440 BOTANY OF THE LIVING PLANT 



is official, the fruits appear as marginal discs. When cut vertically 

 they show a superficial hymenium, with numerous asci arranged as 

 in Peziza (compare Fig. 362, p. 429). Male sexual organs are some- 

 times borne, as minute, non-motile bodies (spermatia), which are 

 produced in flask-shaped spermogonia, contained in the marginal 

 teeth of the thallus. Female organs have been seen in some of 

 the Lichens (Collema, etc.) to have the form of a coiled carpogonium 

 with a receptive trichogyne that projects to receive the non-motile 

 spermatium, as in the Red Seaweeds. But it seems probable that 

 in many of the Lichens, as in many advanced Fungi, the sexual organs 

 even if morphologically present are not functional. 



The establishment of a new Lichen from the germination of the 

 ascospore depends upon the presence of the Algal partner. This 

 is not left to chance, but is provided for, in some cases at least, by 

 " hymenial gonidia " : these are small Algal cells which develop in 

 close relation to the asci. When the fungal spores are ejected 

 some of these adhere to their sticky walls, and thus the two partners 

 germinate together from the first. 



The close similarity in structure, nutritional behaviour, and pro- 

 pagative method with the Ascomycetes makes it probable that the 

 Asco-Lichens arose from a branch of the Ascomycetes, which has 

 adopted a symbiotic relation with Algae, and become actually 

 dependent upon it. None of the Lichen-Fungi are known to lead 

 an independent life, though some of them have been cultivated in 

 nutrient media. On the other hand the Algal cells, if liberated and 

 cultivated, have been found in certain cases to continue their normal 

 life, and even to propagate as the free Algae do. Finally, synthetic 

 experiments have succeeded in building up a Lichen from its two 

 constituents when grown together. The proof that the Lichen 

 consists of a coalition of two organisms living in symbiotic relation 

 seems thus complete. That the symbiosis is a mutual advantage 

 is clear from the healthy growth. It may be held that the Algae 

 contribute fresh organic substance by photo-synthesis, while the 

 Fungus supplies water and soluble salts, which it is specially able 

 to extract and convey. 





