4 TRANSLOCATION IN PLANTS 



loss from the leaves have been repeatedly observed since 

 the time of Malpighi (1679) and Hales (1727). When 

 it was recognized that essential mineral salts are absorbed 

 from the soil solution, it was immediately apparent that 

 absorption of water and its movement through the plant 

 might account for both the absorption and the movement 

 of these salts. Weight was added to the assumption that 

 solutes are absorbed and carried with the transpiration 

 stream, by the observations that, when cut stems are 

 placed in solutions of dyes or other solutes, these are 

 absorbed with the water and carried with it through the 

 xylem. Among some of the earliest experiments dealing 

 with the movement of water through stems are to be found 

 experiments of this kind with dye solutions. Knight 

 (1801) made use of colored solutions in tracing conducting 

 tissues, and Pfeffer (1900, p. 217) speaks of Magnol in 

 1709 and De la Baisse in 1733 as having carried out such 

 experiments. Indeed such experiments have been repeated 

 innumerable times by beginning and advanced students 

 in experimental botany as well as by investigators and have 

 naturally led to the seemingly obvious conclusion that the 

 upward movement of solutes takes place with the trans- 

 piration stream through the xylem. The amount of solute 

 absorption and the rate and direction of its movement are, 

 under these conditions, found to be determined by the 

 amount, rate, and direction of water movement. Studies 

 on transpiration and water absorption and conduction 

 seemed therefore to help in solving the problems of the 

 absorption and upward transfer of solutes. 



It would be perhaps impossible and certainly imprac- 

 ticable to list all the papers which contain data of this 

 nature dealing with the conduction of solutions through 

 stems. Many have used dyes and have traced their 

 movement macroscopically or microscopically. Others 

 have used colorless salts and have traced the movement 

 spectroscopically or chemically (iron, lithium, beryllium, 

 caesium). Colloidal sols also have been used, both under 

 conditions when the material was colored and easily 



