496 TRANSLOCATION OF SOLUTES 



translocated upwards in the xylem, and that it is the diminution in transpira- 

 tion rate which accounts for the smaller amounts of translocation through 

 ringed stems. However, for various reasons which cannot be considered in a 

 brief discussion this interpretation seems less probable than the simple assump- 

 tion that the phloem is the channel of transport. 



IVIoose (1938) has shown that the phloem exudate of several different 

 species contains calcium, magnesium, potassium, lithium, barium, boron, copper, 

 manganese, strontium, and nitrate nitrogen. The presence of such inorganic 

 constituents in the phloem sap is presumptive evidence that at least some 

 translocation of mineral salts occurs through the phloem. 



The evidence at present available does not indicate clearly whether trans- 

 location of mineral salts occurs predominantly in the xylem, whether it occurs 

 predominantly in the phloem, or whether both of these are important path- 

 waj's of transport. Even if conduction of mineral salts occurs chiefly in one 

 of these tissues it seems almost certain that at least some transport occurs in 

 the other. 



Under some conditions the xylem may be the main channel of transport 

 while under others the phloem may serve in this capacity. Some of the con- 

 ditions which conceivably may have an influence on the route of translocation 

 of mineral salts are the following: 



( 1 ) The predominant route of translocation of mineral salts may be 

 different in different species or in herbs as contrasted with woody plants. In 

 herbaceous species such as cotton, for example, most transport of mineral salts 

 may occur in the xylem, while in woody species they may move principally 

 through the phloem. 



(2) Some solutes may niove principally through the xylem, others through 

 the phloem. Furthermore the same element may move along different routes 

 depending upon the type of chemical combination in which it occurs. Nitrogen 

 in the organic form (amino acids, etc.) may be translocated through the 

 phloem, but nitrates may travel in an upward direction through the xylem 

 (Loomis, 1935). In those species in which amino acid synthesis occurs pre- 

 dominantly in the roots, such as the apple (Chap. XXVI), upward trans- 

 location of nitrogen may occur in the phloem, while in those species in which 

 amino acid synthesis occurs principally in the aerial organs, most upward 

 translocation of nitrogen is probably in the form of nitrates and may occur 

 largely in the xylem. The route followed by some of the other elements — 

 especially sulfur and phosphorus — may also depend upon whether they are 

 in organic or inorganic combination. The tissue through which the solute 

 is translocated may also differ with the direction of its movement. As shown 

 in the later discussion even those workers who believe upward translocation 



