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.
A simple way in which this can be demonstrated is to tie a piece of pig’s bladder very firmly over the end of a small funnel (preferably a “thistle funnel”). If the funnel is now filled with a strong solution of (say) cane sugar and is immersed in a vessel containing water, the water will soon start to pass through the membrane into the sugar solution and rise up the stem of the funnel (Fig. 2). If a long piece of glass tubing is attached to the stem, it will be noticed, after a day or two, that a column of water several feet high has risen up the tube. It is important to realise that the height of this column is not a measure of the pressure exerted by the initial sugar solution, since this solution is naturally becoming steadily more dilute as the water enters it. In point of fact the osmotic pressure of a 15 per cent solution of sugar is about 10 atmospheres, or sufficient to support a column of water of well over 300 feet in height!
The walls of the root-hair cells function in exactly the same way as semipermeable membranes, though they allow considerable amounts of the substances dissolved in the root-water to pass through them also. Measurements of the osmotic pressures exerted by the cell-sap of many different plants have been carried out. These have been found to vary considerably, but a value of about 10 atmospheres for normal plants (mesophytes) can be taken as an average figure. As a result of this large suction pressure, it is obvious that a root-hair which is freely supplied with water will soon become distended with diluted sap and develop a corresponding balancing pressure. This is called the “turgor pressure.” An equilibrium between this and the osmotic pressure, resulting in the cessation of the flow of water into the cell, would soon be reached were it not for the fact that water is continually passing from the root-hairs into the root and stem of the plant. It is this movement which maintains a flow of water through the whole plant, the excess water being eliminated largely through the leaves. The process by which water is conducted through a plant is extremely complex, and there is nothing to be gained by attempting to discuss it here. The important thing to understand is that, provided water is in contact with the root-hairs, a steady flow into the root can be maintained. On the other hand, if insufficient water is available in the soil, the plant may not be able to obtain an adequate supply to build up its turgor pressure, with the result that the whole plant becomes limp and is said to “wilt.” Obviously this danger is greater if water is eliminated too rapidly by the leaves, and plants growing in dry habitats are often provided with devices to prevent excessive transpiration.
Measurements of the osmotic pressures exerted by both halophytes and xerophytes have been shown to be, in general, much larger than those characteristic of normal plants. We have mentioned 10 atmospheres as a typical value for mesophytes, whereas 40 atmospheres would appear to be an average value for plants in the former classes. Indeed, some desert plants have been shown to exert pressures running up to the enormous figure of 100 atmospheres and more. Obviously this greatly increased power of suction must be of much assistance to plants growing in arid soils in enabling them to obtain what little water there is. In the case of halophytes, a high osmotic pressure is virtually essential if they are to overcome the considerable pressure of the salt water in which they have to grow. Ordinary sea-water has an osmotic pressure of about 20 atmospheres, but in a salt-marsh the concentration of salt may become very much higher during a spell of dry weather in those areas which are not submerged by every tide. If halophytes were incapable of exerting a greater osmotic pressure than that of the salt water in a marsh, osmosis would take place in the wrong direction and water would be sucked out of the plant into the soil-water. Thus the plant would not only fail to obtain its water-supply, but would lose much of the water it already contained. This effect can easily be demonstrated by putting any normal plant into salt water, when it will be seen to wilt in a very short time.
A good deal of work has been done on the measurement of the osmotic pressures developed by halophytes when growing in salt solutions of varying concentrations. The results show clearly that they fluctuate considerably and are able to alter rapidly to adjust themselves to changes in the concentration of salt in the soil-water. This adaptability accounts for the wide tolerance shown by many halophytes growing in different parts of salt-marshes. The whole problem of the mechanism by which water is absorbed by plants is very complex, and the above account is much simplified in order to explain the main principles.
TRANSPIRATION
Passing now to the other end of the plant, we must say something about the process by which the surplus water is disposed of at the leaves. This is known as transpiration. On the surface of any leaf a number of minute pore-like openings are to be found which are called the “stomata.” Each stoma usually takes the form of a slit between two elongated cells known as “guard cells,” lying side by side (Fig. 3(c).) The opening or closing of the pore is controlled by the swelling or contraction of this pair of cells. Thus, when the turgor pressure of the plant is high, the cell-walls expand and the slit is opened to aid the elimination of water. When the water-supply is less abundant, the turgor pressure falls and the cells contract so that they lie with their walls in contact with each other, thus closing the slit. It should be emphasised that the stomata are not only concerned with the elimination of water-vapour but are also
