[Illustration: FIG. 5.--SECTION ACROSS INROLLED LEAF OF CROWBERRY (EMPETRUM NIGRUM), MUCH ENLARGED.]
the open air only through narrow openings. The leaves of some of our common grasses show these characteristics to great advantage. And again the stomata are often sunk in little pits, by which device they obtain further protection. If we now examine the plants composing the sand-dune or shingle-beach associations in the light of these facts, we shall find them full of interest. The plants are well equipped to meet the adverse conditions of a very porous soil, drying winds, and scorching sun. Note the grey felt of hairs which protects the leaves of the Horned Poppy (_Glaucium flavum_), the tough, waxy skin which covers the Sea Holly (_Eryngium maritimum_), the extensive underground stem-systems of the fleshy-leaved Sea Convolvulus (_C. Soldanella_) and Sea Purslane (_Honkenya peploides_). Even the annual plants display similar characters. In the great desert regions the annuals are often quite normal in structure: that is because they appear during the brief rainy season, and pass away before the fierce heat of summer sets in. But on our shingle beaches the annuals grow throughout the summer, and need protection against drought: so the Sea Rocket (_Cakile maritima_), the Sea Whin (_Salsola kali_), and others are very fleshy plants; their leaves are small, with an impervious skin, their root-systems are better developed than in most annuals. The grasses and sedges of these places, such as the Bent (_Ammophila arenaria_), Sea Wheatgrass (_Triticum junceum_), Sea Sedge (_Carex arenaria_) have underground stems which burrow widely through the sand, with an extensive root-system, and tufts of inrolled leaves beautifully protected against over-transpiration and well worth microscopical examination.
If we turn from the shingle beach to the salt-marsh, where water is very abundant, we shall be struck by the peculiar fact that its vegetation displays characters quite similar to those we have just been studying. How can we reconcile this with the theory that the peculiar characters of the shingle-beach plants are correlated with lack of moisture? The explanation is to be found in the fact that plants have difficulty in absorbing water if it is highly charged with mineral substances in solution. In the salt-marsh the heavy muddy soil is impregnated with common salt (chloride of sodium): the plants absorb it with difficulty; and in consequence they are faced with the same main problem which confronts the Sea Holly and Sea Whin, and they meet it in the same way. Indeed, the salt-marsh plants appear to be more highly specialized, for very few intruders from outside can venture in, while on the beach we may meet with many plants which belong to other formations growing successfully, at least for a time. The salt-marsh flora is very exclusive, and contains but few species which we encounter in other situations. Some of them are also found on dry sea-rocks--the Sea Pink (_Statice Armeria_), Scurvy-grass (_Cochlearia officinalis_), Sea Aster (_A. Tripolium_), and so on; showing that soaking soil is in no way essential to their growth. (The first two reappear among alpine plants on some of our higher mountains, pointing again to an analogy of conditions not altogether understood.) But the salt-marsh formation as a whole is perhaps the most distinctive as regards its composition of any of the plant-groups of our country. It is dominated by such species as the grey leathery-leaved _Obione portulacoides_, the small-leaved, thick-stemmed Sea Pink, the Sea Wormwood (_Artemisia maritima_), which is all covered with a silky coat; the pools are fringed with _Scirpus Tabernæmontani_, a dwarf greyish copy of the Common Bulrush of our lakes, and filled with the narrow-leaved _Ruppia_ and _Zannichellia_; and in the muddiest places are little forests of Glasswort, leafless, very fleshy, the flowers reduced to mere essentials and buried in the fleshy stems (Fig. 2, p. 18).