94 TRANSLOCATION IN PLANTS 



Hirschfeld that solutions may be moving up through certain 

 channels of the xylem and down through adjacent ones, 

 and she suggested that backward movement of solutions 

 might thus carry sugar from the leaves. The fact that 

 injected dyes were carried back in transpiring shoots, even 

 when the plant was well supplied with water, seemed to 

 favor this interpretation. She suggests that under natural 

 conditions the wetting of the leaves with dew or rain might 

 allow for an extensive backward flow, or that unequal 

 insolation of neighboring parts might also bring about 

 considerable back flow. 



Though Madam Birch-Hirschfeld's experiments led her 

 to doubt the effectiveness of the phloem or other living 

 cells in the transport of solutes, and though she definitely 

 suggested the possibility and gave experimental evidence 

 supporting the hypothesis of a backward flow through the 

 xylem, still she felt that the problem was far from settled 

 and suggested that living cells might transport the normally 

 moving substances in a manner different from that in which 

 lithium nitrate is transported. 



16. Findings and Interpretations of Dixon and His 

 Co-workers. — Dixon, in 1922, and Dixon and Ball (1922) 

 more boldly supported the hypothesis that the xylem is 

 the channel through which most of the backward movement 

 of solutes takes place. All evidence based on ringing 

 experiments is summarily dismissed on the grounds that 

 ringing always results in some sort of plugging of the 

 xylem. The strongest plea against a consideration of 

 the phloem as a channel for backward transfer is that the 

 phloem must be inadequate for carrying the amount that 

 is carried, because the narrowness of the tubes, their high 

 colloid content, and frequent cross walls seem to preclude 

 the possibility of rapid transfer through them. In order 

 to estimate the probable rate of normal transfer, Dixon 

 selected a potato tuber weighing 210 g. connected by a 

 stolon with a diameter of 0.16 cm., in which the ''bast" 

 had a maximum cross-sectional area of 0.0042 sq. cm. 

 The tuber contained at least 50 g. of organic matter and 



