470 BOTANY PART n 



surface of the hyphae in the form of crystals or granules. Their supposed use as 

 a protection against snails appears, according to ZOPF, not to hold generally ( 88 ). 



The Lichens are distributed in numerous species over the whole earth. They 

 extend further than even the Mosses towards the poles and towards mountain 

 summits. They attain their maximum development in moist Alpine regions where 

 they sometimes cover the soil, rocks, and tree-trunks with a colonial vegetation 

 or hang in beard-like masses from the branches of the trees. In the Arctic regions 

 they may cover the soil and give rise to extensive tracts of Lichen tundra. 



The simplest Lichens are the FILAMENTOUS, with a thallus con- 

 sisting of algal filaments interwoven with fungal hyphae. An example 

 of such a filamentous form is presented by Ephebe pubescens, which is 

 found growing on damp rocks, forming a blackish layer. 



Another group is formed by the GELATINOUS Lichens, whose 

 thallus, usually foliaceous, is of a gelatinous nature. The algae 

 inhabiting the thalli of the gelatinous Lichens belong to the families 

 of the Chroococcaceae and Nostocaceae, whose cell walls are swollen, 

 forming a gelatinous mass traversed by the hyphae of the fungus. 

 The genus Collema is a European example of this group. 



In both the filamentous and gelatinous Lichens the algae and 

 the fungal hyphae are uniformly distributed through the thallus, 

 which is then said to be unstratified or HOMOIOMEROUS. 



The other Lichens have stratified or HETEROMEROUS thalli. The 

 enclosed algae are usually termed GONIDIA. They are arranged in a 

 definite GONIDIAL LAYER, covered, externally, by a CORTICAL LAYER, 

 devoid of algal cells and consisting of a pseudo-parenchyma of closely- 

 woven hyphae (Fig. 422). It is customary to distinguish the three 

 following forms of heteromerous Lichens : CRUSTACEOUS LICHENS, 

 in which the thallus has the form of an incrustation adhering closely 

 to a substratum of rocks or to the soil, which the hyphae to a certain 

 extent penetrate. FOLIACEOUS LICHENS (Fig. 426), whose flattened, 

 leaf-like, lobed or deeply-cleft thallus is attached more loosely to the 

 substratum by means of rhizoid-like hyphae (rhizines), springing either 

 from the middle only or irregularly from the whole under surface. 

 FRUTICOSE LICHENS (Fig. 424) have a filamentous or ribbon-like thallus 

 branched in a shrub-like manner and attached at the base. They are 

 either erect or pendulous, or may sometimes lie free on the surface of 

 ' the substratum. 



In nature the germinating spores of the Lichen Fungi appear to 

 be capable of continuing their further development only when they 

 are enabled to enter into symbiosis with the proper gonidia. For a 

 few genera of Lichens, however, it has been determined that the 

 fungi sometimes exist in nature without the presence of the algae ; it 

 has been shown that the tropical Lichen Cora pavonia (Fig. 431), 

 whose fungus belongs to the order Hymenomycetes, may produce 

 fructifications even when deprived of its alga ; these have a form 

 resembling those of the fungal genus Thelephora. Mycelia have 



