LICHENES 



35 



sheaths to their cells, and these sheaths have a considerable capacity for retaining 

 water brought to them by the fungus-hyphae. In return they supply the 

 fungus with the greater part of its food, manufactured with the aid of 

 chlorophyll from the carbon dioxide of the air. This mutually dependent 

 existence of the constituents of a lichen is usually regarded as a good example 

 of symbiosis {i.e. living together with mutual advantage), though it has 

 also been suggested that the advantage is wholly on the side of the fungus 

 which, on this view, is simply regarded as a parasite on the alga. 



Fig- 3 1 - 



Leptogium tremelloides (L. f.) Wainio On the branch of a tree. Nat. size. 



For convenience it is customary to classify the lichens according to the 



form and structure of the thallus. In some 



the two constituents are evenly distributed 



through the thallus, which is then said to 



be homoiomerous, while in others the alga 



forms a definite layer (or layers) in the thallus, 



known as the gonidial layer. These 



are known as Heteromerous Lichens. 



The latter are much commoner and are either 



Crust aceous (forming a firm crust closely 



Fig. 32. Leptogium tremelloides attached to the substratum), Foli aceous 



(L.f.) Wainio Section through (flattened and leaf-like, but only loosely 



thallus. (After Zahlbruckner attached to the substratum) or Fruticose 



in E. P.) Much enlarged. (filamentous or band-like, and branched in 



a shrub-like manner). 



The most widely distributed homoiomerous lichens are gelatinous in 

 texture, a common South African species being Leptogium tremelloides*. Here 

 the alga is a species of Nostoc, while the fungus is a discomycete. 



* This plant often occurs on the bark of trees in ravines and other sheltered 

 localities as a flat lump one or more inches in diameter. It is a very pretty thing, the 

 greenish black colour of the frilled thallus forming a strong contrast to the saucer-shaped, 

 rust-coloured apothecia, which are \— \ inch in diameter. The plant fructifies in the 

 neighbourhood of Capetown (Table Mountain) in spring (Fig. 31). M. 



