UPWARD TRANSPORT OF NITROGEN 75 



transpiration, or the means used to alter it, on some other 

 phase of metabolism and growth. 



Mason and Maskell (1931, p. 150) have suggested that 

 one reason there is little relation between transpiration 

 and total ash content is that the salts are retransported 

 from the leaves to the roots where they have a damping 

 effect on further absorption. According to this suggestion, 

 therefore, transpiration would merely hasten circulation 

 within the plant and have little effect on total absorption. 

 Though not an impossible suggestion, it seems rather 

 forced and it has little supporting evidence. If it were 

 correct one could expect large quantities of salts to be 

 trapped above a ring (see Sec. 10, p. 57). 



Some have considered that a certain minimum amount 

 of transpiration is sufficient (this is implied in Livingston's 

 editorial footnote, Palladin, 1926, p. 150) and that, since 

 transpiration has not been completely eliminated in any 

 experiment, this minimum may ahvays have been exceeded, 

 and, for this reason, the experiments on the influence of 

 transpiration on salt absorption have shown no direct 

 relation between water absorption and nutrient absorption. 

 A very striking and direct relation has been observed, 

 however, between the rate of absorption and conduction 

 of dyes and inorganic salts introduced by injection or 

 through cut stems and the rate of transpiration. Further- 

 more, Birch-Hirschfeld (1919) found such conduction in 

 cut stems practically to cease when transpiration was 

 greatly reduced. The suggestion that a certain minimum 

 is always exceeded therefore seems to have no support. 



Though these studies of the influence of transpiration 

 on salt absorption and movement in intact plants could 

 hardly be considered as offering conclusive proof that 

 salts are not carried in the transpiration stream, they do 

 distinctly support the evidence obtained from ringing and 

 xylem-cutting experiments and cannot be criticized on the 

 grounds that the conducting tissues are interfered with. 



Haas and Reed (1927), on the other hand, have pub- 

 lished data which they think demonstrate a distinct 



