MARITIME ASSOCIATIONS 277 



number of factors, the chief of which are : their power of 

 resisting desiccation both during their germination and 

 vegetative growth ; their rate of growth, the more quickly- 

 growing plants forcing others to retire to higher levels ; 

 and on considerations connected with their reproduction. 

 In the deeper water, never exposed to the air even at 

 the lowest spring- tides, Laminarias are dominant. Above 

 this may be a zone of Halidrys siliquosa, only exposed for 

 a short time at low water during spring-tides ; then one of 

 Fucus serratus, uncovered at low water, but during neap- 

 tides for a short time only ; higher still are zones of Fucus 

 vesiculosus and Ascophyllum nodosum, exposed for longer 

 periods ; and then Fucus plcUycarpus, only covered by the 

 highest tides. The spray-washed rocks, seldom sub- 

 merged at all, are covered with the xerophytic fronds of 

 Pdvetia caniculata. In this situation the plant is exposed 

 to drying winds and the heat of the sun throughout the 

 day, and excessive evaporation of water is prevented by 

 the presence of a hard, thick, external layer almost 

 impermeable to water. 



II. Maritime Terrestrial Vegetation. 



The associations of terrestrial maritime plants may be 

 grouped according to the nature of the soil and elevation 

 above sea-level as follows : 



1. Muddy banks of estuaries, salt-marshes, etc. 



2. Sandy seashore. 



3. Pebbly seashore £uid shingle. 



4. Rocks and cliffs. 



5. Exposed slopes facing the sea. 



The associations — with the exception of those of the 

 last group, which are usually pasture-associations — are of 

 an open type ; competition is not keen, and almost every 

 plant, adapted by its structure for life in such surround- 

 ings, has a chance of surviving. One association is often 

 rapidly replaced by another, as on sand-dunes, and 

 changes in the distribution of the plants are always going 

 on. As in other open associations, annuals are very 

 abundant, both in individuals and species, and in some 

 cases constitute the sole flora — e.g., Saiicornia herbacea 

 and strand-a,ssocia,tioi\s. 



