DOWNWARD TRANSPORT THROUGH THE XYLEM 93 



at the rate of 0.003 g. per hour, or a movement of 0.0007 g. 

 per hour per square milUmeter of cross section of entire 

 petiole, 0.0008 g. per square milUmeter of cross section of 

 the vessels, or 0.006 g. per square millimeter of cross section 

 of sieve tube. She calculated the rate at which lithium 

 moved into parenchyma or phloem tissues from a 1 per cent 

 solution and found after 20 hr. that this was only about 

 0.000005 g. per square millimeter per hour, or only about 

 one-hundred-sixtieth as fast as the sugars actually seem 

 to move, assuming the movement through the entire living 

 part of the petiole, or only one-thousandth as fast as it 

 should move if the movement is restricted to movement 

 through the sieve tubes. She points out that sugars could 

 hardly be expected to diffuse even as rapidly or penetrate 

 the cells as easily as lithium nitrate, therefore it seems 

 impossible that these phloem tissues could carry solutes 

 fast enough. Measurements of water movement through 

 phloem tissues also showed extremely low rates and if 

 solutions as strong as 10 to 20 per cent sugar, or even if 

 sugar moved as fast as water alone, gram for gram, the 

 mass movement of sugar would be too slow by more 

 than a hundred times. 



All the experiments on the transfer of introduced dyes 

 or salts indicated without exception that the transfer of 

 these solutes through the phloem or other living tissues 

 is extremely slow, whereas the transfer through the xylem 

 vessels is immensely more rapid and very easily demon- 

 strated. Rapid transfer of solutions within the xylem 

 tissues takes place readily not only from the base toward 

 the transpiring leaves, but a backward flow is also easily 

 demonstrated if solutions are introduced through cut side 

 branches or incisions. (See also Yendo, 1917; Rumbold, 

 1920; and others who observed flow in both directions.) 

 Such a backward movement occurs even when leaves are 

 entirely absent below the point of injection. Since injected 

 dyes move vertically through the xylem and transverse 

 movement is very slight, as pointed out by Strasburger 

 and as shown by her own experiments, it seemed to Birch- 



