UPWARD TRANSLOCATION OF MINERAL SALTS 491 



experiments by dry weight determinations and analyses for sugar. As shown 

 in Table 45 the dry weight and total sugar content per stem were invariably 

 least in the ringed stems. This was also usually true for the percentage 

 of sugar in terms of fresh weight or dry weight. 



TABLE 45 COMPARATIVE EFFECTS OF CUTTING THE XYLEM OR PHLOEM ON GROWTH, DRY 



WEIGHT, AND SUGAR CONTENT OF DEFOLIATED SHOOTS (dATA OF CURTIS, I925) 



In general the evidence seems to warrant the conclusion that the phloem is 

 the principal tissue in which upward translocation of organic solutes occurs, 

 although it seems probable that small quantities of such compounds are, on 

 occasion, translocated in an upward direction through the xylem. 



Upward Translocation of Mineral Salts. — For many years it was uni- 

 versally agreed that the upward translocation of mineral salts ^ occurred 

 through the xylem. Current concepts, however, are not nearly so unanimous. 



2 The term "mineral salts" is necessarily employed somewhat loosely. It is 

 used in the immediately following discussion to include nitrogen which may be 

 translocated upwards in either organic or inorganic forms {cf. discussion in 

 Chap. XXVI on synthesis of amino acids in roots). Some translocation of other 

 elements, especially sulfur and phosphorus, may also occur in the form of organic 

 compounds since these elements may be converted from inorganic to organic 

 combination at any time after they are absorbed. 



