THE FUNGI 



189 



ap 



Distribution. The number of Lichens is very large, and they are 

 of almost universal distribution, their peculiar structure enabling 

 them to live where 

 scarcely any other 

 vegetation is pos- 

 sible, this being 

 especially the case 

 on exposed rocks, 

 where Lichens are 

 among the first or- 

 ganisms to appear. 

 They play a very 

 important role in 

 the decomposition 

 of rocks, being 

 able, by the devel- 

 opment of special 

 solvent substances, 

 to disintegrate even 

 such hard rocks as 

 granite and gneiss. 

 The hyphae of Ver- 



FIG. 153. A, Sticta pulmonacea, a foliose Lichen (natu- 

 ral size) ; ap, apothecia. B, section of the thallus 

 showing the algal cells, a, and the rhizoids, r (X40). 

 C, Usnea barbata, algal cells (Pleurococcus) surrounded 

 by the fungal filaments ( X 450) . 



rucana marmorea 

 have been found 

 to penetrate to a depth of nearly two centimetres into limestone 

 upon which it was growing. Where the Lichen grows closely 

 attached to the smooth bark of trees, as in the so-called crustaceous 

 forms, it is often to a greater or less extent parasitic, penetrating 

 into the tissues of the bark. Such forms are often deficient in the 

 green algal cells. 



The Lichen-thallus 



The Lichens show several well-marked types in the form of the 

 thallus. This may be closely adherent to the substratum (Crusta- 

 ceous) ; flat or leaflike (Foliaceous) ; gelatinous, or bushy (Fruti- 

 cose) . The internal structure also shows more or less variety. 



Gelatinous Lichens. The simplest type is shown in the gelatinous 

 Lichens, where the independence of the two constituents of the 

 thallus is evident. In these forms the Alga is usually a species of 

 Nostoc, as in Collema (Fig. 155, A, B), which very much resembles 

 a normal Nostoc-colony. The gelatinous mass is penetrated by the 

 loose filaments of the Fungus, which finally produces the charac- 

 teristic fruiting-bodies. 



In the more typical Lichens the hyphae are densely interwoven, 

 and form a tough, often leathery thallus, within which the algal 



