Crafts et al. 



166 — 



Water in Plants 



up complicating density currents. Furthermore, such a method is depend- 

 ent on a differentially permeable membrane and the walls of dead xylem 

 elements are not differentially permeable although it is possible that a care- 

 ful removal of bark would leave a layer of living xylem cells which could 

 act like such a membrane. 



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Fig. 45. — DPD measurements on the petioles of squash plants. Solu- 

 tions injected into the petioles tend to assume a concentration having the 

 same DPD as the tissue. DPD values in atmospheres are shown on the 

 vertical axis. (From Stocking, 1945). 



Arcichovskij and Ossipov (1931&) used this method to determine the 

 suction pressure of the Central Asia desert shrub Arthrophytum haloxylon 

 Litiv. DPD's up to 142.9 atmospheres were recorded for this plant and a 

 DPD gradient of as much as 44 atmospheres per meter was observed. The 

 specific gravity of sap flowing from cut twigs was very low in relation to 

 that of the external sugar solution, indicating that the moisture deficit of 

 the tissue was due to high tension rather than osmotic pressure. Figure 

 43 shows the increasing gradient in DPD from the roots up the stem of such 

 a plant as measured by Arcichovskij and Ossipov (1931Z?). Figure 44 

 shows the diurnal fluctuation in transpiration and DPD of Arthrophytum 

 with data on temperature and relative humidity. 



Plants with hollow petioles such as those of squash or pumpkin are con- 

 venient material to use in testing the DPD of the tissue (Stocking, 1945). 



