494 



TRANSLOCATION OF SOLUTES 



was reversed, and there was a rapid increase in the electrolyte concentration 

 of the sap. During the summer months, when the highest rates of transpira- 

 tion occur, the electrolyte concentration of the sap was appreciable. These 

 data furnish presumptive evidence that upward translocation of mineral salts 

 occurs in the xylem but do not prove it to be the only or even the main tissue 

 involved in this process. 



Evidence indicating that in cotton plants upward translocation of nitrogen 

 occurs principally in the xylem is furnished by the work of Mason, Maskell, 

 and Phillis (1936). These workers used plants which had been grown up 

 to the time of the experiment in sand cultures which were deficient in nitrogen. 



At the beginning of the experiment the plants 

 were divided into four groups, one of which 

 was used for the initial analysis for nitrogen 

 content. The phloem was then intercepted 

 in the stems of the plants of the second 

 group, and the xylem intercepted in the stems 

 of the plants of the third group. The fourth 

 group was used as a control. The plants 

 were defoliated, supplied with a solution rich 

 in nitrogen and enclosed within bell jars con- 

 taining wet cotton. After four and eight 

 days analyses for nitrogen were made of the 

 ~e tops and roots from plants of each group. 

 The results (Fig. no) show clearly that 

 nitrogen moved up almost as effectively in 

 the stems in which the phloem was inter- 

 cepted as in the controls, while cutting out a 

 section of xylem greatly reduced nitrogen 

 transport. The roots of the plants from all 

 three groups, on the other hand, accumulated approximately equal quantities 

 of nitrogen. The results of this experiment make it seem very probable that 

 most of the translocation of nitrogen in cotton plants occurs through the 

 xylem. One possible criticism of this experiment, however, is that the rather 

 high concentration of nitrogen in the soil solution may have caused a de- 

 parture from normal in the path of translocation. 



The results of Clements and Engard (1938) also indicate that at least 

 a part of the mineral salts are translocated upwards through the xylem in 

 a number of woody species. Although ringing the stems decreased the quantity 

 of mineral salts translocated, an effect which was attributed by these workers 

 largely to indirect effects on the xylem as a result of ringing, considerable 



4 



DAYS 



Fig. 1 10. Effect of intercept- 

 ing xylem and of intercepting 

 phloem on upward transport of 

 nitrogen in cotton plants. Data 

 of Mason, Maskell, and Phillis 

 (1936). 



