Crafts et al. —106— Water in Plants 



of the cryoscopic or plasmolytic methods fail to reveal the actual capacity 

 of the cells to absorb water. 



The effect of environment on DPD is therefore more pronounced than 

 on OP. The important external factors are soil moisture, atmospheric 

 humidity, and light. Li (1929), using leaves of Syringa ohlata, concluded 

 from statistical studies that from 6: 30 A.M. to 10 A.M. light and relative 

 humidity exert an important effect, while from 10 A.M. to 2 P.M. light 

 becomes insignificant. Temperature was considered relatively unimportant 

 in its effect on DPD. 



Although this statement is true for purely liquid systems where DPD 

 varies as absolute temperature in systems involving the atmosphere, tem- 

 perature, through its relation to the degree of saturation, may have a pro- 

 found effect upon transpiration and hence upon the DPD of the tissues. 

 For further discussion of this point see page 190 and Figure 52, Chapter 

 X. Table 28 indicates the effect on DPD of varying amounts of moist 

 soil. 



Table 28. — Effect of varying soil moisture supply upon the DPD of plant tissues 



(MoLZ, 1926): — 



Wide diurnal fluctuations in DPD values of plant cells have been re- 

 ported as might be expected. The maximum is reached about noon or in 

 the early afternoon (Molz, 1926; Li, 1929; Herrick, 1933) under most 

 conditions. 



Table 29. — Osmotic quantities in the hypocotyl of Helianthus amiuus seedlings, 

 determined at normal volume (data of Beck and Andrus, 1943) : — 



Meyer and Wallace (1941) found DPD values of approximately 8 

 atm. in potato tuber tissue. This closely approached the OP of the ex- 

 pressed sap measured cryoscopically, indicating a low order of turgor. 

 Lyon (1942) obtained values of roughly the same magnitude but found 

 considerably higher OP values, using the minimum cell volume (simplified) 

 method. 



Gradients of DPD are often of interest to the physiologist in studies 

 of translocation and other functions. Beck and Andrus (1943) measured 



