CRYPTOGAMS 



197 



we come to the real Mosses. A section through a Lichen 

 thallus (Fig. 381) shows large numbers of green cells 

 having much the appearance of such unicellular Algae as 

 Pleurococcus and Nostoc, held in the 

 meshes of a tissue made up of filaments 

 resembling Fungus hyphse. These 

 appearances represent the truth of 

 the matter. Lichens are composite 

 growths in Avhich certain unicellular 

 Algse and certain Fungi take part. 

 Figure 332 shows how this union be- 

 gins. The spore of a Fungus has 

 fallen near a cell of Pleurococcus. 

 The young mycelium is already ap- 

 plied to the Alga, which has divided. 

 Further development consists in. the 



Section of a lichen 

 thallus. 



extension and branching of 

 the mycelium, and the multi- 

 plication of the algal cells; the 

 construction, by these means, 

 332. Fh-st stages in the formation of a thallus having certain 

 of the lichen thallus. distinguishing peculiarities of 



BORNET. ,. 



structure, according to the 



kind of Fungus and the kind of Alga concerned ; and 

 finally, the production of a spore-bearing body. In many 

 Lichens this fructification is an apo- 

 thecium (Fig. 329, a) very like that 

 of Peziza, with a hymenium con- 

 taining spore sacs or asci (Fig. 333). 

 Most of the Lichen Fungi are Sac 

 Fungi. They are parasitic upon 

 the Algse and cannot exist without them. The Algae, 

 however, are known to be able to exist perfectly well 

 without the Fungi. 1 



1 Symbiosis (as the word is understood among English-speaking 

 botanists) is the living together of unlike organisms for mutual advan- 

 tage. Many botanists regard Lichens as examples of symbiotic accom- 

 modation. 



Section of an apothe- 

 cium. 



