briefly discussed. As far as man is concerned, his activities are less in evidence along the coast than in most parts of the country, since coastal areas do not lend themselves well to agricultural development. Nevertheless, in a thickly populated area like ours, there is no region where the hand of man has not played some part in modifying the vegetation. For instance, large areas of many salt-marshes are used for the grazing of cattle, which has the effect of restricting some plants but not others. Many old salt-marsh areas, too, have been completely transformed by drainage operations or the construction of sea-walls to exclude the tides, and the laying out of golf-courses has altered the vegetation in sand-dune areas in a number of places. Moreover, in certain districts marram-grass has actually been planted to stabilise shifting sand-dunes, and elsewhere rice-grass has been employed in a similar way for reclaiming salt-marshes, so that it is often impossible to distinguish between natural and partly artificial vegetation. Nor should it be forgotten that the large-scale felling of the native forests all over the country in the past has had the indirect effect of preventing the natural development of the climax vegetation in many suitably undisturbed areas along the coast.
Rabbits are frequently responsible for considerable modification of the vegetation, and are often extremely common in coastal areas. In particular, the grassland on the tops of cliffs is often infested with them, and the older sand-dunes provide a veritable rabbit’s paradise. In all probability, the somewhat stunted vegetation which is so characteristic of such areas results as much from its being continually nibbled by rabbits as from its exposure to strong winds. Some plants, however, are more attractive to rabbits than others, so the actual composition of the vegetation may be considerably altered. Even salt-marshes are not exempt from the attentions of rabbits; in some districts, for instance, it is unusual to see more than a quite small proportion of the sea-aster plants reaching the flowering stage. Birds also sometimes have a marked effect on the vegetation, particularly when large colonies gather on small islands for breeding purposes. Needless to say much excreta is deposited on the cliff-ledges and cliff-tops near their nesting sites, and the increase in the amount of nitrogen and phosphates in the soil produced in this way has the effect of altering the composition of the vegetation considerably.
The above brief summary can do no more than suggest the kind of factors which must be looked for if we are to make any attempt to understand why coastal vegetation is distributed as it is, and why particular species occur in some places and not in others. Our knowledge of these matters is still extremely incomplete, and it is well to realise that much valuable information can still be easily collected by amateur botanists who are prepared to make a fairly detailed survey of the vegetation in a particular habitat and to keep their eyes open for the factors which have been responsible for its composition.
CHAPTER 4 FORM AND HABIT OF COASTAL PLANTS
THE MAJORITY of the plants we find growing round the coast have to contend with unusually harsh conditions, and many of them are specially adapted to enable them to survive in their inhospitable habitats. In this chapter we shall consider some of the characteristic growth-forms they adopt.
Undoubtedly the main problem for most of these plants is to obtain adequate supplies of water, particularly in the early stages of their growth. This applies both to those growing in such obviously dry habitats as sand-dunes, shingle beaches or rocky cliffs, and to those growing in saline ground, such as salt-marshes or brackish swamps, although the reason for the difficulty is quite different in the two cases. The whole question of water-supply is sufficiently fundamental to merit discussion in some detail.
To deal first with the dry habitats; the whole trouble here is that they do not retain sufficient quantities of water in their surface layers, since the “soil” they provide is largely made up of coarse particles. The water-holding power of a soil depends in the first instance on the size of its particles. If these are large, water can percolate easily through them, and will also evaporate more quickly because of the large air-spaces between them. Thus the greater the number of small particles, the longer the soil will take to become dry after rain. Furthermore, it is a well-known fact that water tends to stick on to the outside of all relatively small particles on account of the force known as “surface-tension,” and since the total surface-area of a given weight of small particles is clearly greater than that of the same weight of coarse particles, the finer the soil the greater its powers of retaining water. But in addition to a lack of small particles, there is usually a shortage of humus in all the habitats in question. This important material, consisting of dead plant-remains in the process of decay, has already been briefly referred to (see here). Without discussing the varied forms in which this organic matter can occur, the amount present in a soil can be roughly guessed from its colour. Thus dark-coloured “peaty” soils contain the greatest amount and sands the least. All farmers are familiar with the fact that adequate quantities of this material are necessary in all “light” (i.e. coarse) soils, if they are not to suffer from frequent drought conditions. Humus possesses great powers of absorbing water, chiefly because much of it is usually in the form of very small particles of what are called “colloidal” size (i.e. they are so small that they easily pass through a filter-paper, and take a long time to settle when they are suspended in water). Quite apart from this, it is a valuable source of plant food, partly on account of the nitrogen it contains, but principally because it absorbs valuable salts and prevents them from being washed away.
The plants growing in saline habitats also have trouble with their water-supply, though of a very different kind. Here there is often an abundance of water, but it is, of course, salt water. As a result, the plants may suffer from what has been called a “physiological drought.” This means that, despite an abundance of water in the soil, they are unable to make use of it on account of the high concentration of salt it contains. Many salt-marsh plants, therefore, live under conditions of partial drought rather similar to those encountered in other coastal habitats. As evidence of this, it can often be noticed that they are greatly benefited by the dilution of their soil-water, when a spell of wet weather occurs in the summer. Indeed many, though not all, halophytes can grow luxuriantly in ordinary garden soil.
Another characteristic of coastal habitats is that they are all to a greater or lesser extent exposed to strong winds. The most important result of this, as has already been pointed out, is to increase the rate of evaporation of water at the leaves (transpiration). This causes plants to draw further on their slender water-supplies, and if these are inadequate, wilting may take place. Thus wind aggravates the results of the water-shortage.
In order to understand the various ways in which maritime plants deal with this fundamental problem of water-supply, it is necessary to say a word about two processes, common to all plants, which are specially important in this connection. These are osmosis and transpiration.
OSMOSIS
All plants obtain the water and soluble salts required for their growth through the agency of cells situated near the ends of their roots, which are known as the root-hairs. These cells are filled with sap, which contains small quantities of soluble salts and much larger amounts of soluble organic substances, such as sugars, in solution.
FIG. 2.—Experiment to demonstrate osmosis.
When the root-hairs are in close contact with the soil-water, a suction pressure is developed through the walls of these cells, called the osmotic pressure. As a result of this, water passes into the cell and temporarily dilutes the sap. This is a familiar chemical phenomenon and can easily be demonstrated in a number of ways. If any solution is enclosed in what is called a “semipermeable membrane” (i.e. one which will allow water, but not dissolved substances, to pass freely through it), the osmotic pressure of the solution will cause water to be sucked into the solution through the membrane.
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