CRYPTOGAMS 331 



Can you tell what they are? Compare with your study of 

 the unicellular algae (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 algae 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 



FIG. 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 algae and fungi, but so closely 

 associated as to constitute practically a single individual. 



