134 TRANSLOCATION IN PLANTS 



evidence and the fact that the sieve tubes have a high 

 content of colloidal material, part of which may be very 

 viscous, seemed to preclude the possibility of a unilateral 

 mass flow of sieve-tube contents. 



It is possible, however, that the conditions necessary for 

 such a flow have not been met experimentally. Birch- 

 Hirschfeld attempted to draw solutions through phloem 

 strips by causing rapid evaporation from the leaves. This, 

 however, would tend to place the water columns under 

 tension, and although water would tend to be drawn from 

 the phloem and thus put its contents also under reduced 

 pressure or even tension, it would not insure a pressure 

 gradient through the phloem. In experiments with stems 

 of Philadelphus, from which the xylem was cut out and the 

 phloem left intact, I was not successful in drawing dye 

 solutions by suction through the region lacking xylem 

 (Curtis, 1925). In a later experiment with Rhus, however, 

 it was found possible by the same method of suction to 

 draw a dye solution through the region lacking xylem but 

 this movement seemed to be limited to the latex tubes. 

 It seems that sieve tubes have rather pliable and non- 

 rigid walls, very different from those of the xylem. An 

 attempt to force liquids through such tubes by methods 

 used by Birch-Hirschfeld, or by cutting across them and 

 applying solutions under pressure at one end, may fail 

 largely because of this lack of rigidity. Any one who has 

 attempted to force liquid or gas by either pressure or 

 suction through such a nonrigid, thin-walled, rubber tube 

 will recognize how such flexibility, allowing for the collapse 

 of the tube, may be the chief difficulty that can be over- 

 come only by using some method to hold the tube open. 

 Dixon (1933) by using higher pressures than have previ- 

 ously been used, that is about 3 atmospheres, has reported 

 injection into sieve tubes for distances up to about a 

 centimeter. 



A normal sieve tube with its high solute content is likely 

 to be highly turgid and distended by the internal turgor 

 pressure which will, of course, be dissipated when the 



