Absorption of Water by the Plant Body

Dynamic Relations of Water Absorption.

The observa tions which have been described above only gives us a picture of static equilibrium between the plant and the soil. They show what the conditions are when there is a balance between the forces pulling water into the seed (or root of the plant) and the forces holding the water in the soil, that is they show us the condition when water is no longer taken up. This, however, though helpful, as giving some measure of the forces which the root has to over come, gives us a very imperfect picture of the relationship to the plant. The plant is continually giving off water in the process of transpiration and unless the rate of absorption keeps pace with the rate of loss of water the water content of the plant will begin to fall. The plant will flag when this difference in the two rates has existed for a short period and the plant will be dead long before the rate of absorption has fallen to zero. The equilibrium between absorption on the one hand and loss of water by trans piration on the other is thus, like the equilibrium in the living cell, a dynamic not a static one.

The term transpiration is applied to the process of loss of water vapour from the aerial part of a land plant ; it is of the nature of controlled evaporation. The transpiratory losses take place mainly from the leaf surface since these organs expose a large surface for evaporation and are supplied on the epidermal surface with pores known botanically as stomata. (See above, Anatomy.) These pores interrupt the otherwise continuous layer of cuticle which covers the surface. In its absence the loss of water from the leaf surface would be very large and uncontrollable. The cuticle is, however, not completely waterproof so that we can distinguish cuticular transpiration and stomatal transpiration. The second is very much the larger, being usually 80-97% of the whole, though under special conditions, as in a tropical rain forest, the cuticular transpiration may be as high as the stomatal.

Measurement of Transpiration.

That transpiration occurs can be shown by the loss of weight of a potted plant, when the pot and surface of the soil are protected from water-loss by rubber sheeting; this gives a convenient method for measuring the rate of transpiration. It is not applicable to plants growing in the ground; for such plants paper dipped in a cobalt chloride solution, dried and applied to the leaf may be employed. On the absorp tion of the water vapour coming from the leaf the colour of the cobalt chloride paper changes from pink to blue, and the rate of this change gives some measure of the rate of transpiration.

Size and Arrangement of Stomata.

The stomata are more commonly confined to the lower surface of the leaf—the cuticle on the upper surface being then continuous, though leaves with stomata on both sides frequently occur. The stomata are very

small pores, those of the sunflower leaf being about of a millimeter (i.e., about of an in.) in diameter. They are very numerous, varying from 4o to 30o per square mm. ; a sun flower leaf may thus bear 13,000,000 of them. The pore is bounded by two special cells known as guard cells (as described in the anatomy of plants) and open below into a chamber which is bounded by the walls of the green cells (mesophyll) of the leaf. These cells are full of water and from their wet cell walls water naturally evaporates into the space, and by the physical process of gaseous diffusion passes out of the stomatal pore into the drier air outside. As the stomata occupy a very small propor tion of the total surface it might be supposed that the rate of passage of a gas or vapour out or in would be very slow. It was shown, however, by Brown and Escombe in 1900 that such very small pores allow of a much greater rate of diffusion than might be expected. It is found that provided the pores are not too close to one another the rate of diffusion is proportional to their diam eter and not to their area.

Effect of Various Conditions on the Rate of Transpiration.— Transpiration like most physiological processes is markedly af fected by external conditions. Since the process is essentially one of evaporation, one of the most important factors (the term factor is applied to a particular condition affecting the rate of a process) is the humidity or relative dryness of the air to which the plant is exposed. The drier the air the more rapid, on purely physical grounds, will be the rate of diffusion of the water vapour through the stomatal pore. This is due to the big difference of the water content (humidity) of the air below and above the pore. The proper measure of the evaporating power of the air is not the actual relative humidity (or percentage saturation of the air) but the saturation deficit and this is markedly affected by temperature. Air 70% saturated at 10° C has only about half the saturation deficit of air 70% saturated at 20° C, and so its evaporating power is only half. Temperature itself has very little effect on transpiration or evaporation except in so far as it affects the evaporating power of the air. Atmospheric pressure has only a slight effect since evaporation tends to go up with a fall in baro metric pressure but it is so small that it can under ordinary con ditions be neglected. Wind is a very important factor in trans piration. It is well known that plants suffer very much in windy situations and "wind-breaks" are often put up for plant protec tion. The ill effect is mainly due to the enhanced rate of transpira tion, which is the direct physical effect of the removal by the wind of the layers of air in contact with the leaf.