Crafts et al. — 176 — Water in Plants 



ment to be rapidly absorbed by bean roots and carried upward in the transpiration 

 stream. In further experiments (1941) he found radiosposphorus injected into bean 

 leaves to migrate out, the initial movement being predominately downv^^ard, a fair por- 

 tion reaching the roots during the evening following injection. A small amount moved 

 upward. Such movement might take place by the mechanism mentioned by Rouschal. 

 CoLWELL (1942) used radiophosphorus for translocation studies on squash plants. 

 His results emphasize the care needed to restrict movement to the phloem. When the 

 tracer is so restricted, its movement is correlated with food movement in the plant. 



BiDDULPH and Markle (1944) studied the distribution of radiophosphorus in cot- 

 ton plants following injection into leaves. They found movement out of the leaf and 

 down the stem to occur at a rate of better than 21 cm. per hour. Though movement 

 followed a diffusion pattern, rates indicated an active mechanism. Upward move- 

 ment in the stem was variable. Movement from the phloem into the xylem occurred, 

 resulting in a recirculation within the plant. Rabideau and Burr (1945) used radio- 

 carbon as a tracer for transport studies. Exposing leaves to CO2 containing C^^, 

 photosynthesis resulted in formation of carbohydrates that moved both upward and 

 downward in the plants. Radiophosphorus applied to roots moved readily past killed 

 areas of the stem but photosynthate containing radioactive carbon failed to pass such 

 regions. They conclude that the latter materials move exclusively through the phloem. 

 Studies on translocation of the reproductive stimulus in plants have shown that 

 flower induction is caused by a definite substance that may act locally or may be 

 moved. Loehwing (1938) using a slit panel, so that the tops and bases of soybean 

 plants could be subjected to different photoperiods, showed that whereas flower induc- 

 tion is normally localized in these plants, defoliation brought about transfer from donor 

 to receptor regions. He concluded that flower initiation was not caused by photo- 

 synthates but by some hormone-like substance. Flower initiation on a receptor region 

 subject to long-day treatment could be brought about by defoliation if a donor region 

 consisting of another portion of the plant was given short-day treatment. 



Hamner and Bonner (1938), using Xanthinm pennsylvanicum, found that though 

 the initial perception of the floral stimulus is by mature leaves subject to a short photo- 

 period, the stimulus may be transported from these to other portions of the same plant 

 or that it might move through a graft or across a diffusion contact to another receptor 

 plant. Defoliation is not required in Xanthinm as the stimulus moves normally both up 

 and down the stem to donor regions that have not received induction. They attribute 

 floral initiation to a definite hormone-like substance. 



WiTHROW and Withrow (1943) studied floral induction in the same plant and 

 found translocation between an induced donor plant and a receptor only when tissue 

 union was established as a result of uninterrupted tissue contact for four days or more. 

 The floral stimulus failed to translocate downward through a killed petiole or through 

 functional xylem. It also failed to cross a ring. They concluded that it moved only 

 in the bark, a result in harmony with tliose of Moshkov (1939). Stout (1945), study- 

 ing translocation of the floral stimulus in sugar beet, found that it would move down 

 a donor shoot and into a darkened shoot but not into an illuminated shoot^ where all 

 three shoots came from the same beetroot. This substantiates the defoliation results 

 of Loehwing and indicates that the flower-inducing substance like curly-top virus 

 (Bennett, 1937, 1940a, b) is translocated along with food material. 



Further studies on virus movement (Bennett, 1940a, b; Hildebrand, 1942; 

 Hildebrand and Curtis, 1942; Bennett, Carsner, Coons, and Brandes, 1946) 

 confirm Bennett's earlier convictions (Crafts, 1939b) that rapid transport of virus 

 takes place in the phloem and is correlated with food movement. If this is true,^ and 

 much evidence now indicates that it is, there seem to be only two interpretations: 

 first, that virus particles are bonded to food molecules and that they move together 

 along concentration gradients of food, or, second, that they all move together by mass 

 flow of the assimilate stream. The first possibility seems doubtful in view of the ac- 

 cumulation of curly-top virus in the seed coats of sugar beet seed during storage of 

 carbohydrates in the seeds. There seems to be no logical reason for their separation 

 at this point if they are really tightly bonded. That Bennett favors the second 

 alternative is indicated in his review article (Bennett, 1940&). 



Studies on transport of thiamin in tomato by Bonner (1942) show that this 

 material is synthesized in mature leaves and transported to the young developing leaves, 

 where it concentrates to a high level, and to the roots. It accumulated above a girdle at 

 the second node of the plants and below one made by steaming the stem in a region 

 between the mature leaves and the young expanding ones. These tests indicate a 



