234 STRUCTURE OF LICHENS 



in their growth and the majority of the algae also are usually 

 confined to a definite zone near the sunned surface of the lichen. 

 Thus, in Fig. 143, A, which represents a cross-section of a lichen, 

 it will be seen that the fungus forms a rather firm layer at the 

 top and bottom of the thallus, while the algae are distributed near 

 the upper surface, where they are exposed to the light, and can 

 therefore carry on photosynthesis. Numerous hyphae projecting 

 from the under surface of the lichen serve to anchor it to the 

 substratum and also assist, doubtless, in the absorption of the 

 earth substances. In some of the gelatinous lichens the fungi 

 and algae are more promiscuously arranged. 



While the fungus is dependent lipon the foods manufactured 

 by the algae, the latter are also benefited to an extent by this 

 arrangement since they are protected by the strata of hyphae, and 

 they are also provided with water and crude material which the 

 fungus readily takes up and holds. This mutually helpful rela- 

 tionship of two organisms is a form of symbiosis, termed com- 

 mensalism. The strange feature about this co-partnership is the 

 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 

 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 

 they are usually found, their growing periods are frequently very 

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

 meters — see soils, p. 50. 



{b) Reproduction of Lichens. — Reproduction of the lichens is 

 brought about by means of fragments that are readily detached 

 when the lichens are dry and brittle, or by means of soredia. 

 These latter bodies consist of a few algal cells intertwined with 



