500 



TRANSLOCATION OF SOLUTES 



^^ 



La 



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t 



This hypothesis assumes that the higher turgor pressure of the leaf cells 

 will cause a mass flow of solution downward in the phloem toward the roots. 

 Plasmodesms connecting adjacent cells are supposed to permit mass move- 

 ment of solution from leaf cell to leaf cell, and from leaf cells into the phloem 

 elements. Movement from one sieve tube to another is supposed to be facili- 

 tated by the communicating pores. Some of the solute molecules may be lost 

 to the cambium and other living cells of the stem, but it is assumed that the 

 greater proportion of them are translocated to the roots. The water com- 

 ponent of the downward moving solution 

 is supposed to be exuded back into the 

 xylem from the cambium or other receiv- 

 ing cells. 



This theory will account for translo- 

 cation through the phloem in only one di- 

 rection at a time. Transport might occur 

 at times in one direction and at times in 

 the other. If the phloem acts as a single 

 osmotic system, as assumed, flow will 

 occur from the end at which the turgor 

 pressure is higher towards the other. In 

 the spring, for example, if it is postulated 

 that the turgor pressure of the supplying 

 cells in the stem is greater than that of the 

 growing stem tips upward flow would 

 usually occur. Later in the season, as the 

 turgor pressure of the leaf cells increases, 

 a reversal in the direction of flow would 

 be expected. 



The principal evidence which has been 

 cited in support of this hypothesis is that 

 sap will often exude rapidly from a cut 

 made into the phloem of a stem (^Vliinch, 

 1930; Crafts, 1936). The latter investigator has demonstrated that a con- 

 tinuous exudation of phloem sap will occur from stems of squash for a period 

 of at least 24 hours. Because the exudate rapidly coagulates in contact with 

 air, thus plugging the phloem, the flow is maintained only if a fresh cut is 

 made at frequent intervals. Exudation occurs at rates varying from O.Oi to 

 0.1 cc. per minute. This suggests the occurrence of a mass flow under pres- 

 sure in the phloem elements of intact stems. It has not been positively shown, 

 however, whether the exudation is occurring from the sieve tubes or other 



Fig. 112. Diagram to illustrate 

 mechanism of solute translocation ac- 

 cording to the Miinch hypothesis. 

 Redrawn from Crafts (1931). 



