186 TRANSLOCATION IN PLANTS 



strated that other solutes are rapidly carried through the 

 phloem. Schumacher (1933) has offered rather conclusive 

 proof that fluorescein is transported through sieve tubes 

 with great rapidity, 5 to 6 mm. per minute, and that this is 

 probably transported exclusively by the protoplasm (see 

 Sec. 37). He suggests, however, that protoplasmic move- 

 ment itself does not carry the dye. 



d. If materials are transported through the phloem by 

 protoplasmic streaming it would seem that different 

 materials would move in opposite directions simultaneously, 

 and it has not been clearly demonstrated that transport 

 through the phloem does take place in both directions 

 simultaneously. The exudation from cut phloem undoubt- 

 edly results from a unidirectional flow, but this may be 

 strictly abnormal and due merely to opening a system 

 which is under high pressure. The observations of Hartig 

 (1861), however, that exudation ceases when a second 

 cut is made above the first, but does not cease when the 

 second cut is below, strongly points to a normal flow in 

 one direction. If, on the other hand, simultaneous move- 

 ment in both directions is demonstrated, the mechanisms 

 which demand a unidirectional flow, such as those of 

 Miinch and Crafts, would have to be abandoned. If, 

 however, unidirectional transport is proven the proto- 

 plasmic streaming hypothesis falls down or would require 

 modification. It is, of course, conceivable that the proto- 

 plasm moving in one direction is different, perhaps bearing 

 a different charge from that moving in the opposite direc- 

 tion and may thus carry a given substance in one direction 

 only. There is some evidence strongly pointing to a 

 unidirectional movement of specific substances. Bennett 

 (1927) reports experiments in which the leaf curl virus of 

 raspberries is shown to move through the phloem in but 

 one direction, and this direction seems to be determined 

 by the direction of major movement of carbohydrates. 

 Caldwell (1931), working with a virus on tomato, has 

 demonstrated simultaneous movement in both directions 

 and at approximately equal rates. However, it seems 



