MECHANISM OF SOLUTE TRANSLOCATION 501 



living cells of the phloem. In some species such as cotton, which have been 

 examined for this phenomenon, it has not been found possible to demonstrate 

 phloem exudation. Some investigators have suggested that such exudations 

 are abnormal flows resulting from cutting open the phloem system, but it 

 seems difficult to apply this interpretation to some of Crafts' results. In one 

 of his experiments the sap exuded in 24 hours was equivalent to the total 

 phloem volume in 189.9 cm. of stem. Furthermore phloem exudation could 

 still be shown in stems which were distinctly wilted, indicating that it is very 

 unlikely that the flow of sap from the phloem could be due to the pressure 

 exerted by adjacent cells as is sometimes suggested. 



Although such phloem exudations are undoubtedly very real phenomena, 

 and their occurrence has been demonstrated in many species, it cannot yet be 

 stated with certainty that the actual mechanism of such flows is that postulated 

 in the mass flow theory of translocation. 



Curtis (1935) has pointed out some very serious objections to this hypoth- 

 esis. Several of the more important of these will be summarized. 



1. Resistance to the mass flow of solution through a tissue such as the 

 phloem would undoubtedly be very great. The solution must flow through 

 numerous cross walls, and through the cytoplasmic membranes of the sieve 

 tubes. If the solution moves en masse through the plasmodesms of the root 

 and leaf cells, as postulated by Miinch, this would add greatly to the resistance 

 which must be overcome. Resistance to passage through parenchyma or com- 

 panion cells would be even greater than in sieve tubes. It seems unlikely 

 that turgor pressures develop in leaves or other supplying tissues of sufficient 

 magnitude to move a solution for any very great distance against the resistance 

 it would encounter in the phloem tissue. 



2. The IVIiinch hypothesis requires that the supplying cells have a higher 

 turgor pressure than the receiving cells if there is to be a pressure gradient 

 from the former to the latter. Such a gradient could seldom be maintained 

 unless the osmotic pressure of the supplying cells is greater than that of the 

 receiving cells. The investigations of Curtis and Scofield (1933) indicate 

 that this is not always true. According to their results, in the onion, potato, 

 bean, and other species, the osmotic pressures of the receiving tissues are greater 

 than those of the storage tissues at times when translocation is occurring from 

 the latter to the former. This is also true in the sugar beet in which the 

 sugar content of the root cells is much higher than that of the leaf cells or 

 the phloem. This objection to mass flow theory may be met, however, by the 

 assumption that the receiving cells often or generally are capable of accumulat- 

 ing organic solutes against a concentration gradient. On the other hand 

 Pfeiffer (1937) has made an extensive investigation of the osmotic pressures 



