Grafts et al. — 108— Water in Plants 



tion TP = OP — DPD ; and at water saturation, the cryoscopic value is 

 a measure of the degree of turgor. In both of these instances, the accuracy 

 of the measurement depends on the extent to which expressed sap is rep- 

 resentative of vacuolar contents. 



Bennet-Clark and Bexon (1940) investigated turgor pressures of 

 leaves by determining the amount of added pressure necessary to express 

 juice from the cells. In this method a round piece of leaf is placed hori- 

 zontally between two solid brass cylinders. Weights are added to the upper 

 cylinder until liquid is first exuded by the cells into the intercellular spaces. 

 A horizontally placed microscope is employed to detect this point. The 

 pressure imposed was believed to be equal to the turgor pressure (and 

 hence equal also to OP) of those cells possessing the lowest osmotic pres- 

 sure. For this reason it was termed the "minimum hydrostatic pressure" 

 of the fully turgid leaf. 



The method operates on the assumption that cell sap exudes only when 

 the externally imposed pressure equals the osmotic pressure of the vacuole. 

 However, this can be true only when the cells are initially in a turgorless 

 condition; otherwise the observed values would be low. It must also be 

 assumed that the elastic stretching of the wall is equal throughout the tissue. 

 The authors state that "... application of a pressure lower than the hydro- 

 static pressure of the contents of the fully turgid tissue expresses no juice." 

 It would seem that any amount of pressure, when applied to fully turgid 

 cells, would express some liquid, although admittedly more from cells hav- 

 ing low osmotic pressures. Therefore, the method of Bennet-Clark and 

 Bexon can hardly be considered critical for determining turgor pressure. 

 Furthermore, the technical difficulties involved would seem almost insur- 

 mountable. 



The term plasmoptysis designates the bursting of cells due to the in- 

 ability of the cell wall to resist turgor pressure. The phenomenon has 

 been observed on immersing cells in water (pollen grains, ripe pulp cells of 

 watermelon fruit, certain marine algae). Some solvents such as alcohol 

 may cause bursting due to rapid entry into the cells. In other instances, 

 swelling of the protoplasmic and cell wall colloids may be the cause. This 

 has been demonstrated in barley root hairs by immersion in a weak acetic 

 acid solution (Strugger, 1935). 



Intercellular Movement of Water: — It has been clearly pointed out 

 (Meyer, 1938) that the DPD gradient existing between two cells is the 

 cause of water movement from one to the other; the osmotic pressure 

 difference may not enter the problem. Under certain conditions it is pos- 

 sible for water to move from a cell with a relatively high to one with a 

 relatively low osmotic pressure due to the existence in the first cell of a 

 higher turgor than in the second. The diagram of Figure 35 illustrates 

 an example. 



Ursprung and Blum (19165') pointed out that the concentration of 

 osmotically active solutes in a cell is not necessarily the determiner of water 

 movement, for if the cell is saturated with water it has no further ability 

 to absorb ; water, on the other hand, may pass through such cells hindered 

 only by the resistance to movement through the membranes involved. Ab- 

 sorption of water by roots undoubtedly involves movement across tissues 

 that vary in osmotic concentration, and finally, the water is delivered into 

 xylem vessels where the concentration of solutes, though higher than in the 

 soil, may be much lower than in the cortical cells across which the water has 



