STORAGE AND TRANSLOCATION 283 



vious workers and believes that organic materials are transported 

 upward in the phloem, the upward movement from the roots and 

 stems of stored materials is generally conceded to take place in 

 the xylem along with the water and inorganic salts. In the sugar 

 maple it is a familiar practice to tap the xylem (sapwood) and 

 withdraw a portion of its contents, which, with the included sugar, 

 is boiled down into the maple syrup of commerce. 



Movement across the stem from pith to cortex and intermediary 

 tissues is facilitated by the rays, which consist of living parenchyma 

 cells, which at the end of the growing season are rich in stored foods. 

 These cells are not only connected with the cortex and pith, but 

 are also in communication with the xylem vessels, into which their 

 stored products can be easily transferred. 



The chief object of dispute in connection with translocation 

 has been the region of downward movement. Sachs (1863) thought 

 that the starch sheath (endodermis) of stems was the region of 

 translocation since the cells there were all found to be filled with 

 this storage product. Because a material is found in a given layer 

 of cells, does not prove that it is carried in that region, however, 

 but may simply mean storage. That this is the case with the 

 starch sheath was later shown by Heine (1888), who removed 

 the endodermis in a set of careful girdling experiments without 

 seriously damaging the other tissues, and found that such treat- 

 ment neither hindered the development of the plants nor inter- 

 fered with the storage of starch in the sheath above the wound. 

 This eliminates the endodermis as the region of downward move- 

 ment. 



Although Dixon (1922) and Arndt (1929) have recently revived 

 arguments in favor of the xylem, since the time of Heine workers 

 have fairly well agreed that the region in question is the phloem 

 or inner bark, which is found immediately outside the cambium. 

 That the phloem is the region in which the downward movement 

 takes place, has been concluded from four different lines of evi- 

 dence, which will be discussed in their relative importance. 



1. Evidence from deduction. The pith is dead and commonly 

 filled with gases. The wood is occupied with the upward move- 

 ment of water and materials in solution. The outer bark or cork 

 is also composed mostly of dead cells in which no longitudinal 

 movements have been detected. So with all the rest of the stem 

 eliminated, the phloem is left as the only remaining possibility, 



