Absorption of Water by the Plant Body

ABSORPTION OF WATER BY THE PLANT BODY Water plants are surrounded by a very dilute solution of salts, as in the case of fresh water or a somewhat stronger solution in the case of sea water. Being surrounded by a watery medium the loss of water, and accordingly the need for absorption of water, does not play any very large part in their economy. The condi tions are very different with land plants which, in the case of higher plants, often expose very large surfaces to the air and consequently lose great quantities of water in the form of vapour, which passes through the pores in their leaves in the process of transpiration (see below). Such land plants must therefore take up larger quantities of water from the soil and the rate at which they can take up water will depend upon the forces with which the water is held in the soil for it is these forces which the suction pressure of the cells in contact with the soil have to overcome.

Soil Conditions.

The soil is a very complex material con sisting of (a) mineral particles of various sizes and varied chem ical composition, (b) of organic matter usually in the form of a dark brown material known as humus which is derived for the most part from the decomposition of remains of dead plants which formerly grew upon the soil, (c) water, which is held by colloidal adsorption in films on the mineral and humus par ticles and also, to a certain extent, within spaces of the soil, (d) air which is found in the spaces between the soil particles. The air can, of course, be displaced by water, as in the case of a water-logged soil where all the pore spaces of the soil are filled with water. Soils differ very much in what is called their me chanical composition, i.e., in the relative amount of particles of large and small size. A sandy soil has a large proportion of large particles, a clay soil has a large proportion of small particles, and to this is due their great difference in behaviour both in relation to water-retention and their response to mechanical treatment (e.g., ploughing) and to fertilizers. The relation of size of soil particles to the water holding power of the soil is particularly important in reference to the physiology of the water absorption of the plant from the soil, and attempts have been made to esti mate the relative forces with which the water is held by different soil constituents. Briggs and McLane working in America deter mined the amount of water held back by various soils when they were whirled round with enormous rapidity in a centrifuge; the centrifugal force resulting from the rapid rotation at first causes water to be thrown off but it is finally balanced by the water retaining forces of the soil particles. By comparing the behaviour of a number of different soils of different compositions they deduced that if the water holding capacity (moisture equiva lent) of the particles of size 2.0 to 0-05 mm. diam. (i.e., coarse and

fine sand) be taken as unity, the corresponding equivalent for the particles of below 0.05 to 0.005 mm. diam. (silt) is 12, while that for the particles of less than 0.005 mm. (i.e., clay) is 57. The silt is partly colloidal, and the clay highly colloidal, and the high water-holding power is due to the colloidal properties. The humus also is of colloidal nature and of marked water-re taining power and its capacity in that respect may be taken as about the same as that of the colloidal clay. The forces with which the water is held in different soils and in the same soil with different amounts of water has been investigated by Shull by what may be termed a biological method. It is well known that dry seeds will take up water actively and often with considerable force, the amount they can take up depends upon the force with which the water is held in the medium from which it is being taken. The method consists then in placing dry seeds in different soils with different content and determining the amount of water taken up after a given period, say 48 hours. Similar seeds are then placed at the same temperature, (a) in concentrated solutions of lithium chloride and sodium chloride of high osmotic pressure, or (b) over sulphuric acid of different concentrations. If the seed takes up in the same time the same amount of water from a given soil as from the salt solution or from the moist air over sulphuric acid the water retaining power of the materials in which the seed is placed should be the same. Now the osmotic pressures of the solutions—which represent the forces against which the water absorbing forces of the seeds are acting—are known and so the water holding power of the soil can be expressed in atmospheres. The results show that the water absorbing power of the seeds is very high, for they will take water from a saturated solution of lithium chloride which has an osmotic pressure of nearly a thou sand atmospheres. They show also, as was to be expected, that in an ordinary loam soil the forces with which the water is held go up markedly as the water becomes reduced in amount. In a silty loam investigated the forces holding water were negligible when there was 20% of water in the soil, but when it was reduced to 15% the forces were several hundred atmospheres and when reduced to about i o% of the order of 1,000 atmospheres pres sure. With sand the condition is very different for the material is non-colloidal and the water is only loosely held ; only when the water content is reduced to a very small amount does the pressure become considerable.