202 SOME BRITISH PLANT GROUPS 
and sea plants, such as we may make on rambles 
along the coast, supplies us with some interesting 
material. On sandy shores, the wave-trampled beach, 
shifting under the influence of winds and currents, 
offers a stretch of “no-man’s-land”—a desert strip 
untenanted alike by terrestrial or marine plants. The 
former do not descend below spring-tide mark, if they 
go so far; the latter cannot obtain foothold on the 
unstable substratum. The peculiar characters of the 
terrestrial beach plants has been referred to on a 
previous page (p. 36). On rocky shores the “ desert ” 
strip is much narrowed, and a certain overlap may 
often be found, for the Lichens—essentially a terres- 
trial group—descend from the plant-covered slopes 
into the spray-swept zone below, and on to mix with 
the Seaweeds which occupy the belt under high-water 
mark, some of them, species of Verrucaria and 
Arthropyrenia, continuing downward till the low- 
water mark of spring tides is reached. On steep 
rocky shores the dividing-line between the Flowering 
Plants and the Seaweeds is quite narrow, and varies 
in elevation with the exposure. On cliffy coasts open 
to the Atlantic waves the uppermost Seaweeds, such 
as Pelvetia, which only asks to be wetted periodically 
by spray, occur far above high-water mark, the lowest 
Seed Plants perching on the rocks much higher still— 
sometimes not venturing to within 100 feet of the 
water-level. Under such extreme conditions none of 
the higher land plants venture down towards the 
unfriendly sea. To see the overlap of the terrestrial 
and maritime vegetation well developed we seek con- 
ditions entirely different, where amid shallow inlets 
and salt-marshes land and sea merge imperceptibly. 
