ILLUMINATION OF LICHENS 243 



shade it assumes a grey-green hue and yields only small quantities of 

 parietin. Placodium elegans, normally a brightly coloured yellow lichen, 

 becomes, in the strong light of the high Alps, a deep orange-red. Rhizo- 

 carpon geographicum is a vivid citrine-yellow on high mountains, but is 

 almost green at lesser elevations. 



c. SHADE LICHENS. Many species grow where the light is abundant 

 though diffuse. Those on tree-trunks rarely receive direct illumination and 

 may be generally included among shade-plants. Wiesner found that corti- 

 colous forms of Parmelia saxatilis grew best with an illumination between 

 and jiy of full light, and Pertusaria amara from -fa to -fa; both of them could 

 thrive from to ^g-, but were never observed on trees in direct light. Physcia 

 ciliaris, which inhabits the trunks of old trees, is also a plant that prefers 

 diffuse light. In warm tropical regions, lichens are mostly shade-plants: 

 Wiesner records an instance of a species found on the aerial roots of a tree 

 with an illumination of only ^<j- 



In a study of subterranean plants, Maheu 1 takes note of the lichens that 

 he found growing in limestone caves, in hollows and clefts of the rocks, etc. 

 A fair number grew well just within the opening of the caves; but species 

 such as Cl. ceruicornis, Placodium murorum and Xanthoria parietina ceased 

 abruptly where the solar rays failed. Only a few individuals of one or two 

 species were found to remain normal in semi-darkness : Opegrapha hapalea 

 and Verrucaria muralis were found at the bottom of a cave with the thallus 

 only slightly reduced. The nature of the substratum in these cases must 

 however also be taken into account, as well as the light influences: lime- 

 stone for instance is a more favourable habitat than gypsum ; the latter, being 

 more readily soluble, provides a less permanent support. 



Maheu has recorded observations on growth in its relation to light in 

 the case of a number of lichens growing in caves. 



Physcia obscura grew in almost total darkness; Placodium murorum 

 within the cave had lost nearly all colour; Placodium variabile var. deep 

 within the cave, sterile ; Opegrapha endoleuca in partial obscurity ; Verrucaria 

 rupestris f. in total obscurity, the thallus much reduced and sterile ; Verru- 

 caria rupestris in partial obscurity, the asci empty; Homodium {Collemd} 

 granuliferum in the inmost recess of the cave, sterile, and the hyphae more 

 spongy than in the open. 



Siliceous rocks in darkness were still more barren, but a few odd lichens 

 were collected from sandstone in various caves : Cladonia squamosa, Parmelia 

 perlata var. ciliata, Diploschistes scruposus, Lecidea grisella, Collema nigrescens 

 and Leptogium lacerum. 



d. VARYING SHADE CONDITIONS. It has been frequently observed 

 that on the trees of open park lands lichens are more abundant on the side 



1 Maheu 1906. 



16 2 



