386 



ECOLOGY 



(4) High shingle. The term indicates shingle aggregated into banks 

 lying well above all except the highest tides. A large percentage of sand 

 may be mixed with the stones and if no humus is present and the stones of 

 small size, lichens may be absent altogether. Those occurring in the "loose 

 shingle" are saxicolous. In the "bound shingle" where there is no grass 

 the stones, fixed in a mixture of sand and humus, are well covered with 

 lichens. With the presence of grass, a thin layer of humus covers the stones 

 and a dense lichen vegetation is developed both of shingle and of dune 

 species. 



(5) Low shingle. This last association lies in the hollows among plants 

 of Suaeda fruticosa. Stability is high and tidal immersions regular and 

 frequent. The dominant factor of the association is the quantity of humus 

 and mud deposited around and over the stones. The lichens cover almost 

 every available spot on the firmly embedded pebbles. The characteristic 

 species of such areas are Lecanora badia and L. iPlacodmni) citrina which 

 effect the primary colonization. To these succeed Lecanora atra and Xan- 

 thoria parietina. In time the mud overwhelms and partly destroys the 

 lichens, so that the phase of luxuriant growth is only temporary. 



Lecanora badia is conspicuously abundant at the sand end of this forma- 

 tion. Lecanora {Placodiuvi) citrina disappears as the mud is left behind. 

 Collema spp. also occur frequently on the mixture of mud and sand round 

 the stones. The species on "low shingle" are those most tolerant of sub- 

 mersion : Verrucaria ■)naura is confined to this area, where it is covered by 

 the tide several hours each day. 



Formation 



Association 



Principal Species 



Dune 



Shingle 



I. Bare Sand 



1: 



-2. Grey Dune 

 Derelict Dune 



High Shingle 

 Loose ■ 



Bound 



5. Low Shingle 



Cetraria aculeata f. acanthella 

 CladoiiicL fiircata 



Cladonia rangiferina, Peltigera rufescens 

 Cladonia furcata^ CI. alcicornis 

 Cladonia furcata, Parmelia fuliginosa 

 Rhizocarpon confervoides 

 [Lecanora atra, L. galactina 

 With sand \ Rhizocarpon confervoides 

 {Lecanora citrina 



[Physcia tenella, Lecanora citrina, Xanthoria parielina 

 . Without sand \ Squamaria saxicola 



I Parmelia saxatilis, P. fuliginosa 

 TTT-,.. ) Cladonia rangiferina, CI. furcata, CI. pungens 



^ [ Cetraria aculeata 



(Xanthoria parietina^ Biatorina chalybeia^ Lecanora atra 

 Without grasses XAspiciliagibbosa^Buelliacolludens^ Verriicariamicrospora 

 \Physcia tenella^ Lecanora atrojlava 

 Rhizocarpon confervoides^ Lecanora citrina var. incrustans 

 L. badia^ L. atra, Xanthoria parietina 

 Verrucaria matira 



McLean adds that Xanthoria parietina in its virescent form on Suaeda 

 fiuticosa also endures constant immersion ; Lecanora badia does not occur 



