CHAPTER IV 



EVIDENCE INDICATING DOWNWARD TRANSPORT 

 THROUGH THE XYLEM 



15. Findings and Interpretations of Birch -Hirschf eld. — 



Anatomical studies and experiments on ringing since the 

 time of Malpighi (1679) and Knight (1801) and especially 

 since those of Hartig (1858, 1861, 1862) and Hanstein 

 (1860) have led botanists to agree that downward trans- 

 location of solutes occurs almost exclusively in the phloem 

 tissues. Luise Birch-Hirschfeld (1919) seems to have 

 been the first seriously to question the validity of this 

 conclusion. Her skepticism with regard to a movement 

 through phloem resulted chiefly from experiments, prin- 

 cipally with lithium salts, which indicated that introduced 

 solutes moved with extreme slowness through these tissues. 

 Strips of phloem tissues from both woody and herbaceous 

 plants were dipped into solutions of lithium nitrate, mostly 

 of 0.13 M concentration, and it was found by spectro- 

 scopic tests that the lithium had moved along the strips a 

 distance of only 1.5 to 2.5 cm. in 24 hr. This rate of 

 movement was approximately the same as that through 

 unspecialized parenchyma tissue as found in potato, kohl- 

 rabi, rutabaga, and the living pith from Sambucus and 

 Sylphium. Since it had been suggested that living cells 

 were concerned in hastening translocation, she studied the 

 effect of killing the tissue but found that the distance 

 lithium moved in a given time, instead of being decreased, 

 was increased about a third when the tissues were killed 

 by boiling. This increased movement was probably due 

 in part to decreased resistance to diffusion in dead tissues 

 and in part, as demonstrated by the experiments of Steward 

 (1930), to driving out gases from the intercellular spaces 

 and thus increasing the area for diffusion. Since it had 



