296 
PLANT STUDIES 
If a Lichen be sectioned, the relation between the sym- 
bionts will be seen (Fig. 272). The fungus makes the bulk 
of the body with its interwoven mycelial threads, in the 
meshes of which lie the Algae, sometimes scattered, some- 
FIG. 272. Section through thallns of a lichen (Stictd), showing holdfasts (r), lower (u) 
and upper (o) surfaces, fungus hyphse (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- 
