Aspects of plant life; with special reference to the British flora

by R. Lloyd Praeger

highly impervious skin, so that they act as veritable cisterns. In plants like the Cacti water storage in the stems is carried very far indeed; while in such genera as the Stonecrops (Sedum) the leaves are often so swollen and charged with water that they lose up to 98 per cent. of their weight if they are dried. Prevention of excessive loss of water by transpiration is effected in plants of dry places mainly by reduction in the size of the leaf and by protection of its surface. Leaf reduction is very marked in many dry countries. If we compare the flora of the Mediterranean region (a dry area) with that of Middle Europe or of England, we shall be struck with the prevalence in the former of small-leaved twiggy plants—Lavender (Lavandula) and Rosemary (Rosmarinus officinalis) will serve as examples. Often leaf-reduction is carried much farther, and we need not go beyond our own commons to find a good example, for in the Gorse (Ulex) flat leaves are entirely absent and the branches are shortened and converted into prickles, thus largely reducing the surface exposed to the sun and wind. The seedling Gorse has little trifoliate leaves, which remind us of its affinity to the Trefoils and Brooms, but they are discarded almost at once, to fit the plant better for life in the dry, breezy localities which it favours. Reverting to the Mediterranean flora, a characteristic of its plants is the prevalence of a grey hue in their stems and leaves, such as we see in the Pinks and Achilleas of our rock gardens. This is due to a coat of wax, as in the Pinks (Dianthus), or a felt of hairs, as in the Achilleas, designed to check excessive transpiration. The coatings of hairs are often of great beauty and complexity, and form an almost impenetrable covering to the leaf surface, protecting the upper side from the fierce rays of the sun, and on the underside sheltering the stomata, or minute openings through which the plant exhales the surplus water drawn up from the roots and inhales carbon dioxide. Another very beautiful device for protecting the underside of the leaf, and one which may be studied in many of our commonest plants, consists of the inrolling of the edges, often combined with a wrinkling or ridging of the underside, so that the stomata are set in deep hollows, communicating with 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

Fig. 5.—Section across Inrolled Leaf of Crowberry (Empetrum nigrum), much enlarged.

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).

Again, it is easy to trace the relationship existing between plant form and soil conditions in the bogland flora; and these relations, unexpectedly enough, turn out to be analogous to those obtaining in the case of the salt-marsh. The sodden peat, sour and badly aerated, and poor in mineral salts, is poor also in the bacteria which feed upon and destroy dead vegetable matter, with the consequence that acid humus compounds collect in the half-decayed vegetable mass; water charged with these substances is as unsuitable for plants as is the water of the salt-marsh. In spite of the wetness of the peat, water is in this case also a desideratum; and the moorland plants, like those of the sea fringe, possess special adaptations for economizing it. This usually takes prominently the form of a reduction of leaf-surface. The dominant plants, such as the Ling (Calluna vulgaris) and Purple Heather (Erica cinerea), have minute leaves with reflexed edges and special structure to protect the stomata. The grasses and sedges which abound have similar characteristics; the whole vegetation tends to be small-leaved and long-rooted. A few of the plants, such as the Eyebright (Euphrasia), eke out the scanty food-supply by a semi-parisitism, robbing their neighbours of portions of their hardly-won sustenance; one or two others, such as the Bladderwort (Utricularia), which floats in the bog-pools, and the Sundew (Drosera), which fringes their edges, entrap insects and digest their juices, helping out their scanty rations with an animal diet. On the moors the peculiar soil conditions determine definitely the type of vegetation, which, over large areas, is as uniform and monotonous as that of the salt-marsh.

We see, then, that the peculiar character of several of the most marked of native plant formations—those of shingle, of salt-marsh, and of moor—are due primarily to scarcity of water. They are drought formations, produced either by physical drought, as in the case of shingle, which fails to retain water, or by physiological drought, as in the salt-marsh or bog, where, though water is present in abundance, it is not in a condition in which plants can readily make use of it.

Let us now go to the opposite extreme, and consider the plant formation which characterizes lowland lakes and rivers, where water suitable for plant use is