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 platycarpus, only covered by the 
highest tides. The spray-washed rocks, seldom sub- 
merged at all, are covered with the xerophytic fronds of 
Pelvetia 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 : 
. Muddy banks of estuaries, salt-marshes, etc. 
. Sandy seashore. 
. Pebbly seashore and shingle. 
. Rocks and cliffs. 
. 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., Salicornia herbacea 
and strand-associations. 
Or wd 
