GENERAL NUTRITION 235 



part of an old stained window in a chapel that was obscured by a lichen 

 growth which adhered tenaciously. When the window was taken down and 

 cleaned, it was found that the surface of the glass was covered with small, 

 more or less hemispherical pits which vvere often confluent. The different 

 colours in the picture were unequally attacked, some of the figures or draperies 

 being covered with the minute excavations, while other parts were intact. 

 It happened also, occasionally, that a colour while slightly corroded in one 

 pane would be'uninjured in another, but the suggestion is made that there 

 might in that case have been a difference in the length of attack by the 

 lichen. The selection of colours by the lichens might also be influenced by 

 some chemical or physical characters. 



Bachmann^ found that on granite there is equally a selection of material 

 by the hyphae : as a rule they avoid the acid silica constituents, while they 

 penetrate and traverse the grains of mica which are dissolved by them 

 exactly as are lime granules. 



On another rock consisting mainly of muscovite and quartz he^ found 

 that crystals of garnet embedded in the rock were reduced to a powder by 

 the action of the lichen. He concludes that the destroying action of the 

 hyphae is accelerated by the presence of carbon dioxide given off by the 

 lichen, and dissolved in the surrounding moisture. Lang^ and Stahlecker-* 

 have both come to the conclusion that even the quartz grains are corroded 

 by the lichen hyphae. Stahlecker finds that they change the quartz into 

 amorphous silicic acid, and thus bring it into the cycle of organic life. Chalk 

 and magnesia are extracted from the silicates where no other plant could 

 procure them. Lichens are generally rare on pure quartz rocks, chiefly, 

 however, for the mechanical reason that the structure is of too close a grain 

 to afford a foothold. 



D. Supply of Organic Food 



a. From the Substratum. The Ascomycetous fungi, from which so 

 many of the lichens are descended, are mainly saprophytes, obtaining their 

 carbohydrates from dead plant material, and lichen hyphae have in some 

 instances undoubtedly retained their saprophytic capacity. It has been 

 proved that lichen hyphae, which naturally could not exist without the 

 algal symbiont, may be artificially cultivated on nutrient media without the 

 presence of gonidia, though the chief and often the only source of carbon 

 supply is normally through the alga with which the hyphae are associated 

 in symbiotic union. 



A large number of crustaceous lichens grow on the bark of trees, and 

 their hyphae burrow among the dead cells of the outer bark using up the 



^ Bachmann 1904. ^ Bachmann 1911. ^ Lang 1903. •• Stahlecker 1906. 



