Grafts et al. 



170 — 



Water in Plants 



Table 44. In this early work the electroscope and photoplates were used 

 as analytical tools. Apparently analyses with the Geiger counter of the 

 movement of radioactive phosphorus in the phloem and xylem of an intact 

 plant are useful only when there are large concentrations of radioactive 

 material in a given portion of the plant (Colwell, 1942). 



Heat. — The ingenious thermoelectric method first used by Rein (1928, 

 1929) for determining the rate of flow of blood and developed by Huber 

 (1932) for measuring plant stream velocities, allows determinations to be 

 made on intact plants with a minimum of injury. Repeated and localized 

 measurements are possible on the same plant; this is a distinct advantage 

 over any method previously used. The method consists essentially of the 

 brief local application of moderate heat and of measurement by means of 

 thermocouples of the rate of transfer upward or downward. In rapidly mov- 

 ing streams the thermocouples are placed at a distance sufficient to prevent 

 interference by radiation or conduction. This is from 4 to 25 cm. from the 

 heating element. 



Fig. 47. — Arrangement of heating element and thermocouples used by Huber and 

 Schmidt (1937) for measuring slow rates of movement of sap streams. Cu — Cu 

 represents the thermocouple points. 



Measurement of velocities in more slowly moving streams, i.e., in conifers 

 or in the phloem, is more difficult. Dixon (1937) modified the thermo- 

 electric method by placing the thermoindicators 1 cm. apart and the near 

 junction 1 cm. from the heater. Both the junctions and the heater were 

 placed on the undisturbed bark. Determinations took from 'one-half to a 

 whole hour. Huber and Schmidt (1937) modified the thermoelectric 

 method by bringing the thermocouples closer to the source of heat. They 

 placed one thermocouple on either side of the heat source with the lower ele- 

 ment somewhat closer than the upper, i.e., 16 and 20 mm. By measuring 

 the rate of heat transfer both in a normally transpiring plant, and a control 

 in which, due to very humid conditions, there was supposedly no movement 

 of the transpiration stream, they were able to measure rates of sap move- 

 ment in the neighborhood of 5 cm. per hour. Figure 47 shows the ar- 

 rangement of the heating element and thermocouples used by Huber and 

 Schmidt. 



