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 ANNUALS
To conclude this chapter something must be said concerning the highly characteristic collection of ephemeral plants which often appear on the relatively bare sand of partially fixed sand-dunes. Very few of these species are confined to the coast and most of them are liable to turn up in waste ground anywhere. In marked contrast to the majority of perennial dune plants, these annuals have quite shallow roots, and a depth of more than 6 in. is unusual. A diagram of the root systems of some typical dune annuals is shown in Fig. 12 (N.B. 15 cms. = 6 in). These roots are devised to utilise the surface-water only, and it may be pointed out that such small amounts of humus as have accumulated at this early stage will be found near the surface.
FIG. 12.—Root-systems of dune-annuals (from Fritsch & Salisbury, 1946). a. Myosotis hispida; b. Cerastium semidecandrum; Phleum arenarium.
In this connection it is worth mentioning that these “ephemerals” often occur more thickly near rabbit droppings, which have locally provided some additional humus. The surface-sand rarely remains moist sufficiently long during the summer months to enable such shallow-rooting plants to exist, and they aim at completing their life-cycle while it is still damp from the winter and spring rains. They may be described as “drought-escaping” plants, but are more usually referred to as “winter annuals,” since they germinate in the autumn and persist as small seedlings through the winter. In early spring they start to grow quickly, then flower, and finally set their seed before the heat of the summer makes their habitat untenable. Their life-cycle comes to a rapid end with the setting of their seeds, and by the middle of the summer only their dried-up remains can usually be found.
Winter annuals are much more in evidence on the surface of dunes in some years than in others. Should an unusually long dry spell of weather occur in the late autumn, or should the surface be seriously disturbed by a series of especially violent gales when the seedlings have just germinated, most of next year’s population may well be blotted out. They are, however, mostly common plants with light seeds, which are widely distributed by the wind and, granted favourable conditions for their germination, can generally be found in abundance in a bare habitat like this, where there is little competition from taller plants. Typical examples are whitlow-grass (Erophila verna
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