This section is from the book "British Wild Flowers - In Their Natural Haunts Vol2-4", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
As one passes from the west of England to the east one is struck by one important feature at least, apart from the difference in the character of the coast, the one rocky and elevated, the other flat. For owing to the beneficent influence of the Atlantic Drift, which flows to the west of Ireland, and is felt in the St. George's Channel, the temperature of the west coast is much higher than that on the east coast. This of course makes the west coast much moister or more humid. The hills are nearer the west than the east coast, hence the rainfall is again much heavier in the west than the east.
On the east coast the climate is cold; winds reaching the coast from the Atlantic are already deprived of their moisture, and the easterly winds from the Continent are dry. The east is much drier as a whole, therefore, and there is much less rain.
In South-west Ireland the temperature in winter is like that of the Mediterranean. The coldest area is in Central England northwards to Scotland. The rockiness of the west and south coast is another factor. Hence we have on the west coast rupestral plants such as Welsh Poppy, Thrift, and Scurvy Grass, on the muddy sandy east coasts Sea Lavender, Sea Kale, Saltwort, etc.
All these factors - higher temperature, more humid conditions, and a rocky substratum - cause the western plants to differ from the eastern, as may be illustrated by lists of plants from each. The same applies largely to the north and south coasts. The western plants are Atlantic types, the eastern Germanic.
There are one or two features of maritime regions, so far as the British Isles are concerned, which influence the vegetation they support that ought to be pointed out, as they are peculiar to this type. In the first place, as briefly mentioned, the coastal border is near the sea-level. The temperature therefore is more or less uniform, and were it not that there is a marked difference on the west to that on the east, and relatively between that on the north and south coasts, the effect of altitude would be negligible in this case, and the sole influence upon temperature would be owing to latitude.
As a whole the altitude at which maritime plants grow, except a few inland types, as Thrift, Sea Plantain, and Scurvy Grass, varies between 1 and 100 ft., so that all the plants are of the ascending type, though they do not actually encroach upon other types owing to the necessity of saline conditions in the soil and the restriction of this to the sea border. Another feature is the generally uniform aspect of plants on each of the coasts north and south, east and west. Each seaboard is more or less restricted to certain classes of wind, even if the uniform land and sea breezes were not equally influential in this respect.
Again, the sequence of changes from one type of maritime zone to another is quite definite, and there are few sea-coast types that are to be found inland under other conditions, except some Grasses (as Seaside Manna Grass), Buckshorn Plantain, Woad, and Centaury, and these are chiefly sandy-soil types. The vegetation of the sea-coast is thus as a whole more or less specialized, and does not grade into inland types. This is an important feature upon which due emphasis should be laid. The occurrence of many inland plants on the sea-coast on the contrary is an equally noticeable feature, e.g. Scarlet Pimpernel on shingle, etc.