Grafts et al. — 162 — Water in Plants 



peared to be instantaneous. Working with plants having lower deficits and 

 observing xylem elements of different sizes, it was possible to produce 

 bubbles that expanded rapidly or slowly and, using plants with little or no 

 water deficit, bubbles could be formed that gradually contracted and disap- 

 peared. These experiments parallel those of Bode and illustrate the role 

 played by capillarity in the healing of broken water columns under condi- 

 tions of reduced stress in the xylem. The simple fact that vapor bubbles 

 may be formed in uncut xylem vessels and that these continue to expand 

 after being formed is proof that tension exists, for the vapor phase would 

 not continue to expand if the pressure were above that of saturated water 

 vapor at the temperature of the experiment, that is around 17.5 mm. of mer- 

 cury. The extremely rapid expansion observed in vessels of plants that 

 are permanently wilted proves that the tension is high for in so expanding 

 the water originally present, except for the small amount involved in forma- 

 tion of vapor, must move out at high rates, overcoming inertia and the resist- 

 ance to flow through capillaries. 



Tensions Developed in the Xylem; Direct Measurements: — 



Difficulties in technique have thus far prevented the direct measurement 

 of tensions that occur in the xylem sap. Use of manometers attached directly 

 to cut branches, stumps, and stems entails severing of the conducting tracts. 

 Tensions greater than one atmosphere cannot be developed within a sys- 

 tem containing gas for the gas will expand indefinitely under subatmos- 

 pheric pressure. The attachment of manometers to bulk tissue rather than 

 to individual vessels permits the drawing of air into the system through 

 the intercellular spaces with the result that only pressures above the zero 

 atmosphere level can be measured. 



BoEHM (1892) and Thut (1932a) were able to demonstrate that if 

 branches were carefully prepared a rise of mercury above the 76 cm. level 

 could be obtained. The use of conifers that possess tightly packed tissue, 

 few intercellular spaces, and short elements of the tracheid type tends to 

 prevent the occurrence of gas movement out of the wood and into the 

 manometer. Boehm was able to obtain a height of 90.6 cm. and Thut 

 recorded a maximum rise of mercury of 101.8 cm. in one experiment with 

 Juniperus. To obtain such a rise the base of the stem was boiled to re- 

 move air. Such experiments, while clearly demonstrating the ability of 

 the leafy shoot to lift mercury above the barometric level, do not pretend 

 to measure the actual tensions that exist within the conducting tracts of 

 intact plants. 



Calculations by Dixon (1914) on the resistance to flow through xylem 

 conductors indicate that a force at least twice that represented by the 

 weight of the transpiration columns would be required to move water through 

 stems at a normal rate of flow. If this approximation is valid there should 

 exist tensions of 20 to 24 atmospheres or more in the uppermost parts of 

 tall trees during periods of rapid transpiration. 



Potometer Experiments: — Renner (1912) used the potometer to 

 study tensions in the xylem of a plant. Uninjured branches, tops of shoots, 

 or twigs were installed in simple potometers. Freshly cut surfaces provided 

 for water uptake, and the resistance of the stem was increased by the use of 

 a clamp, or by double notching. After the rate of water uptake was ob- 

 served the branch above the potometer was cut. The stump still in the 

 potometer was taken into the laboratory and the rate of water uptake under 



