EXPORT OF SOLUTES FROM LEAVES 497 



of mineral elements occurs in the xylem agree that outward transport of such 

 compounds from leaves occurs mostly in the phloem. 



(3) The concentration of solutes in the medium in which the plant is 

 rooted may affect the path of transport. If a given solute is present in low 

 concentrations most of it may move up through the phloem. With higher 

 concentrations a large proportion of the solute molecules or ions may "leak" 

 past the phloem and be carried up in the xylem. The possibility that inorganic 

 solutes are actually secreted into the xylem ducts (Chap. XVII) must also be 



considered. 



(4) IVletabolic conditions within the various organs of the plant, especially 

 the roots, may have an influence on the path of transport. If, for example, 

 the root cells are relatively well stocked with carbohydrates a large proportion 

 of nitrogen absorbed may be utilized in the synthesis of amino acids in the 

 roots, at least in some species. If, on the contrary, the roots are deficient 

 in carbohydrates very little of the nitrogen absorbed will be converted mto 

 organic forms in the roots. The previous discussion has indicated that the 

 path of translocation of nitrogen may depend upon the kind of chemical 

 combination in which it exists. Since ringing the stem of a plant below the 

 foliage sooner or later leads to carbohydrate starvation of the roots this factor 

 must be considered in the evaluation of such ringing experiments. It is quite 

 possible that ringing, under some conditions at least, may induce movement 

 of solutes in a tissue in which it normally does not occur. 



The results of Gustafson and Darken (i937) support the view that at 

 least some mineral elements are translocated through both the xylem and the 

 phloem. Phosphorus which had been made artificially radioactive was supplied 

 to plants as an 0.5 per cent solution of KH2PO4. The presence of such 

 phosphorus in plant tissues can be detected by means of an electroscope. Rooted 

 cuttings of willow, geranium, bryophyllum, and a species of Sediun were used 

 as the experimental material. Various types of experiments indicated that the 

 phosphorus ascended the stems of these species in both the xylem and the 

 phloem. As nearly as a quantitative estimate could be made it appeared that 

 these two tissues were about equally effective in conducting phosphorus under 

 the conditions of these experiments. 



Export of Solutes Other than Carbohydrates from Leaves and Other 

 Lateral Organs. — Outward translocation of solutes from leaves and other 

 lateral organs is often referred to as "export" of solutes. Leaves not only 

 export carbohydrates, but at times at least, mineral elements in either organic 

 or inorganic combination. According to Mason and Phillis (i937) nitrogen, 

 phosphorus, potassium, sulfur, magnesium, and chlorine all may be exported 

 from leaves, while calcium is practically immobile. Nitrogen, phosphorus, 



