252 



GROWTH OF LICHENS 



marked change produced in the nature of the two symbionts. As 

 long as they are independent of each other, very special conditions 

 are necessary for their welfare, but associated, they form the most 

 resistant plants known. This accounts for their distribution from 

 the equator to the pole and their association upon crystalline 

 rocks, baked earth, bark of trees and other places where no other 

 plant life is possible. Under unfavorable conditions the lichen 

 becomes dry and brittle, in which resting condition it is able to 



Fig. 177. 



Fig. 177. Structure of the Lichen: A, section of a lichen, showing the 

 compact arrangement of the hyphae at the top and bottom, also the anchor- 

 ing fungal threads on the underside and the dark algal cells, a, near the top. 

 B, enlarged view of the algae to show their relation to the hyphae. C, dia- 

 gram of one of the powdery particles, sorodium, appearing upon certain 

 lichens, showing the hyphae and algae. These bodies are scattered by the 

 wind and form new lichens. 



Fig. 178. Sexual reproduction of the lichen: A, section of an ascocarp, 

 the hymenium appearing as a dark band in the mouth of the cup. B, en- 

 larged view of the asci, a, and paraphyses, p, of the hymenium. 



meet any extreme temperature and drought. With the return of 

 suitable moisture and heat they become leathery or gelatinous and 

 renew their growth with considerable rapidity. Thus they live 

 on from year to year, but owing to the exposed places in which 



