Chapter IX — 157 — Uptake and Movement 



The long time periods between injection and observation by the above work- 

 ers were believed by Baker and James to have led to error. Their own 

 work resulted in radial transfer of dye from one annual ring to another. 

 Because most dyes are poisonous to living plant cells, it seems possible that 

 MacDougal, Overton, and Smith might have caused injury to their 

 material, resulting in death of cells, invasion of air, and formation of tyloses. 



It is commonly observed that when an intact stem of a transpiring plant 

 is cut beneath a dye solution, the dye rushes in, going both up and down the 

 stem. Arndt (1929) interpreted the downward movement of eosin solu- 

 tion applied to cut ends of Coffea arabica stems as indicating that the 

 descending sap in the xylem should provide an adequate mechanism for the 

 downward transport of food. Dixon (1924) had similarly interpreted 

 backward flow of eosin in potato shoots. Postulating that different xylem 

 tracts might be isolated, he proposed that release of tension within a leaf 

 might result in backward movement of foods in solution at the same time 

 that water and salts were moving upward in other isolated tracts. Dixon 

 later reversed his opinion concerning food movement (1933). 



The immediate penetration of dye into a stem of an actively transpiring 

 plant that has been cut beneath the solution may result from several actions : 



i) If the liquid is under tension, a strain is developed in the conducting elements 

 tending to reduce their diameter. Bode (1923) has shown by direct observation that 

 this may amount to a reduction of several per cent in the total diameter, and MacDougal 

 (1925) proved by dendrographic measurements that a contraction of up to one per 

 cent in the whole stem may be found during the day. Actual vessel contraction may 

 greatly exceed this value since vessels make up only a fraction of the woody tissue and 

 living parenchyma would probably shrink less. Immediately upon cutting a contracted 

 vessel there occurs an elastic expansion which causes a rapid inward flow of dye at 

 both cut surfaces. 



2) Simultaneous with the rupture of a tensile water column in the xylem the 

 diffusion pressure of water within that column increases and water moves out of the 

 conducting elements into adjacent living cells which were under a saturation deficit, but 

 in approximate equilibrium with the tensile water columns before cutting. Even root 

 cells in which a saturation deficit occurred would contain water with a lower diffusion 

 pressure than that in the xylem after cutting. Because of this dye may be drawn down- 

 ward through a cut stem into the roots underground (Kennedy and Crafts, 1927). In 

 fact, this mechanism is employed for the destruction of deep-rooted perennial weeds by 

 the acid-arsenical method (Kennedy and Crafts, 1927; Crafts, 1933a, 19336, 1937). 

 In this method, instead of cutting, the foliage is sprayed with an acid solution con- 

 taining arsenic. The acid kills the foliage and renders it permeable, whereupon the sap 

 from the leaves and stems, plus the arsenic from the spray solution, is drawn down into 

 the roots. Root systems of wild morning-glory (Convolznilus arvensis) and Russian 

 knapweed (Centaiirea repens) have been killed to depths of six feet in some instances 

 and often as high as 95 per cent of a population may have its roots destroyed to a 

 depth of four feet. Sodium chlorate, ammonium thiocyanate and other chemicals may 

 be translocated by this same mechanism (Crafts, 1935; Robbins, Crafts, and Raynor, 

 1942). Individual plants may be treated by cutting off their tops under one of the 

 above solutions, or by simply immersing them in a container of solution and allowing 

 them to stand until the tops are killed and rendered permeable. This latter method is 

 more effective when used on individual tops of plants that are interconnected under- 

 ground. Here the undipped tops continue to transpire and thus draw solution through 

 the dipped plant into all plants that are attached through the root system. A detailed 

 discussion of the mechanics of this method, its limitations, and its advantages will be 

 found in Robbins, Crafts, and Raynor (1942). 



2) If the xylem elements contain gas under subatmospheric pressure, the dye 

 solution is forced into them as the gas contracts under the pressure of the solution at 

 atmospheric pressure. If the gas is water vapor, it will contract and condense com- 

 pletely at ordinary temperatures. 



4) Air will be slowly forced into solution by the additional pressure resulting from 

 capillary forces. Surface tension would cause water to be forced into a tube with a 



