dunes, although under normal conditions in garden soil they are soft and slender.
FIG. 3.—Types of stomata: a. Transverse section of leaf with a thin cuticle: b. Transverse section of leaf with a thick cuticle, showing a sunken stoma; c. Surface view of a stoma.
In many plants the stomata are protected by being placed in grooves or hollows sunk well below the surface of the leaf (Fig. 3(b)). In the dune grasses, marram-grass (Ammophila arenaria) and sea lyme-grass (Elymus arenarius), the stomata are mostly confined to the bottom and sides of the deep grooves in their leaves. This protection is much improved by the tendency of the leaves to roll up into a narrow tube in dry weather, which has the effect of maintaining a layer of air, largely saturated with water-vapour, between the stomata and the outside air, and thus reducing evaporation. The manner in which air is enclosed when the leaf rolls up is clearly shown in Fig. 4, and the corrugated inner (i.e. upper) surface is due to the deep grooves along which the stomata are scattered. The outer (i.e. under) surface is furnished with a thick cuticle and is devoid of stomata. This habit of rolling the leaf under dry conditions is shared by many plants, and is a good example of the way they can adjust themselves to variations in their water-supply. When water is plentiful, the blade opens out and becomes flat, thus exposing a greater surface for transpiration. The fresh appearance of marram-grass on open sand-dunes after abundant rain is quite distinct from its parched look after a long period of dry weather, and on closer inspection will be found to be due to the unfolding of its leaves.
FIG. 4.—Transverse section of marram-grass leaf when rolled (from Fritsch & Salisbury, 1946).
Another common way in which the stomata are protected is by the growth of hairs on the surface of the leaf. These are often associated with sunken stomata and are very effective in maintaining a damp atmosphere round the opening, since moisture tends to condense on them. The stiff hairs protecting the furrows on the upper surface of the marram-grass leaf will be noticed in Fig. 4. Many seaside plants have hairy leaves, and some are covered with a thick down. The yellow horned poppy (Glaucium flavum) (Pl. IX), the sea stock (Matthiola sinuata) and the buck’s horn plantain (Plantago coronopus) (Pl. XXXVI) are good examples of coastal plants with hairy leaves, while the leaves of sea-wormwood (Artemisia maritima) (Pl. XXXI) and the tree-mallow (Lavatera arborea) are markedly downy. The characteristic silvery foliage of the sea-buckthorn (Hippophae rhamnoides) (Pl. XX) and sea-purslane (Halimione (Obione) portulacoides) (Pl. 16) is also due to scale-like (peltate) hairs covering the surface of the leaves. These hairs are usually dead when the leaf is mature, and contain only air. Apart from aiding the retention of moist air near the surface, they reflect much of the sun’s heat. Some leaves possess simple unbranched hairs, but those on many others are branched and occur in very different forms. Some typical covering hairs from the leaves of coastal plants are shown in Fig. 5. Like the thickness of the cuticle, the degree of hairiness shown by individuals of the same species often varies with availability of the water-supply in the habitat. Thus the sand-dune form of silverweed (Potentilla anserina) commonly shows a thick felting of silvery hairs on the upper surface of its leaves, as well as on the lower.
FIG. 5.—Typical covering-hairs on various leaves: a. Plantago coronopus; b. Cynodon dactylon; c. Erophila verna; d. Matthiola sinuata; e. Hippophae rhamnoides.
Still another way in which relatively damp air is maintained over the surface of the leaves is by the plant adopting a dense mat habit, so that the transpiring surfaces of the leaves are kept in close contact with each other. Alpine plants often mass their foliage in this way, but amongst coastal plants thrift (Armeria maritima) provides one of the best examples, since its habit varies considerably with the place in which it is growing. Thus the close rosette form is typical when it is growing on rocky cliffs and other dry habitats, or when it is heavily grazed, whilst with a better water-supply it assumes a much more open habit (Fig. 6). Many sand-dune plants spend most of the year in the form of a rosette, only sending up a vertical stem during the flowering season. In this way, only the upper surface of the leaf is exposed to the wind, the under surface being kept closely pressed against the surface of the sand, where it is fully protected from both sun and wind and consequently remains cool and moist.
Transpiration is discouraged in a large number of widely differing plants by a reduction in the actual surface of the leaves. Many conifers furnish examples of this; pines have needle-shaped leaves, and cypresses have scale-like leaves, which are closely pressed to the stem over part of their surface. Among coastal plants, tamarisk (Tamarix gallica) (Pl. 8), now a well-established alien in Britain, has numerous little scale-like leaves, and in the glassworts (Salicornia) the rudimentary leaves are only just visible as tiny scales which are firmly attached to the joints of the succulent stems (Fig. 9(b)). In some plants the same result is achieved by the leaves taking the form of spines. Gorse (Ulex spp.) is the best-known example in this country, but in desert regions the majority of the xerophytic plants show this modification, the Cacti being a familiar case. Our native xerophytes more frequently develop spiny margins to their leaves, thistles furnishing the obvious example. The most striking seaside plant to show this development is the sea-holly (Pl. 1), though the leaves of the prickly saltwort (Salsola kali) (Pl. I) also terminate in stout spines. The tendency to form woody tissue in the form of spines appears to be closely related to a shortage in the water-supply. A number of plants which produce spines when growing in dry habitats do not possess any when moisture is abundant.
Occasionally the function of the leaf is taken over by specially modified branches known as “cladodes.” The only coastal plant exhibiting this modification is the wild asparagus (Asparagus prostratus), a rare plant found on sandy shores in a few localities only in this country. If the familiar feathery foliage of the garden asparagus is examined, it will be seen to consist of tufts of short leaf-like branches arising from the axils of minute scaly leaves (Fig. 7). It is difficult to see exactly what advantage a plant can gain from the substitution of a leaf-like stem for an ordinary leaf—possibly the tissue of the cladode is more resistant to shrinkage when the plant is suffering from a shortage of water.
FIG. 6.—Different forms adopted by thrift: a. Rosette form under grazing or in dry ground; b. More diffuse habit when protected from grazing and with a good water-supply (from Tansley after Yapp, 1917).
FIG. 7.—Part of a branch of asparagus, showing cladodes and scale-leaves (s).
Quite apart from these permanent alterations in leaf-form, the shape and size of the leaves of many common plants vary greatly with the conditions under which they grow. For example, the first leaves of the red-fruited dandelion (Taraxacum laevigatum), when growing in a moist hollow among sand-dunes, are often quite entire (i.e. with smooth edges); later in the season, when the sand has become dry, it produces