Chapter IX 



— 165 — 



Uptake and Movement 



have reviewed the results; Molz (1926) has made many determinations. 

 The method has been described in Chapter VII. 



Ernest (1931, 1934&) and Oppenheimer (1930a) have severely criti- 

 cised the plasmolytic methods for determining DPD. Some of these criti- 

 cisms are given in Chapter VII. Though many of them may be vaUd, the 

 method has provided an approximate picture of the water balance in cells 

 and tissues that is extremely useful in explaining the behavior of plants. 



Newer Methods of Estimating Tensions in Xylem: — Arcichov- 

 SKij and associates (1931), in a series of five papers, discuss the results of 

 investigations on suction pressure measurements in plants. Six methods 

 are considered, i.e., gravimetric, potometric, "Schlieren," optical, mechan- 

 ical, and electrical methods. Fundamentally all of these methods are based 

 on determining the concentration of a solution which has a DPD equal to 

 that of the test material. 



! 



^ /9 



Fig. 44. — Diurnal fluctuation in transpiration and in DPD at diflferent heights 

 on the stem of Arthrophytum haloxylon. The heavy dashes represent transpiration, 

 the heavy lines DPD, the Hght dashes relative humidity and the light line tempera- 

 ture. From Arcichovskij and Ossipov (1931&). 



The "Schlieren" method appears to be the most useful. When a twig 

 with a ring of exposed xylem is immersed in a sugar solution, water will 

 move into or out of the cells, depending on whether the immersion solution 

 has a lower or higher DPD than that of the tissue. This water movement 

 will be apparent in the solution due to the differences in the refractive in- 

 dices. If the solution has a lower DPD water will move into the stem and 

 will be indicated by concentration currents falling away from the twig. Con- 

 versely, water moving out of the twig into a solution of higher DPD will be 

 detected by the rising of the diluted solution. If the experimental twig is 

 horizontal and the test area is in a glass chamber containing a sugar solu- 

 tion, the streaking of the density currents in the solution can be observed 

 and a solution having an equivalent DPD determined. 



Certain objections to this method immediately come to mind. Such 

 an exposure of xylem caused by removing the bark must expose much rup- 

 tured phloem tissue, and phloem exudation into the test solution would set 



