DOWNWARD TRANSPORT THROUGH THE XYLEM 95 



this must have been transported through the stolon in less 

 than 100 days. Assuming that the solution moving 

 through the phloem had a concentration of 10 per cent 

 sugar, the volume moving into the tuber must have been 

 500 cc. to carry the 50 g. of sugar. Therefore the rate of 

 flow must have been at an average of about 50 cm. per 

 hour if the entire cross-sectional area were available for 

 transfer or twice this amount if allowance is made for 

 a returning stream. This, however, seems impossible 

 because one can hardly conceive of this amount of solution 

 being forced through the narrow sieve tubes with their 

 frequent cross walls and high colloid content. Dixon 

 assumed a solution of 10 per cent but says this is excessive 

 because the sap bleeding from cut stems seems never to 

 reach 4 per cent (see also Dixon 1924). As high as 8 per 

 cent has been reported in the sap from the maple by Jones 

 et al. (1903), and, of course, one is not justified in assuming 

 that the solution bleeding from the xylem has the same 

 concentration as that in the sieve tubes. The figures 

 assumed for the amount of organic matter transported 

 and the time allowed for transportation, as well as the 

 assumed concentrations are sufficiently liberal, however, 

 to indicate that minor corrections could not appreciably 

 change the conclusion. 



Dixon (1922) claims to have been the first to have made 

 such calculations on the rates of transport and this point 

 is again mentioned by Mason and Maskell (1928a), but 

 Dixon evidently failed to read all of Birch-Hirschfeld's 

 paper, even though he cites her experimental evidence 

 as strongly supporting his hypothesis, for she made very 

 similar calculations based on the relation between the 

 amount of photosynthate made in a given leaf area and 

 the cross-sectional area of the phloem of the petiole through 

 which such carbohydrate must be transported. 



Dixon also estimates the probable amount of daily 

 removal of photosynthate from the leaf of Tropaeolum 

 ma jus making calculations very similar to those of Birch- 

 Hirschfeld, and concludes that if two-thirds of the photo- 



