UPWARD TRANSPORT OF NITROGEN 57 



evidence, from the experiments in which larger stems were 

 ringed and leaves were left above the ring, that ringing 

 does interfere with upward transport of salts, even when 

 there is no injury to the xylem. It is true that all of these 

 experiments are open to the criticism that transpiration 

 from the leaves above the ring is likely to be lessened, but, 

 as pointed out earlier in this section, this difference in 

 transpiration would seem inadequate to account for the 

 great differences observed, especially in view of the trapping 

 effect of the ring in preventing reexport from the parts above it. 



If transport through the phloem is unidirectional at any 

 one time and place (see Chap. V), as assumed by Miinch 

 (1930) and by Crafts (1931), then it is obvious that, while 

 carbohydrates are moving down from the leaves to the 

 roots, any upward transport of salts from the soil must 

 take place exclusively through the xylem. Under such 

 conditions it would not be possible for organic or inorganic 

 material of any sort to move up through the phloem. If, 

 on the other hand, simultaneous movement in both direc- 

 tions through the phloem of the main stem is demonstrated, 

 then, although small amounts of any solutes that may 

 leak into the xylem may be carried in the transpiration 

 stream, still the major part of the upward transport may 

 take place through the phloem at the same time that a 

 downward movement is taking place through the same 

 tissues. 



If carbohydrates are carried almost exclusively through 

 the phloem — and even those that may be stored in abun- 

 dance in the xylem regions are not transported through 

 the xylem (see Sees. 4 and 5), and the evidence points very 

 strongly to such a conclusion — it seems unlikely that the 

 plant can have developed a mechanism that would carry 

 one type of solute exclusively through the xylem and 

 another through the phloem. 



The findings of Auchter (1923), although not so clear-cut 

 in their indications as to the tissues concerned in transport 

 as are these ringing experiments, yet at the same time may 

 be considered to support the contention that the movement 



