coastal habitats all over Britain and, as a result, the general type of vegetation found in each of them is strikingly similar wherever it occurs, although there are naturally many local differences of detail.
This similarity can best be appreciated if the botanist, when observing coastal plants, pays special attention to those growing together most frequently in a particular habitat, instead of merely searching for individual (and perhaps rare) species. If he does this, it will soon become apparent to him that many of them habitually occur in well-marked and easily recognised communities. Incidentally, the plant communities found along the coast are, for the most part, simpler in composition than those found inland. This makes them ideal material for study by anyone who is getting beyond the stage of just collecting and identifying plants (a very necessary stage for all field-botanists), and is beginning to take an interest in the vegetation as a whole.
Let it, however, be said at once, for the benefit of the pure plant-hunter, that the coastal regions do in fact provide happy hunting grounds for many relatively rare plants, largely because they have been so little disturbed. This is particularly true of sand-dune areas, which often carry a rich flora, although the rarities found there are usually not confined to the coastal belt. At the same time, I feel strongly that many naturalists would enjoy their botanising even more if they cultivated the habit of looking at the whole vegetation as a unit, noticing how the individual species of which it is composed are grouped, and trying to discover why particular plant communities grow where they do. After all, by doing this, one is not denied the excitement of coming across an unusual plant, and it has the great advantage that the common plants become once more interesting, since their relative frequency is of fundamental importance in determining the nature of the general vegetation. The relation of vegetation to its surroundings is known as plant ecology, and nowhere can a start at this way of botanising be better made than along the coast. In the hope that some of my readers may be stimulated to look at seaside plants in this broader way, the characteristic vegetation of the main coastal habitats is discussed from a general ecological standpoint in the later chapters of this book. With this end in view, I have devoted Chapter 3 to a brief explanation of some of the main principles of plant ecology.
It is important to remember that the coastline never remains completely static, but is continually in process of being either eroded or built up. The ways in which this can take place are discussed in some detail in the next chapter, but it is clear that both erosion and deposition can provide virgin ground, suitable for colonisation by plants. One of the most interesting features of coastal vegetation is that it is often possible to observe a whole series of successive stages whereby these bare habitats acquire a comparatively stable plant-cover, from the first tentative seedlings to such complex communities as are typical of heathland or scrub. This fascinating process in which one community is replaced by another, and this in turn by others, can be witnessed and understood far better along the coast than anywhere else. Unfortunately nowhere is it possible to observe the production under natural conditions of the final or “climax” stage (see page 29) in the development of the vegetation.
But perhaps the best reason of all for studying the flowers of the coast is that by so-doing one visits delightful and exciting places, always within sight and sound of the sea. For my own part, I frankly admit that this was why I became specially interested in seaside plants. Although I have botanised in all sorts of country, and in particular greatly enjoy doing so amongst mountains, I have never experienced anywhere the same variety and pleasure as I have had when looking for flowers along the coast.
A complete list of all the plants growing round the coast of the British Isles would be an extremely long one, but the great majority would be species found equally frequently in quite different habitats far away from the sea. In particular, sand-dunes, cliff-tops, and shingle that is no longer exposed to wave-action, can all support a wide variety of inland plants. It is, however, with those plants which are confined to the coastal belt that we are chiefly concerned in this book, and these fall into two main groups, only the first of which consists of genuine maritime species. This includes practically all the plants found in regularly submerged salt-marshes, and also a number of others occurring in habitats which are commonly exposed to a certain amount of sea-spray, such as beaches and cliffs. The plants belonging to this group are called halophytes (Greek halos=sea salt), and may be roughly described as plants which can live in soils where the water is salt. The main difference between these and normal plants is that their protoplasm (living material) is not destroyed by exposure, either externally or internally, to salt solutions.
One is tempted to divide this group further into “true halophytes,” chiefly confined to salt-marshes and thriving under conditions where the actual water-table remains permanently saline, and what may be called “spray halophytes,” characteristic of habitats exposed to salt spray, but not normally submerged by sea water. Annual seablite (Suaeda maritima) (Pl. 3) and common sea-lavender (Limonium vulgare) (Pl. 5) are examples of true halophytes, while spray halophytes are exemplified by samphire (Crithmum maritimum) (Pl. 10) or sea-rocket (Cakile maritima) (Pl. XVI). We hardly know enough yet about the requirements of many of these plants, however, to make such a distinction and it is therefore inadvisable to carry it far. Some true halophytes, for instance, appear actually to require the presence of salt in their root-water for their full development, but probably all spray halophytes and certainly some “true” ones (e.g. thrift) can grow equally well in ordinary garden soil, provided that competition from taller and faster-growing plants is excluded. Indeed the absence of competition from other plants, which are unable to endure the conditions, is the chief reason for the abundance of spray halophytes on exposed cliffs and similar habitats. Since the term “halophyte” is used to describe plants of such differing requirements, it is probably best to consider that the one essential feature they have in common is that they can tolerate a certain amount of salt water, rather than that they actually require its presence. Moreover, it should not be forgotten that sea-water contains small quantities of many other salts besides sodium chloride, and it is quite possible that these may also have important effects on the growth of plants, which we do not at present understand. The majority of halophytes are perennials and often develop extensive woody rootstocks. But their most obvious physical feature is usually their fleshy or succulent leaves, a characteristic which appears to be closely connected with the absorption of salt water, since it is also shown by a number of inland plants when they grow near the sea or inland salt areas.
The second group of plants confined to the coastal belt consists of those found only on sand-dunes. There is, of course, nothing inherently maritime about a sand-dune, but it so happens that in the British Isles the main dune areas are coastal. The largest areas of blown sand in the world are found in desert regions far from the sea, and in North America extensive “coastal” dunes exist round the Great Lakes, where the water is fresh. Even in this country considerable patches of blown sand, carrying some typical dune plants, can be seen in the Breckland. All plants capable of establishing themselves in mobile sand bear a general resemblance to each other wherever they occur, and the majority of them are known as xerophytes (Greek xeros=dry). Most of them possess unusually extensive roots or rhizomes (buried stems), and their leaves are often adapted in some way or other to cut down excessive loss of water from them. Many xerophytes have succulent leaves and stems, similar to those belonging to halophytes. This likeness, is, however, largely accidental and is due in this case to the development of water-holding cells, called collectively “aqueous tissue” (see page 47), for the purpose of storing water. Xerophytes are by no means confined to dune areas in Britain but may also be found in other dry habitats inland. Those confined to dunes, besides possessing an unusual ability to withstand drought, are also able to deal with the problem of occasional burial in loose sand by their exceptional powers of sending up new shoots as soon as they are covered over—marram-grass (Fig. 10) possessing this power to a quite