LICHENS 259 



The Algae found within the thalli of Lichens are forms which 

 can also grow independently in the terrestrial habitats in which 

 the latter occur ; examples are afforded by the unicellular green 

 Cystococcus (found in the Lichen Cladonia) and the filamentous 

 blue-green Nostoc (found in Peltigera). The Fungi concerned 

 are, on the other hand, unable to lead an independent existence 

 in nature, and are only capable of growth when associated with 

 their appropriate Alga. A single species of Alga may be coupled 

 with several different Fungi to form as many distinct Lichens, 

 but each Fungus has only one algal associate. It has even 

 proved possible to produce a Lichen artificially by sowing the 

 spores of its fungal constituent among the appropriate algal cells. 

 The vast majority of Lichen Fungi belong to the Ascomycetes, 

 as shown by their fructifications, which closely resemble 

 small rather flattened Peziza-cups (Fig.- 141, D). In Cladonia 

 (Fig. 141, E), where the thallus itself is often inconspicuous, 

 the apothecia are borne on the edges of variously shaped 

 upgrowths. 



Since the hyphae at the two surfaces of the thallus are usually 

 densely compacted and thick- walled (Fig. 142, C), the Alga (al.) 

 embedded in the interior is well protected during times of drought. 

 As a general rule, moreover, tufts of hyphae project from the 

 underside of the thallus into the substratum, absorbing moisture 

 which is held there by capillarity, and thus the algal cells are 

 kept supplied during dry periods. In the case of Lichens growing 

 on rocks, the penetration of these hyphal tufts is facilitated by 

 the secretion of solvents, and it is to this property that such 

 Lichens owe their important role in the disintegration and 

 colonisation of bare rock-surfaces. 



Whilst the Alga thus obtains protection and a supply of 

 moisture, the Fungus no doubt profits by absorbing part of the 

 photosynthetic products of the former, with whose cells some 

 of its hyphae come into intimate contact (Fig. 142, B). In some 

 cases, in fact, the hyphae are stated actually to penetrate the 

 algal cells, so that the partnership borders closely on parasitism. 

 Despite this intimate relation, the enveloping hyphae are usually 

 loosely arranged, so that a well-marked system of intercellular 

 spaces admits of the inward diffusion of carbon dioxide (Fig. 142, 

 C, hy.). In transverse sections the algal cells are seen as patches 



