GENERAL NUTRITION 237 



Lecanora thallus. It kills its host in patches and the dead material mostly 

 drifts away. On any strands that are left Candellariella vitellina generally 

 settles and evidently profits by the dead nutriment. It does not spread to 

 the living thallus. Lecanora polytropa also forms colonies on these vacant 

 patches, with advantage to its growth. 



Even the larger lichens are attacked by these quick-growing crusts. 

 Pertusaria globulifera spreads over Parmelia perlata and P. pJiysodes, 

 gradually dissolving and consuming the different thalline layers; the lower 

 cortex of the victim holds out longest and can be seen as an undigested 

 black substance within the Pertusaria thallus for some time. As a rule, 

 however, the lichens with large lobes grow over the smaller thalli in a purely 

 mechanical fashion. 



c. From other Vegetation. ZukaP has given instances of association 

 between mosses and lichens in which the latter seemed to play the part of 

 parasite. The terricolous species Baeomyces rnfus {Spkyridium) and Biatora 

 decolorans, as well as forms of Lepraria and Variolaria, he found growing 

 over mosses and killing them. Stems and leaves of the moss Plagiothecium 

 sylvaticum were grown through and through by the hyphae of a Pertusaria, 

 and he observed a leaf of Polytrichum commune pierced by the rhizinae of 

 a minute Cladonia squamule. The cells had been invaded and the neigh- 

 bouring tissue was brown and dead. 



Perhaps the most voracious consumer of organic remains is Lecanora 

 tartarea, more especially the northern form frigida. It is the well-known 

 cud-bear lichen of West Scotland, and is normally a rock species. It has 

 an extremely vigorous thickly crustaceous and quick-growing thallus, and 

 spreads over everything that lies in its path — decaying mosses, dead leaves, 

 other lichens, etc. Kihlman^ has furnished a graphic description of the way 

 it covers up the vegetation on the high altitudes of Russian Lapland. More 

 than any other plant it is able to withstand the effect of the cold winds that 

 sweep across these inhospitable plains. Other plant groups at certain seasons 

 or in certain stages of growth are weakened or killed by the extreme cold 

 of the wind, and, immediately, a growth of the more hardy grey crust of 

 Lecanora tartarea begins to spread over and take possession of the area 

 affected — very frequently a bank of mosses, of which the tips have been 

 destroyed, is thus covered up. In the same way the moorland Cladoniae, 

 C. rangiferina (the reindeer moss) and some allied species, are attacked. 

 They have no continuous cortex, the outer covering of the long branching 

 podetia being a loose felt of hyphae ; they are thus sensitive to cold and 

 liable to be destroyed by a high wind, and their stems, which are blackened 

 as decay advances, become very soon dotted with the whitish-grey crust of 

 the more vigorous and resistant Lecanora. 



1 Zukal 1879. ^ Kihlman 1890. 



