492 TRANSLOCATION OF SOLUTES 



Some contemporary authorities contend that the existing evidence supports the 

 thesis that the xylem is the principal channel through which upward transport 

 of such solutes occurs. Others are convinced that the available data indicate 

 that the phloem is the principal tissue concerned. These contradictory views 

 may ultimately be reconciled in the conclusion that both of these tissues serve 

 as important routes of transport. 



Formerly the impression was generally prevalent among botanists that 

 mineral salts are carried into the plant along with the water since both are 

 absorbed by the younger parts of roots. As the discussion in Chap XII in- 

 dicates it is now widely recognized that the absorption of water and mineral 

 salts are largely if not entirely independent processes. This does not neces- 

 sarily indicate, however, that translocation of water and translocation of 

 mineral salts are also independent processes. It seems entirely possible that 

 salts may pass into the xylem ducts and be translocated upwards in the rising 

 water columns. 



Studies of the sap from xylem vessels show that it usually contains at 

 least traces of both organic and inorganic solutes. The previous discussion 

 has indicated that the occurrence of soluble carbohydrates in the xylem sap 

 cannot be accepted as convincing evidence that the xylem is the principal 

 channel in which upward translocation of such compounds takes place. The 

 similar presence of inorganic elements cannot, however, be so heavily dis- 

 counted. In proportion to the total quantities utilized, the concentration of 

 inorganic constituents in the xylem sap is usually relatively higher than that 

 of organic solutes. Furthermore appreciable concentrations of mineral salts 

 are often present in the sap of vessels at seasons when upward flow of water 

 is occurring at its most rapid rates. At such times the xylem sap contains 

 little or no organic material in solution. 



The presence of inorganic constituents in the vessel sap is shown clearly 

 by the work of Anderssen (1929). He obtained samples of tracheal sap 

 from branches of the pear and other species by displacement with gas. The 

 concentration of some of the minerals tested for was found to be distinctly 

 higher in the tracheal sap than in the soil solution (Table 46). 



The same investigator studied the seasonal changes in the electrolyte con- 

 centration of the tracheal sap of pear branches by determining its specific 

 resistance from month to month during the year. The greater the specific 

 resistance, the less the electrolyte concentration of the sap. As shown by his 

 data (Fig. 109) the maximum concentration of electrolytes occurred shortly 

 after full bloom, following which it slowly and mostly consistently decreased 

 until the minimum was attained in February. In the early spring the trend 



