Knowles — The Maritime mid Marine Lichens of Hoioth. 85 



II. FoRMAIlONS AND ASSOCIATIONS. 



Lichens are found growing on many different kinds of sub-strata, but 

 there are three on which they occur more abundantly than on any others, 

 viz. rocks, bark of trees and wood, and the soil. Considering them from 

 this point of view, lichenologists have arranged them into the three following 

 groups : — 



1. Saxicolous lichens — those growing on rock and stones. 



2. Corticolous lichens — those found on bark of trees and on wood. 



3. Terricolous lichens — those that grow on the soil. 



According to the most recent conceptions of plant ecologists as to the 

 meaning of the term 'formation,' these three groups might, perhaps, represent 

 three formations. But as many of the details of the lichen-communities of 

 the Howth coasts are not sufficiently worked out to enable one to arrive at 

 an accurate estimate of their standing, I have thought it better, until some 

 other area has been investigated, to abstain from employing the terms 

 'formation' and 'association,' and, for the purpose of describing the lichen- 

 vegetation of these coasts, to make use of tlie three long-established groups 

 above mentioned. All three groups are found on the Howth shores, the 

 saxicolous being the most abundant. The Blackthorn thickets on the cliffs 

 above the Broad Strand afford the only habitat for the corticolous forms. 

 Those that grow on the ground, the terricolous, are largely represented, and 

 have been studied mainly on the earth cliffs at Earlsclift'e and at GHenaveena. 



1. The Lichens of I he Rocky Coast (Sccxicoiotis) . 

 i. Silicioiis Rocks. 



The greater part of the coast is composed of silicious rocks ; but, as has 

 been stated, there is a small outcrop of limestone on the shore at Balscadden 

 Bay. As several lichens not found on the silicious strata grow on this 

 limestone area, its vegetation will be considered separately. The zonal 

 distribution of the lichens on the sea-shore, to which Warming has drawn 

 attention, and which has been referred to already at page 83, is very well seen 

 along the greater part of th*e Howth shores. As one walks along the top of 

 the headlands when the tide is low, a dark baud, which seems like a stain on 

 the rock-surface, can be distinctly traced on the cliff-faces and on tlie rooks 

 of the seashore. At liigh spring-tide this dark band is almost hidden by tlie 

 water, but it becomes wider as the tide falls ; and at low spring-tide it is seen 

 to end, on the cliffs and high rocks, in a well-defined line running parallel 



