CRYPTOGAMS 331 
Can you tell what they are? Compare with your study of 
the unicellular alge (337) and with Fig. 429. Does this 
throw any light on their real nature? 
381. The lichen thallus a composite body. — You will 
probably have no difficulty in making out that these small 
round bodies are green alge of some kind, but of what species 
will depend upon the kind of lichen with which it is associated. 
In Cladonia and the bearded lichen (Fig. 473), it is a proto- 
coccus ; in other forms, a pleurococcus or a nostoc —and so 
on, each species of lichen fungus being specialized to a cer- 
tain form of alga. The great botanist, De Bary, showed 
Fic. 471. — Artificial lichen mycelium, m, made by sowing spores of a fungus, 
sp, among alga cells, a. 
that it is even possible to produce a lichen thallus artificially 
by sowing the spores of a fungus among the cells of the par- 
ticular alga with which it is able to unite. The spores will 
germinate without the alga, but soon perish unless they come 
in contact with the right one. It is thus made clear that the 
lichen plant as a whole is a combination of elements belong- 
ing to two distinct orders, the alge and fungi, but so closely 
associated as to constitute practically a single individual. 
