Absorption

To understand the methods for reducing transpiration, as proposed in this chapter, it is necessary to review briefly the manner in which plants take water from the soil.  The roots are the organs of water absorption.  Practically no water is taken into the plants by the stems or leaves, even under conditions of heavy rainfall.  Such small quantities as may enter the plant through the stems and leaves are of very little value in furthering the life and growth of the plant.  The roots alone are of real consequence in water absorption.  All parts of the roots do not possess equal power of taking up soil-water.  In the process of water absorption the younger roots are most active and effective.  Even of the young roots, however, only certain parts are actively engaged in water absorption.  At the very tips of the young growing roots are numerous fine hairs.  These root-hairs, which cluster about the growing point of the young roots, are the organs of the plant that absorb soil-water.  They are of value only for limited periods of time, for as they grow older, they lose their power of water absorption.  In fact, they are active only when they are in actual process of growth.  It follows, therefore, that water absorption occurs near the tips of the growing roots, and whenever a plant ceases to grow the water absorption ceases also.  The root-hairs are filled with a dilute solution of various substances, as yet poorly understood, which plays an important tent part in the ab sorption of water and plant-food from the soil.

Owing to their minuteness, the root-hairs are in most cases immersed in the water film that surrounds the soil particles, and the soil-water is taken directly into the roots from the soil-water film by the process known as osmosis.  The explanation of this inward movement is complicated and need not be discussed here.  It is sufficient to say that the concentration or strength of the solution within the root-hair is of different degree from the soil-water solution.  The water tends, therefore, to move from the soil into the root, in order to make the solutions inside and outside of the root of the same concentration.  If it should ever occur that the soil-water and the water within the root-hair became the same concentration, that is to say, contained the same substances in the same proportional amounts, there would be no further inward movement of water.  Moreover, if it should happen that the soil-water is stronger than the water within the root-hair, the water would tend to pass from the plant into the soil.  This is the condition that prevails in many alkali lands of the West, and is the cause of the death of plants growing on such lands.