DEVELOPMENT OF PLANTS 



239 



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 meet any 

 extreme of temperature and drought. With the return of suit- 



t ly, ( \ ■ 



K: 



Fig. 178. 



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, diagram of one of the powdery particles, soredium, 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, />, of the hymenium. 



able 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 

 they are usually found, their growing periods are frequently very 

 short and their total annual growth may not exceed a few milli- 

 meters. 



