Flowers of the Coast. Ian Hepburn

deeply into the crevices between the rocks.

      Extensive root-systems are also a characteristic feature of xerophytes in all parts of the world. In sand-dunes and shingle this enables the plants to utilise the moisture which is always present some way below the surface (see here). In addition, the elaborate root-systems developed by many dune plants perform the important function of binding blown sand. All pioneer colonists on sand-dunes, and to a lesser extent those on mobile shingle, have to contend with the possibility of being periodically swamped by loose sand or shingle. Most of them have, in varying degrees, the ability to form fresh shoots easily when they are submerged in this way, and to grow up through this covering. Marram-grass easily outstrips all other plants in the vigour with which it can do this. When once established in loose sand, it soon produces a mass of underground runners from which new shoots continually spring. Where these new shoots occur, fresh adventitious roots are produced under them; as the stems and leaves become buried by sand, further shoots and leaves are produced at a higher level on the stem (Fig. 10). The thick tufts of leaves and young shoots are very stiff and offer a considerable obstruction to the wind, causing it to drop some of its load of sand round them. Provided the blown sand does not accumulate too rapidly over the plants, marram-grass can continue to grow upwards through many feet of sand. In this way dunes rising to considerable heights can be produced, their size depending on the supply of available sand and the strength of the prevailing winds. The whole interior of the dune remains closely penetrated by a mass of rhizomes (underground stems) and fine roots which bind it together. As these rhizomes and roots are constantly being renewed at higher levels, the lower ones gradually die off, but their dead remains persist in the lower regions of a dune for a long time and continue to exercise a stabilising effect on it (Pl. XXI).

      FIG. 10—Leaf production at different levels shown by marram-grass (from) Firistch & salisbury, 1946).

      To a lesser extent the sea couch-grass (Agropyron junceiforme) can produce a similar result. This plant has much the same habit as ordinary couch-grass or twitch (Agropyron repens), which is an all too familiar agricultural weed. Like marram-grass, it produces a mass of rhizomes from which new shoots spring up at frequent intervals, but its runners tend to spread more rapidly in a horizontal direction than vertically when covered by loose sand. As a result, the dunes it produces are comparatively low compared with those formed by marram-grass. None the less, its powers of binding sand are considerable, and single plants have been shown to cause dunes as much as 20 feet across during a few years’ growth. The sea lyme-grass (Elymus arenarius) has a similar root-system and occasionally forms low dunes of the same type on certain parts of the coast.

      Many other pioneer plants on shifting sand can accumulate small amounts of sand round them to form miniature dunes, provided they are sufficiently virile to shoot up again when they become buried. For example, the sea-sandwort (Honckenya (Arenaria) peploides) is a low-growing plant not more than a few inches high, but it possesses surprisingly extensive creeping roots and readily produces fresh shoots when it is covered. Even those common pioneers of sandy beaches, the sea-rocket (Cakile maritima) (Pl. VII) and the prickly saltwort (Pl. I), which are only annuals, can collect tiny dunes round their long trailing branched stems. Their dead remains usually persist for a considerable time in the winter and continue to hold the sand, although the principal agent here is the stem rather than the roots.

      Marram-grass has relatively little stabilising effect on the surface sand, and it is the later colonists which establish themselves between the clumps that are responsible for its eventual consolidation. Notable amongst these are the sand-sedge (Carex arenaria) and the sand-fescue (Festuca rubra var: arenaria), both of which produce horizontally creeping roots just below the surface. Tufts of foliage arise from these runners at frequent intervals, and in the case of the former often appear spaced out along a nearly straight line for many feet, showing clearly the course of its immense roots (Pl. X).

      Plants growing on shingle similarly have to endure periodical swamping by stones. Very unstable shingle is usually devoid of vegetation, but during storms those less mobile parts of the beach, which carry quite a large amount of vegetation, may also be disturbed. The shrubby seablite (Suaeda fruticosa) (Pl. XXIII), a local plant which is found on Chesil Bank and on some shingle beaches along the Norfolk coast, adapts itself in an interesting way to these conditions. When it grows in perfectly stable shingle it is a plant of erect habit, 3-4 feet high, but where the shingle is more mobile it assumes a quite different, semi-prostrate habit. Under the latter conditions, the stem is tilted forwards as shingle flows over it from above and the lower part becomes imbedded. New branching shoots then arise from the buried stem, and at the same time fresh tufts of roots are produced under each shoot. When growing on very unstable shingle banks, the original axis of many of these plants may become quite horizontal, and a complex system of prostrate branches, covered everywhere with fibrous roots, will be found in the form of a dense mat just beneath the surface. The aerial shoots from these extensive buried stems grow vertically upwards, usually to a height of a foot or two, but much of the elaborate underground structure is generally dead and already in the process of decay (Fig. 11).

      FIG. 11.—Typical habit of the shrubby seablite, when growing in shingle (from Tansley, after Oliver & Salisbury, 1913; drawing by Sarah M. Baker).

      Several other characteristic shingle-plants are capable of enduring a certain amount of shingling-over. In particular, it has often been noticed that when the surface of the shingle has been unusually disturbed by storms during the winter both the sea-campion (Silene maritima) (Pl. XXXIII) and the sea-sandwort seem to grow more vigorously and flower more freely during the following season. The shrubby seablite is also much happier in mobile shingle, and it seems possible that the stimulus of having to form new shoots when their stems become buried has a beneficial effect on the virility of many shingle plants.

      We have already referred to the tendency of coastal plants to adopt a close mat-habit to reduce transpiration, and to the rosette form assumed by many dune plants. Shingle beaches and the lower portions of sea-cliffs, in addition to being exposed to strong winds, are also subjected to considerable amounts of spray. The low habit of growth shown by many plants in these habitats serves to protect them from spray as well as from excessive transpiration. If a plant produces an erect flowering stem, it usually dies away as soon as the seeds are ripe so that the foliage should not be destroyed by the spray-saturated gales of the winter. Thus the young leaves of that characteristic shingle plant, the maritime variety of the curled dock (Rumex crispus var: trigranulatus), remain covered by a mass of withered foliage of the previous season’s growth during the winter months, which is often effective in protecting them from spray. In a similar way, the yellow horned poppy (Pl. IX) spends most of the year in the form of a compact rosette closely pressed to the stones and protected by thick hairs. Many plants adopt a largely prostrate habit when growing in exposed places, but this may often be caused by the wind retarding growth on one side of the stem and producing unequal growth (see here). In particular, blackthorn (Prunus spinosa) and broom (Sarothamnus scoparius var: prostratus) (Pl. XXIV) sometimes grow completely flat along the ground on shingle, and a particularly well-marked prostrate variety of the woody nightshade (Solanum dulcamara var: marinum) (Pl. 2a) may be seen on some south coast beaches.

      DUNE