230 TRANSLOCATION IN PLANTS 



while those with lower turgor and nearer the source of supply fail to receive 

 the solutes. (4) The proposed mechanism seems not adapted to account 

 for movement of specific substances to specific cells or tissues, but all must 

 move together. (5) The removal of water from the receiving cell if it is 

 in the position of the cambium may be easily explained, but removal from 

 apical growing points or from cortical storage tissues is not easily accounted 

 for. (6) If the proposed mechanism is correct, it is difficult to account for 

 the failure of leaves, or stems bearing leaves, to empty carbohydrate when 

 the cut petiole or stem is placed in water. Under such conditions the pres- 

 sure gradient through the phloem should be steepened because the distance 

 of flow is shortened, the phloem is cut open reducing the resistance, and the 

 supply of water through the xylem is increased. (7) Local chilling of the 

 petiole or stem does not stop exudation from cut phloem, yet it does prevent 

 emptying when the leaf or stem is attached to the plant. The method of 

 flow, rate of flow, and direction of flow in cut phloem may be strictly abnor- 

 mal. (8) A pressure that would cause a unidirectional mass flow through 

 plasmodesma or sieve pores would probably cause a flow of the protoplasm 

 that lies across these pores, would not allow for its return, and would not 

 allow for transport of the vacuolar contents where the osmotic pressure is 

 supposedly developed. Because of its many weaknesses the Miinch hypo- 

 thesis seems untenable. 



31. From measurements of size of pores of sieve tubes. Crafts calculated 

 pressure gradients much in excess of those available would be necessary to 

 cause a flow of exudate at the rates actually observed with cut stems. Since 

 the total cross-sectional area of cell walls is greatly in excess of that of the 

 sieve pores, and even of the sieve tubes themselves, and since their walls in 

 the fresh condition are thick and greatly hydrated, he first suggested that 

 the hydrated walls acted as the path of transport. Because of the great 

 resistance to flow through hydrated walls and since the protoplasm seemed 

 completely permeable to solutes, he later suggested that flow takes place 

 principally through the sieve-tube lumen, but that flow takes place freely 

 across all side and end walls and is not restricted to flow through sieve pores. 

 The actual flow through sieve tubes is supposed to take place by a mechan- 

 ism similar to that proposed by Miinch with the exception that it is assumed 

 that neither flow out of supplying cells nor entrance into receiving cells 

 takes place through plasmodesma. The receiving cells are assumed to be 

 capable of absorbing against a concentration gradient, and absorption is 

 assumed to be independent of turgor. 



32. Most of the weaknesses attendant upon the hypothesis of 

 Miinch apply equally well to that of Crafts. Attempts to obviate 

 certain weaknesses of the former are based on assumptions with 

 little supporting evidence and have introduced other weaknesses 

 equally serious. The claim of low resistance to flow through walls is not 

 substantiated, nor is the claim that the protoplasm of sieve tubes is com- 

 pletely permeable. Although his suggestions and conclusions seem not 

 to be tenable, he gives valuable data on phloem anatomy and his observa- 

 tions and calculations on exudation, as well as those of others, can be inter- 

 preted to indicate that exudation from phloem and other cells is not normal 

 but results from sudden release of pressures by cutting. Various claims by 



