THE ABSORPTION OF WATER BY ROOT SYSTEMS OF PLANTS

Abstract

Botanists are generally agreed upon the manner in which water moves after it has entered the conducting vessels of the plant, but they are by no means agreed upon the manner in whicl it passes from the soil to the conducting vessels of the roots. Water may move by mass flow for hundreds of feet through the xylem vessels or tracheids, drawn in a state of tension by the removal of water from the xylem vessels of the leaves during transpiration. However, in passing from the soil to the xylem vessels of the it must cross a mass of living cells, a fact which introduces problems not encountered in its movement through the cavities of the dead vessels or tracheids of the xylem. In approaching this problem it should be remembered that there usually are important differences between the conditions found in the xylem vessels of actively transpiring plants and those found when little or no transpiration is occurring. In actively transpiring plants the water in the xylem vessels is usually under tension, i.e. exhibits negative When transpiration is slow and the soil contains an abundance of moisture, as in the spring of the year, the water in the conducting vessels often exhibits a positive resulting in bleeding from cut or broken stems. This positive is referred to in the literature by such terms as 'root pressure, 'sap pressure, and exudation pressure. In view of these facts it seems very probable that the complex of factors bringing about the absorption of water by rapidly transpiring plants may be quite different from that bringing about the absorption of water when little transpiration is occurring. From the earliest days of plant physiology, workers have taxed their ingenuity to explain the absorption of water and the development of root pressure in plants, but few of the early theories were based on actual experimental work. The earliest workers were unable to give any very definite explanations of the processes involved, though capillarity was supposed to play an important part in the absorption of water and solutes. In the igth century more definite theories began to appear. DeCandolle (I832) applied the term to the absorbing regions of the roots. He believed the spongioles were actually contractile and, aided by capillarity, sucked in water from the soil. Dutrochet (I837), who is generally credited