Roots. 37 



are not being withdrawn from solution within the plant, or 

 are not being used by the plant in the manufacture of new 

 compounds. The more a substance is being employed or 

 transformed by a plant, so that its concentration is continu- 

 ally diminished, the more it will enter from without ; in 

 this way the supply is adjusted to the demand. 



If, on the other hand, a substance which is able to pass 

 the plasma membrane is not being transformed by the 

 plant, it cannot continue to enter after the concentration 

 of its molecules and ions within the plant is equal to that 

 in the soil water; in this way useless materials are kept 

 from accumulating. This is true of the substances in solu- 

 tion, but the solvent, which in this case is water, passes 

 most rapidly from the region of less to that of greater 

 concentration (see Experiment 45). The passage of water 

 through membranes from regions of lower to those of 

 higher concentration is known as osmosis. The cell sap 

 of the root hairs is of greater concentration than the soil 

 water; and since this condition is maintained by evapo- 

 ration from the leaves and other above-ground parts, and 

 by the employment of some of the water in the manufac- 

 ture of plant food, the water continues to enter the plant 

 from the soil. If the water is abundant, it may enter the 

 plant even faster than it is evaporated or used, in which 

 case the plant cells become stretched and turgid. In this 

 way rigidity is given to herbaceous stems and leaves. But 

 when the water in the soil runs low, evaporation may be in 

 excess of its movement into the plant, and wilting results. 



24. Importance of Water. Water, which is taken by 

 land plants almost exclusively from the soil, is the solvent 

 and vehicle of transport for all substances which enter the 

 plant, and for those compounds as well which are manu- 

 factured within the plant ; it contributes its own substance 



