366 A TEXTBOOK OF THEORETICAL BOTANY 



to synthesize a Lichen. For this purpose he took the ascospores of the 

 Lichen CoUema and sowed them in a pure culture of the Blue-green Alga 

 Nostoc. As a result he obtained a Lichen, although he was not able to induce 

 it to fruit. He further observed that the fungal elements died off when no 

 Nostoc was forthcoming. 



In 1886 Bonnier succeeded in synthesizing various Lichens from the two 

 components and showed that it was possible to obtain fruiting bodies from 

 the association of fungal hyphae and true algal cells taken from the open. 

 Even at the present time it has not been found possible to isolate the Fungus 

 and grow it to maturity in pure culture, except in a single instance in the 

 Basidiolichen Cora, where the Fungus, Thelephora, is known to occur in 

 nature apart from the Lichen. 



The Lichen thallus may be either homoiomerous or heteromerous, 

 according to whether the algal cells or gonidia are scattered uniformly among 

 the fungal elements or whether they are restricted to a single layer. There 

 are three main types of development of the Lichen thalli : crustaceous, 

 when they form incrustations on the surface of stones or rocks ; foliaceous, 

 when they are flattened and leaf-like ; and fruticose, or shrub-like, when 

 they are attached by the base and grow either vertically or pendulous from 

 the point of attachment. 



The fungal hyphae are closely woven together into fairly compact " false 

 tissues," and in the best developed species the following strata can be 

 distinguished : — 



1. The Upper Cortex of closely compacted hyphae. 



2. The Gonidial Layer with algal cells. 



3. The Medulla, which is less conipact. 



4. The Lower Cortex, Vvhich is often absent. 



5. The Hypothallus, a thin sheet of hyphae which often has a different 



colour from the rest and persists chiefly at the margins. 



One or more of these layers is missing in many species. There is no 

 special surface layer of cells either above or below. 



The algal elements are most frequently unicellular Chlorophyceae, which 

 are often difficult to identify. Chlorella, which has been proved to be 

 associated with a number of Lichens, is very similar to Pleiirococcus. It 

 increases within the thallus by means of aplanospores. Pleurococcus itself 

 often occurs, and associated genera such as Cystococciis and Palmella are 

 also found in symbiosis with fungal hyphae formiing Lichens. The chief 

 filamentous Alga is Trentcpohlia, but a curious parasitic filamentous Alga, 

 Cephaleuros, is found in the leaf-growing Lichen Strigula, which is very 

 common in the tropics. Among the Cyanophyceae are a number of genera 

 which contribute to the formation of Lichens. Mention may be made of 

 Gleocapsa among the unicellular types, and of Nostoc and Rwularia among 

 the filamentous types. 



Although the algal constituent is less affected by the symbiosis than the 

 fungal, certain modifications are found ; for example, Nostoc sometimes 



