54 THE BIOLOGY OF FLOWERING PLANTS 



The osmotic pressure on which the suction force depends 

 is also related to the requirements of the individual and 

 varies with changing external conditions. It is, moreover, 

 specific in the sense that plants habitually growing in extreme 

 conditions have a characteristically high osmotic pressure. 



Soil and the Water Supply. — ^Ne may now consider the 

 way in which the forces which hold water in the soil are 

 related to absorption by the plant. It has already been 

 implied that the whole of the water in the soil is not equally 

 available for plant growth. The water which the plant can 

 readily absorb has been called growth water, available water, 

 or physiological water-content ; the water not easily available 

 has been called the non-available water, or expressed as the 

 wilting coefficient. In the terminology of Clements (1905), 

 the total water content of the soil is the holard, the water 

 available for plant growth is the chresard, and the 

 non-available water is the echard. 



The point at which the plant seriously fails to cover its 

 water requirements by absorption from the soil is indicated 

 by the onset oi permanent wilting, that is, wilting from which 

 the plant does not recover if placed in a saturated atm.osphere. 

 In the case of ordinary mesophytes, the flagging of the 

 leaves can be readily observed, though even with them the 

 determination of the wilting point is, to a certain extent 

 subjective. In the case of plants with stiff leathery leaves, 

 or with leaves of the needle or heath types, the determina- 

 tion of wilting by inspection is much less certain, or may 

 even be impossible. Such plants, if potted, may be 

 balanced along a steelyard with the plant on one side of 

 the fulcrum and the pot on the other. As long as the 

 plant draws sufficient water from the soil, the pot end of 

 the system rises ; when the plant end of the system rises 

 it is indicated that the plant no longer draws sufficient 

 water from the soil to cover its losses, and that a condition 

 equivalent to wilting has set it. This method has been 

 applied to the cactus among others. 



Wilting depends not on complete failure of the water 

 supply, but on failure to obtain a sufficiently rapid supply. 



