DOWNWARD TRANSPORT THROUGH THE XYLEM 91 



movement through the xylem tissues is immensely more 

 rapid. That water and solutions can be rapidly carried 

 not only upward through the xylem but also backward 

 through these same water conducting tissues has been 

 recognized for a long time and was reported by Hales 

 (1727), Strasburger (1891, pp. 582 and 936), and others. 

 That this backward movement may be considerable was 

 clearly indicated in the experiments of Yendo (1917). 

 Birch-Hirschfeld, however, seems to have been the first 

 among modern botanists* to suggest that there may be a 

 normal backward flow through the xylem and that it may 

 account in part at least for the backward translocation of 

 dissolved materials. 



To test the frequency, extent, and speed of the backward 

 movement and the factors influencing it, she carried out 

 various experiments using principally lithium nitrate which 

 was introduced through cut side twigs, or even in a few 

 instances through uncut tissues, by dipping leaves into 

 lithium solutions, or placing drops of strong solutions on 

 the surfaces of leaves with thin cuticle, or by placing small 

 crystals of the salt between the bark and the wood. 



The experiments were carried out under a variety of 

 conditions. In some the leaves were present both below 

 and above the point of introduction. In these, even when 

 cut branches were used and the base immersed in water, 

 the lithium reaction was apparent in 23^^ to 8 hr. at a 

 distance of 10 to 16 cm. below the point of introduction, 

 and chiefly on the side where the solution was introduced 

 through a cut side twig. For this experiment a rather 

 strong solution (0.5M) was supplied. In one experiment 

 with two leafy shoots of Liguster amurense, drops of 0.75ilf 

 lithium nitrate solution were placed on the surface of a 

 leaf near the middle of a side twig. The cut shoots were 

 standing in water, one free in a room and the other in a 

 moist chamber. For the twig in the moist chamber in 

 which the side twig was 8 cm. long, the lithium reaction 



* At the time of Hales and other early investigators there was speculation 

 concerning the flow and ebb of sap through the wood. 



