236 PHYSIOLOGY 



material with which they come in contact. Others live on dead wood, palings, 

 etc. where the supply of disintegrated organic substance is even greater ; or 

 they spread over withered mosses and soil rich in humus. 



b. FROM OTHER LICHENS. Bitter 1 has recorded several instances ob- 

 served by him of lichens growing over other lichens and using up their 

 substance as food material. Some lichens are naturally more vigorous than 

 others, and the weaker or more slow growing succumb when an encounter 

 takes place. Pertusaria globulifera is one of these marauding species; its 

 habitat is among mosses on the bark of trees, and, being a quick grower, it 

 easily overspreads its more sluggish neighbours. It can scarcely be considered 

 a parasite, as the thallus of the victim is first killed, probably by the action 

 of an enzyme. 



Lecanora subfusca and allied species which have a thin thallus are 

 frequently overgrown by this Pertusaria and a dark line generally precedes 

 the invading lichen; the hyphae and the gonidia of the Lecanorae are first 

 killed and changed to a brown structureless mass which is then split up by 

 the advancing hyphae of the Pertusaria into small portions. A little way 

 back from the edge of the predatory thallus the dead particles are no longer 

 visible, having been dissolved and completely used up. Pertusaria amara 

 also may overgrow Lecanorae, though, generally, its onward course is 

 checked and deflected towards a lateral direction; if however it is in a young 

 and vigorous condition, it attacks the thallus in its path, and ahead of it 

 appears the rather broad blackish line marking the fatal effect of the enzyme, 

 the rest of the host thallus being unaffected. Neither Pertusaria seems to 

 profit much, and does not grow either faster or thicker; the thallus appears 

 indeed to be hindered rather than helped by the encounter. Biatora (Lecided) 

 quemea with a looser, more furfuraceous thallus is also killed and dissolved 

 by Pertusariae ; but if the Biatora is growing near to a withering or dead 

 lichen it, also, profits by the food material at hand, grows over it and uses it up. 

 Bitter has also observed lichens overgrown by Haematomma sp. ; the growth 

 of that lichen is indeed so rapid that few others can withstand its approach. 



Another common rock species, Lecanora sordida (L. glaucoma), has a 

 vigorous thallus that easily ousts its neighbours. Rhizocarpon geographicu m, 

 a slow-growing species, is especially liable to be attacked ; from the thallus 

 of L. sordida the hyphae in strands push directly into the other lichen in a 

 horizontal direction and split up the tissues, the algae persist unharmed for 

 some time, but eventually they succumb and are used up; the apothecia, 

 though more resistant than the thallus, are also gradually undermined and 

 hoisted up by the new growth, till finally no trace of the original lichen is 

 left. Lecanora sordida is however in turn invaded by Lecidea insularis 

 1 ' L. intumescens) which is found forming small orbicular areas on the 



1 Bitter 1899. 



