80 



PLANT STRUCTURES 



If a Lichen be sectioned, the relation between the sym- 

 bionts will be seen (Fig. 72). The fungus makes the bulk 

 of the body with its interwoven mycelial threads, in the 

 meshes of which lie the Algae, sometimes scattered, some- 



Fio. 72. Section through thallus of a lichen (Stictd), showing holdfasts (r), lower (u) 

 and upper (o) surfaces, fungus hyphae (m), and enmeshed algae (g). After SACHS. 



times massed. It is these enmeshed Algae, showing through 

 the transparent mycelium, that give the greenish tint to 

 the Lichen. 



In the case of Lichens the symbionts are thought by 

 some to be mutually helpful, the alga manufacturing food 

 for the fungus, and the fungus providing protection and 

 water containing food materials for the alga. Others do not 

 recognize any special benefit to the alga, and see in a Lichen 

 simply a parasitic fungus living on the products of an alga. 

 In any event the Algae are not destroyed but seem to thrive. 

 It is discovered that the alga symbiont can live quite inde- 



