150 TRANSLOCATION IN PLANTS 



at least, much or all of the nitrogen is converted into amino- 

 acids or proteins before leaving the roots. The only way 

 for these to reach the leaves would be through their resecre- 

 tion into the transpiration stream, but their amounts in 

 the xylem sap are extremely low and I know of no evidence 

 showing that living cells of higher plants readily excrete 

 organic nitrogen; although Mason and Maskell (1931, 

 p. 149) observed the organic nitrogen content of tracheal 

 sap taken from the cotton plant to be 4.43 mg. per 100 cc, 

 while the nitrate nitrogen content was 5.36, and ammonia 

 nitrogen 2.37. When the nitrogen supply in the soil is 

 high, it is conceivable that some nitrate would more easily 

 get across the living tissues into the transpiration stream 

 and be carried to the leaves, but when it is low it would 

 seem that the living cells of the roots and those along the 

 conducting tract would remove all available nitrogen from 

 the nonliving transpiration stream (see Sec. 14). The 

 readiness with which hving cells remove nitrate and certain 

 other ions from a very weak solution has been demon- 

 strated by Osterhout (1922), Hoagland (1923), Steward 

 (1932), and others. 



2. a. The pressure gradient that would seem necessary 

 to account for the movement especially for any great dis- 

 tance, as for example from the leaves of a tree to the roots 

 at a distance even as low as 10 m., would seem to preclude 

 the possibility of the movement being solely dependent 

 upon such a mass flow. Calculations by Crafts (1931) 

 indicate the necessity of great pressures to cause such a 

 flow. 



b. If solution enters the receiving cells by mass flow 

 through plasmodesma, as postulated by Miinch, this 

 resistance would be added to that of flow through sieve 

 plates, and still more pressure would be necessary. Fur- 

 thermore, in addition to all these resistances that must be 

 overcome by pressure to cause a mass flow, it would be 

 necessary to develop suflB.cient pressure in the receiving 

 cell to force water from this cell into the xylem. This 

 pressure must exceed the difference between the osmotic 



