394 



THE NON-VASCULAR PLANTS 



Fig. 



IQI. 



- A lichen growing upon the bark 

 of a tree. 



of plants besides bacteria which are included under fungi 

 but which are very different from ordinary fungi. They 

 are different in that they completely envelop the host from 



which they draw nourish- 

 ment. They are com- 

 posed of fungi and algae 

 which grow together. 

 The principal part of the 

 structure is formed by 

 the hyphae of the fungi, 

 and enmeshed among 

 them are found the cells 

 of the alga. (See Figure iq2.) The fungus of the lichen 

 derives its nourishment from the cells of the alga, while 

 the alga derives its moisture and solutes from the hyphae 

 of the fungus. This arrangement makes it possible for 

 lichens to live where 

 either algae or fungi 

 alone could not Kve. 

 We find Hchens on 

 bare rocks. They 

 are the first plants 

 to appear in such 

 places. By their 

 growth and decay 

 they gradually pro- 

 duce a little soil. 

 Then other plants 

 come in and crowd the Hchens out. Thus the hchens are 

 the first on rocks of what are called pioneer plants. Some- 

 times you may find upon lichens httle saucer-shaped out- 

 growths in which spores are produced. 



Fig. 192. — Section through the body of a lichen 

 showing the cells of an alga (shaded) within it. 



