502 TRANSLOCATION OF SOLUTES 



in a number of tree species, and, with only negligible exceptions, finds them 

 to be distributed in accordance with the requirements of the Miinch theory. 



3. The mass flow theory will account for translocation in the phloem in 

 only one direction at a time. There are some indications, however, that 

 translocation in the phloem is simultaneously bi-directional. Phillis and 

 Mason (1936a) claim to have demonstrated that organic forms of nitrogen 

 travel up in the phloem of the cotton plant, at the same time that carbohy- 

 drates are moving in a downward direction through the same tissue. Palmquist 

 (1938) has apparently demonstrated that the dye fluorescein will move in 

 one direction through the phloem of bean leaves while soluble carbohydrates 

 are moving through the same phloem in the opposite direction. 



If bi-directional movement of solutes occurs in the phloem this probably 

 means bi-directional movement in individual sieve tubes. In view of the 

 intimate inter-relationships between phloem elements it seems improbable that 

 solutes could move upward only in certain cells or elements and downward 

 only in others. 



2. The Streaming of Protoplasm Theory. — De Vries (1885) and other 

 nineteenth century investigators postulated that streaming of the protoplasm 

 in the cells of the phloem might explain the relatively rapid rate of transport 

 of solutes. In recent years this theory has been supported by Curtis (1935). 

 The basic assumption is that rotational streaming of the protoplasm occurs in 

 the sieve tube elements, and that solute molecules, caught in the protoplasmic 

 matrix, are carried by this protoplasmic movement from one end of the ele- 

 ment to the other. The molecules are usually assumed to pass from one sieve 

 tube to the next by diffusion, presumably largely through the sieve plates. 

 Diffusion over such short distances can occur very rapidly even if the molecules 

 are moving along a diffusion gradient which is not very steep. Some advocates 

 of this theory have even postulated that streaming protoplasm may be con- 

 tinuous from sieve tube to sieve tube through communicating pores. This 

 theory would account for simultaneous movement of solutes in both upward 

 and downward directions in the same sieve tube. 



Although this theory has been rather strongly advocated the positive evi- 

 dence in its favor is not very convincing. Various types of experiments in- 

 dicate that the cells of the phloem must be alive in order for translocation 

 to occur. For example, Curtis and Herty (1936) have demonstrated that 

 movement of carbohydrates out of bean leaflets is markedly reduced when 

 the petiole is chilled to a temperature between 0.5 and 4.5° C. At 7-11° C. 

 translocation was faster than at temperatures close to 0° C. and at 17-24° C. 

 it occurred still more rapidly. Curtis (1929) showed that enclosing the stems 

 of bean plants in an atmosphere of nitrogen checked transport, indicating thai 



