Читать книгу Flowers of the Coast - Ian Hepburn - Страница 15

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.