498 TRANSLOCATION OF SOLUTES 



and sulfur are probably exported in organic combination, while potassium, 

 chlorine, and magnesium apparently move out in inorganic form. Apparently 

 translocation of all of these substances out of the leaf occurs through the 

 phloem. In general those elements which are readily redistributed in plants 

 (Chap. XXV) are the ones which are readily translocated out of leaves. 



There is some evidence that a daily variation in the mineral content of 

 leaves may be of regular occurrence. For example, Penston (1935) found the 

 potassium content of mature potato leaves to reach a peak about 3 :oo p.m. 

 and to decline during the evening hours. This was apparently also true of 

 other mineral elements. This investigator considers that potassium is prob- 

 ably transported to the leaves in the transpiration stream and is exported from 

 them through the phloem. 



Prior to leaf abscission a large proportion of the mineral elements and 

 nitrogen in the foliage of tree species is translocated back into the stems and 

 roots (Deleano, 1936). A similar phenomenon occurs in flowers. Shortly 

 before abscission of the petals of cotton blossoms, nitrogen, magnesium, phos- 

 phorus, potassium, chlorine, and magnesium are translocated out of the petals 

 into other parts of the plant (Phillis and Mason, 1936 b). 



Mechanism of Translocation of Solutes in the Xylem. — Although at 

 present we cannot estimate with certainty the proportion of the mineral salts 

 translocated upwards through the xylem it is unquestionably true that at least 

 some of the translocation of inorganic compounds occurs through this tissue. 

 Under certain conditions small amounts of organic solutes may also be moved 

 upward in the xylem. Any solutes conducted through the xylem are carried 

 along with the ascending streams of water which are pulled up through the 

 plant according to the mechanism which has already been discussed. In the 

 leaves these solutes move out of the xylem conduits into adjacent living cells. 

 Some solutes may also be lost from the xylem stream by diffusion into living 

 cells of the stem which abut on vessels or tracheids. The rates at which 

 solutes are translocated upwards through the xylem of the stem will correspond 

 with the rates of translocation of water (Chap. XV). 



Mechanism of Translocation of Solutes in the Phloem. — It has long 

 been recognized that simple diffusion is inadequate to account for the known 

 rates of translocation through the phloem. This realization has led to the 

 postulation of other theories of the mechanism of transport in this tissue. 



I. The Mass Floiu Hypothesis. — This theory of translocation was first 

 proposed by Munch (1927, 1930) and in modified forms has also been strongly 

 advocated by Crafts (1931, I932, I933, 1938). The principles involved in 

 this hypothesis can most easily be clarified by reference to a simple physical 

 system (Fig. 1 11). 



