132 TEXTBOOK OF PLANT PHYSIOLOGY 



people assume on the basis of such meager remains as are revealed 

 when a plant is dug out in the usual way. The greater part of the 

 most active fine rootlets are usually torn off in the digging. Rot- 

 mistroff , in Russia, and Weaver, in America, were the first to give a 

 correct idea of the real extent of the root system, when all of its 

 small branches remain intact. These authors have shown that 

 even in cereals, as oats and wheat, the root system is not confined 

 to the tilled layer. It penetrates into the soil to a depth of 1.5 to 2 

 meters and spreads extensively in all directions (Fig. 50). The 

 roots of alfalfa and other plants of dry habitats penetrate consider- 

 ably deeper. Even in small plants, the total length of the roots 

 with all their branches measures several hundred meters. In 

 larger plants this length has to be measured by kilometers. The 

 absorbing surface of the roots is increased several times by the 

 development of root hairs. Such an immense system enables the 

 plant to utilize perfectly even a scantily distributed soil moisture 

 and equally meagerly distributed mineral substances. 



The enormous amount of water that is utilized by plants must 

 not only be absorbed from the soil but it has to be conveyed to the 

 evaporating leaves. Sometimes, especially in trees, the water cur- 

 rent has to ascend to a height of several meters. The water dis- 

 tribution of the plant consists, therefore, of three principal proc- 

 esses : the absorption of water from the soil, its translocation to the 

 place of consumption, and its loss in the process of transpiration. 

 Each of these three processes will be discussed separately. 



42. Laws Governing the Entry of Water into the Cell. ' Suc- 

 tion Tension and Methods of Its Determination. — The absorption 

 of water by the cell from the surrounding medium is determined 

 by the osmotic properties of the cell. If a plant cell is immersed 

 in water, first the colloids of the cell wall and then the protoplasm 

 become saturated with water. The protoplasmic sac containing 

 the cell sap checks the passage of the substances dissolved in water, 

 but allows the latter to pass through. Therefore the cell sap 

 absorbs water (see Art. 36), and, increasing in volume, exerts a 

 pressure on the protoplasm, and through the latter on the cell wall. 



If the cell walls were of unlimited extensibility, this absorption 

 of water by the cell would continue until the concentrations of 

 the inner and outer solutions became equal. Distending under 

 the influence of the entering water, the cell wall exerts on the 

 cell contents an elastic counterpressure, opposite in its direction to 



