CRYPTOGAMS 



197 



331. 



Section of a lichen 

 thallus. 



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

 thallus (Fig. 331) shows large numbers of green cells 

 having much the appearance of such unicellular Algse as 

 Pleurococcus and Nostoc, held in the 

 meshes of a tissue made up of filaments 

 resembling Fungus hyphcC. These 

 appearances represent the truth of 

 the matter. Lichens are composite 

 growths in which certain unicellular 

 Algge 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 



extension and branching of 

 the mycelium, and the multi- 

 plication of the algal cells; the 

 construction, by these means, 

 of a tliallus having certain 

 distinguishing peculiarities of 

 structure, according to the 

 kind of Fungus and the kind of Alga concerned ; and 

 finally, the production of a spore-bearing body, hi 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 Algse, 

 however, are known to be able to exist perfectly well 

 without the Fungi. ^ 



A B 



332. First stages in the formation 

 of the lichen thallus. — 



BORNET. 



333. Section of an apothe- 

 cium. 



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. 



