Grafts et al. 



— 164 



Water in Plants 



As previously mentioned Preston ( 1938) studied the rate at which dyes 

 were injected into cut ash stems. He calculated the tensions which must 

 have existed at zero time, to cause the rate of injection observed, to be in 

 two instances of the order of 3 atmospheres. This value represented very 

 approximately in his estimation the tension present in the vessels im- 

 mediately after cutting. It is questionable whether this can be taken to rep- 

 resent the tension in the vessel system before cutting since, as Preston 

 points out, the equations which he used are not valid at zero time. 



/s 



/6 



/4 



/2 



/O 



-/50 



-50 



^O 



/50 



£50 



J50 



Fig. 43. — DPD gradient from the root up the stem of 

 Arthrophytum haloxylon. The vertical axis is DPD gradient in 

 atmospheres per meter, the horizontal axis centimeters. The 

 zero point represents the crown and all negative values are 

 measured downward along the root. From Arcichovskij and 

 Ossipov (19316). 



"Suction Tension" Measurements : — If there is a direct water con- 

 tact between the liquid in the xylem and that of the neighboring cells, the 

 diffusion pressure of water in the cells should approach quite closely the 

 diffusion pressure of water in the nonliving xylem conductors. Calculation 

 of the tension in the xylem should be possible from a knowledge of the dif- 

 fusion pressure deficit of water in the living cells and the osmotic pressure 

 of the tracheal contents. 



A large amount of work has been done in measuring the DPD (suction 

 tensions) of plant cells and tissues. Ursprung (1935) and Beck (1928) 



