CRYPrOGAMS 



197 



we come to the real Mosses. A section tlu'ougli a Lichen 

 thallus (Fig. 331) sliows hirge numbers of green cells 

 having much the appearance of such unicellular Algie as 

 Pleurococcus and Nostoc, held in the 

 meshes of a tissue made up of hlaraeiits 

 resembling Fungus hypha'. These 

 appearances represent the truth (jf 

 the matter. Ijichons arc composite 

 growtlis in which certain uniccUidar 

 AlgiE and certain Fungi take jtart. 

 Figure 332 shows how tliis union l>e- 

 gins. The spore of a Fiuigus has 

 fallen near a cell of Pleurococcus. 

 The young mycelium is already ap- 

 plied to the Alga, whicli has divided. 

 Further development consists in the 



extension 



3:!l . 



Section ol' a liclieu 

 tliiinus. 



A B 



332. First stages in tlio I'omiatiiin 

 of the lichen thallus. — 



BOBNET. 



and lii'iindiing of 

 the mycelium, and (lie midti- 

 jilicatiou of the algal cells; the 

 construction, l)y these means, 

 of a tliallus having certain 

 distinguishing peculiarities of 

 structure, according to the 

 kind of Fungus and the kind of iMga concerned ; and 

 finally, the prodtietion of a spure-ljearing l)ody. In many 

 Lichens this fructification is an apo- 

 thecium (Fig. 3211, «) 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 Algte and cannot exist without tliem. The Alg;e, 

 however, are known to be able to exist perfectly well 

 without the Fungi. ^ 



333. Section of an apfiilie- 

 ciuni. 



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

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

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

 modation. 



