262 BIONOMICS 



benefited materially by the more nutritive substratum : their apothecia were 

 more abundant and their thallus more luxuriant. The gonidia especially 

 had profited; they were larger, more brightly coloured, and they increased 

 more freely. Hoffmann offers the explanation that the strain on the algae of 

 providing organic food for the hyphal symbiont was relaxed for the time, 

 hence their more vigorous appearance. 



Arthonia subvarians is always parasitic on the apothecia of Lecanora 

 galactina, and Almquist' discovered that the hymenium of the host alone is 

 injured, the hypothecium and excipulum being left intact. 



The " parasitism " of Pertusaria globulifera on Parmelia perlata and 

 P.physodes, as described by Bitter'-', may also be included under antagonistic 

 symbiosis. The hyphae pierce the Parmdia thallus, break it up and gradually 

 absorb it. Chemical as well as mechanical influences are concerned in the 

 work of destruction as both the fungus and the alga of the victim are dissolved. 

 Lecanora tartarea already dealt with as a marauding lichen'* over decaying 

 vegetation may spread also to living lichens. Fruticose soil species, such as 

 Cetraria aculeata and others, die from the base and the Lecanora gains 

 entrance to their tissues at the decaying end which is open. 



Arnold* speaks of these facultative parasites that have merely changed 

 their substratum as pseudo-parasites, and he gives a list of instances of such 

 change. In many cases it is rather the older thalli that are taken possession 

 of, and, in nearly every case, the invader is some crustaceous species. The 

 plants attacked are generally ground lichens or more particularly those that 

 inhabit damp localities, such as Peltigera or Cladonia or certain bark lichens. 

 Drifting soredia or particles of a lichen would easily take hold of the host 

 thallus and develop in suitable conditions. To give a few of the instances 

 observed, there have been found, by Arnold, Crombie and others : 



on Peltigera canina: Callopisrna cerina, Rinodina turfacea var., pUimbia 

 obscurata and Lecanora aurella; 



on Peltigera aphthosa: Lecidea decolorans; 



on Cladoniae: Bilimbia microcarpa, Bacidia Beckhausii and Urceolaria 

 scruposa, etc. 



Urceolaria {Diploschistes) has a somewhat bulky crustaceous thallus which 

 may be almost evanescent in its semi-parasitic condition, the only gonidia 

 retained being in the margin of the apothecia. Nylander^ found isolated 

 apothecia growing vigorously on Cladonia squamules. 



Hue" describes Lecanora aspidophora f. errabunda, an Antarctic lichen, as 

 not only a wanderer but as a "shameless robber." It is to be seen everywhere 

 on and about other lichens, settling small glomeruli of apothecia here and 



1 Almquist 1880. 2 s.xVitx 1899. 3 55ee p. 237. ^ Arnold 1874. 



° Nylander 1853. Hug 191 5. 



