CHAPTER XXVIII 
MARITIME ASSOCIATIONS 
On the seashore and the margin of estuaries the soil 
varies considerably ; it may be sandy, shingly, rocky, or 
muddy, and the vegetation exhibits a corresponding 
diversity. The flora of a mud-bank is quite distinct 
from that of a sand-dune, and this in turn differs from the 
flora of rocks and cliffs. In all cases the land-plants are 
typical xerophytes, but the xerophytic character may be 
due to a variety of causes. It may be an adaptation to 
physiological dryness, as in the plants of a mud-bank 
where the water is strongly saline ; to physical dryness, as 
in sand-dune plants where the soil contains very little 
water ; or to the action of strong drying winds, as in 
those of cliffs. In this chapter, then, a number of distinct 
formations will be described. They are brought together 
merely for convenience, and not necessarily because they 
are related ecologically. 
As is the case with other plants, the maritime plants 
group themselves naturally in two divisions—the aquatic 
vegetation and the terrestrial. 
I. Maritime Aquatic Vegetation. 
On the open seashore and in estuaries, where thé 
water-level is altering with each tide, the aquatic vegeta- 
tion is almost restricted to one group of plants—the sea- 
weeds, or alge. The absence of other plants is due chiefly 
to the difficulty of bringing about fertilization. The 
periodic rising and falling of the water makes it necessary 
that fertilization should be effected under water. Aerial 
flowering stems would be of no use, for the flowers are 
submerged at high tide and ordinary pollen destroyed ; 
275 
