Grafts et al. — 168— Water in Plants 



insight into plant physiological processes, to the time of Stephen Hales 

 (1738), nevertheless the rate at which materials are moved through the tis- 

 sues of transport in the plant has been an elusive problem. Confronted, in 

 higher plants, by an organism which possesses in contrast to the higher ani- 

 mals no specialized pumping organ nor large, easily recognized conducting 

 tubes, the plant physiologist has been slow to understand the mechanisms 

 of transport in the plant. 



The rate of movement of the ascending "transpiration stream" has been 

 studied in three ways : 1 ) observation of quantity of material moved and 

 cross sectional area of the tissue of transport, 2) by the use of indicators, and 

 5) by direct observation {see discussion above). Attempts have been made 

 using these methods to study the movement of the "assimilation stream" 

 as well. 



Quantity of Material Moved and Area of Transport : — It is 



obvious that if the amount of material moving in a given time through a 

 conductor of known cross sectional area is determined, the rate of flow can 

 be calculated. This method has, until the recent introduction of radioactive 

 tracers, been the chief method of studying the movement of materials in the 

 phloem. Such a method, while improved with our increasing knowledge 

 of the anatomy of the conducting tissues, at best is limited to average values 

 over relatively long periods of time. 



Indicators: — The introduction of indicators into the moving stream 

 has been the most widely used method of measuring the rate of flow in the 

 xylem. These indicating methods have employed the use of four different 

 types of indicators: 1) salts, 2) dyes, 3) radioactive material, and 4) heat. 

 Some results of early work with indicators are presented in Table 44. 



Salts. — While salt absorption, as often pointed out (Hoagland, 1944), 

 may be more dependent on climatic factors determining carbon fixation, 

 respiration, and other biochemical processes than on the amount of water 

 absorbed, still the transpiration stream provides an efficient and rapid means 

 of transporting to the rest of the plant body materials absorbed by the roots. 

 Salt movement may not necessarily follow exactly the flow of water, but in 

 the xylem elements where water movement is usually many times in excess 

 of diffusion rates, salt movement may approximate that of the water. 



Sachs early pointed out the fallacy in using cut branches or plants with 

 injured roots in experiments on sap movement in plants, criticizing the re- 

 sults of early workers (McNab, 1871 ; and Pfitzer, 1878). His determina- 

 tions were made by use of lithium salts applied directly to the roots in solu- 

 tion or to soil. The rate of movement of the lithium through the plant was 

 tested by means of the typical lithium flame test. 



Dyes. — While the use of dyes facilitated the detection of the tracer 

 materials, the possibility of adsorption and consequent reduced rate of move- 

 ment made such determinations open to criticism, particularly if strongly 

 adsorbed dyes were used. Strugger (1943) and others have used fluo- 

 rescent materials in studying the movement of the transpiration stream. 

 Special emphasis was placed by Strugger on the movement of the fluo- 

 rescent material via the cell walls from the i^ascular bundles through the 

 intervening tissue to the evaporating surface. While stressing the fallacy 

 of Sachs' imbibitional theory in respect to intravascular movement of wa- 

 ter, he places great emphasis on imbibition as a factor in the extravascular 

 movement. Materials passed rapidly through the walls of the mesophyll 

 and palisade tissue without entering the living cells. 



