UPWARD TRANSLOCATION OF ORGANIC SOLUTES 487 



through the phloem, but is not in any sense a proof, since storage tissues also 

 contain relatively high concentrations of foods. 



Another reason frequently advanced for considering the phloem as the 

 tissue which is primarily concerned in the downward translocation of organic 

 solutes is that the general direction of the movement of sap in the xylem is 

 upward. It is not inconceivable, however, that there might be descending 

 streams in certain xylem ducts, even while the majority are occupied with up- 

 ward moving columns of water. In fact theories of the downward transloca- 

 tion of solutes involving this very concept have been advanced from time to 

 time, although currently they find little support. If a dye is injected into 

 the xylem of a woody stem it usually moves both upward and downward. 

 This has sometimes been cited as evidence of downward currents in the xylem. 

 Such downward movements of injected dyes are probably due to the effects 

 of tension or to the entrance of the dye into vessels which are occupied only 

 by gases at a subatmospheric pressure, and is scarcely proof that downward 

 movements normally occur in the xylem of intact stems. 



While it is possible that backward movement of liquid may occasionally 

 occur through the xylem, all the evidence indicates that this tissue plays 

 no important role in the normal downward conduction of solutes in plants. 



Upward Translocation of Organic Solutes. — Under many conditions an 

 upward ^ translocation of organic solutes takes place in plants. This occurs, 

 for example, in the stems of woody species when the buds resume growth in 

 the spring. The tissues of the new shoots are constructed largely out of 

 foods which move in an upward direction from the storage tissues of the 

 stems, as during the early stages in their expansion the leaves do not photo- 

 synthesize at a rate sufficient to supply all the foods used in the growth of 

 the shoot which bears them. Upward translocation of foods from the older 

 leaves on a given shoot to developing leaves situated closer to its apex also 

 probably occurs. As the leaves mature there is a reversal in the direction of 

 translocation of carbohydrates; they are then translocated from the leaves 

 into the stems in which they generally move in a downward direction. 



A number of other examples of the upward transport of foods in plants 

 can be cited. Developing fruits are often attached to stems in such a position 

 that some or all of the food translocated into them moves through the stems 

 in an upward direction. In the early stages of the development of seedlings 

 upward translocation occurs from the endosperm or cotyledons towards the 



1 The terms "upward" and "downward" as applied to translocation phenomena 

 should not be interpreted too literally. As a rule translocation in the general 

 direction of roots to leaves or other apical regions is termed "upward transloca- 

 tion" ; movement in tlie reverse direction, "downward translocation." 



