SECT. I CRYPTOGAMS 417 



themselves such an agreement in their structure and mode of life, 

 and have been so evolved as consortia, that it is more convenient to 

 treat them as a separate class. 



In the formation of the thallus the algal cells become enveloped 

 by the mycelium of the fungns in a felted tissue of hyphte (Fig. 354). 

 The fungus derives its nourishment saprophytically from the organic 

 matter produced by the assimilating alga ; it can also send haustoria 

 into the algal cells, and so exhaust their contents (•'''). The alga, on 

 the contrary, derives a definite advantage from its consortism Avith 

 the fungus, receiving from it inorganic substances and water, and 

 possibly organic substances also. From the symbiosis entered into by 

 a Lichen fungus with an alga, a dual organism results with a dis- 

 tinctive thallus, of which the form (influenced by the mode of nutrition 

 of the independently assimilating alga) differs greatly from that of 

 other non-symbiotic Eumycetes. 



The thalli of the Lichens exhibit a great variety of forms. 

 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 i^resented by Ephebe pubescens, which is 

 found growing on damp rocks, forming a blackish layer. 



Another group is formed by the gelatinous Lichens, Avhose 

 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 CoIIema is a European example of this group. 



Li both the filamentous and gelatinous Lichens the algae and 

 the fungal hyphae are uniformly distributed through the thallus, 

 Avhich is then said to be unstratified or homoiomerous. 



The other Lichens have stratified or heteromerous thalli. The 

 enclosed algae are usually termed C40NIDIA. 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. 354). 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. 355), 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. 356) 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 



2 e 



