LICHEN COMMUNITIES 



379 



indifferently on any kind of shore rock either siliceous or calcareous. 

 Wheldon and Wilson^ noted this indifference to substratum on the Arran 

 shores, where a few calcicolous species such as Verrucaria nigrescens, V. 

 maculiformis, Placodium tegularis and PI. lobulatum, grow by the sea on 

 siliceous rocks. They suggest that the spray-washed habitat affords the 

 conditions, which, in other places, are furnished by limestone. 



The greater or less proximity of the salt water induces in lichens, as in 

 other maritime plants, a distribution into belts or zones which recede 

 gradually or abruptly according to the slope of the shore and the reach of 

 the tide. Weddell^ on the Isle d'Yeu delimited three such zones : (i) marine, 

 those nearest the sea and immersed for a longer or shorter period at each 

 tide ; (2) semi-marine, not immersed but subject to the direct action of the 

 waves, and (3) maritime or littoral, the area beyond the reach of the waves 

 but within the influence of sea-spray. In the course of his work he indicates 

 the lichens of each zone. 



Fig. 122. Ramalina siliqtiosa A. L. Sm. Upper zone of barren plants (after M. C. Knowles, 



R. Welch, Photo.]. 



In Ireland, a thorough examination has been made of a rocky coast at 

 Howth near Dublin by M. C. Knowlesl She recognizes five distinct belts 



^ Wheldon and Wilson 19 13. 



2 Weddell 1875. 



' Knowles 1913.^ 



