60 



ELEMENTARY STUDIES IN BOTAN1 



This means that in this position the Algae manufacture food 

 enough for themselves and for the fungus, for otherwise they 

 would be destroyed. 



This association has led to some very important results. It 

 makes it possible for the two plants to exist in conditions that 

 would be impossible for either plant alone. For example, 



lichens are abundant on 

 bare rocks, from which 

 all other plants are 

 absent. In ascending 

 mountains, after all other 

 vegetation has disap- 

 peared, the lichens per- 

 sist on the most exposed 

 rocks. In such places 

 the alga could not grow 

 alone because of lack of 

 moisture, and the fungus 

 could not grow alone be- 

 cause of lack of food; 

 but in the sponge-like 

 body of the fungus the 

 alga gets its moisture, 

 and from the enmeshed 

 alga the fungus gets its 

 food. This fact is im- 

 portant, because lichens can thus start soil-formation on 

 bare and exposed surfaces. The materials of their dead 

 bodies give to other plants a chance to grow, and so a soil 

 gradually accumulates. 



At certain times disk-like or cup-like bodies are borne by 

 lichens, with brown or black or more brightly colored lining 

 (Figs. 44 and 45). When this lining is examined, it is found 

 to be made up largely of delicate sacs containing spores 

 (Fig. 48). This shows that the lichen fungus is a sac-fungus 



FIG. 47. - Section of a lichen, showing the in- 

 terwoven mycelium of the fungus (m) and 

 the enmeshed alga (g). After SACHS. 



