Grafts et al. — 1 10 — Water in Plants 



find the freezing point of living cells. From the freezing point lowering the osmotic 

 pressure can be calculated by a simple formula. 



Errors in cryoscopy involve supercooling and absorption of heat by the apparatus. 

 Methods are available to correct both of these; by a calibration technic they may be 

 avoided. 



A vapor pressure method has been devised to determine the osmotic pressure of 

 small samples of sap. Another sensitive method involves the temperature low^ering 

 due to evaporation. 



A simplified tissue method involves measuring tissue strips in series of solutions 

 of graded concentrations and calculating to the original volume. Another method 

 involves balancing root pressure against an external solution of known concentration. 

 This method has led to detection of a discrepancy postulated to result from active 

 water uptake by root cells. 



Most plants have osmotic concentrations in the range of 5 to 30 atmospheres. An 

 extreme value of 202.5 atm. has been recorded. The distribution of osmotic pressure 

 values in the leaf assumes the following pattern: palisade parenchyma > spongy 

 parenchyma > upper epidermis > lower epidermis. Xylem sap is very dilute. Phloem 

 exudate has a higher concentration. 



Diffusion pressure deficits of cells and tissues may be determined by surrounding 

 them with solutions which cause no change in volume or in weight. Another method 

 is to inject a solution into the hollov^f petiole or stem and follow changes in concentra- 

 tion. Variations in DPD resulting from diurnal fluctuation in transpiration may be 

 followed by this method. Several other methods have been described. 



Changes in DPD values are greater than in OP values. They result from varia- 

 tions in soil moisture, humidity, and light. Cell walls of some xerophytes show little 

 elasticity, hence DPD values fluctuate sharply and water is conserved. 



Turgor plays an important role in growth and development of plants. Quality of 

 vegetables depends on turgor. Growth hormones may act through turgor expansion. 



Direct measurement of turgor is difficult. Values from 3 to 10 atm. have been 

 recorded. Intercellular water movement follows DPD gradients; it may be inde- 

 pendent of OP. 



