DOWNWARD TRANSPORT THROUGH THE XYLEM 97 



be forced through the xylem led Dixon and his co-workers 

 to conclude that Birch-Hirschfeld's suggestion of backward 

 flow through the xylem was more satisfactory than the 

 commonly accepted one that the phloem is the channel of 

 backward movement. 



Dixon and Ball (1922) give additional experimental 

 evidence that the xylem may carry solutes backward. 

 They carefully dug a large potato plant, and before any 

 wilting became visible cut off the apex of a leaf under 

 an eosin solution. Within an hour the veins of all leaves 

 as well as the stems and roots were stained, while the tuber 

 also, when sectioned the next day, showed staining. The 

 dye was carried exclusively through the vessels of the xylem. 

 The following quotation from Dixon (1922) gives added 

 evidence for a possible backward flow. 



Another very striking experiment may be carried out with the 

 imparipinnate leaf of Saynbucus nigra. Its petiole is spht longitudinally 

 for a few centimeters and half removed. The remaining half is set in 

 a solution of eosin. The solution is rapidly drawn up the wood capil- 

 laries of the intact half-petiole and soon appears in the veins of the 

 pinnae on the same side of the leaf, beginning with the lowest, and 

 gradually working up into the upper ones. Finally it appears in the 

 terminal pinna. All this while the veins of the pinnae on the other 

 side remain uncoloured. Now, however, the eosin begins to debouch 

 into the base of the uppermost of these pinnae and spreads through its 

 veins ; finally it makes its way down the offside of the rachis to the bases 

 of the lower pinnae, and from thence spreads into their veins. In 

 this case we see very clearly how transpiration actuates an upward 

 current on one side and a downward current on the other. It is inter- 

 esting to note that if the terminal pinna and its stalk are removed, the 

 eosin does not appear in the pinna of the second side, or only after a 

 considerable time when the small anastomosing conducting tracts are 

 utilised. 



For evidence that normally occurring solutes may be 

 carried backward in uncut stems, Dixon appeals to the 

 backward conduction of a stimulus, such as that in Mimosa 

 causing the folding of pinnules. Evidence has been given 

 that the transference of the stimulus is due to the transport 

 of something of the nature of a hormone which is carried 

 in the transpiration stream. That this substance is carried 



