composed of the elements of two different kinds of plants, a fun- 

 gus and an alga. The plant body of the lichen is made up of 

 fungus threads in the meshes of which the algae are enclosed. 

 Many of the lichens are greenish in color, this color being im- 

 parted to the lichen thallus by the algal cells underneath the 

 outer layer of fungus threads. Others are brown, reddish, etc. 

 Their method of nutrition is interesting and illustrates one of the 



Fig. 248. 

 Foliaceous lichen (Physcia stellaris). Natural size. 



forms of symbiosis (paragraph 207). The algal cells perform the 

 function of photosynthesis, so that the fungus element as well as 

 the algal is provided with the necessary carbohydrates. The 

 fungus is thus entirely dependent on the alga for its organic or 

 carbohydrate food. On the other hand the fungus, being exter- 

 nal in most cases, and forming the "rhizoids" and holdfasts, 

 supplies the solution of mineral and nitrogenous substances, pro- 

 tects the alga during dry seasons and holds it in place on steep 

 slopes. Many lichens are multiplied and propagated naturally 

 by small specialized bits of the lichen thallus (soredia) which 



