118 TRANSLOCATION IN PLANTS 



solution was indicative of the composition of the material 

 supplying the growing point. Though these particular 

 investigators may no longer hold to the hypothesis that an 

 analysis of this solution will throw light on what sub- 

 stances are essential for maintaining growth of the shoots, 

 it seems desirable that the point be considered somewhat 

 further since the notion is widespread that these solutions, 

 bleeding from stumps, are solutions naturally moving to 

 and nourishing the growing shoots. 



It should be pointed out that the time of most vigorous 

 bleeding and highest sap composition coincides with a 

 period when there is little or no loss of water from the tops, 

 when there is little or no growth, and therefore at a time 

 when there is probably little or no natural flow even of 

 water. The concentration of bleeding sap rapidly falls at 

 about the time of the opening of the buds. This rapid dis- 

 appearance at this season has been noted by a number of 

 investigators among them Schroeder (1871), Jones et al. 

 (1903), and Moreau and Vinet (1923). It has not been 

 clearly established, however, whether the disappearance 

 precedes, accompanies, or follows the establishment of a 

 moving stream through the xylem. The commonly 

 accepted hypothesis is that the solution moves up to the 

 developing shoots which use the sugar and other solutes in 

 their growth, but it is not clear whether the disappearance 

 precedes or is a result of the flow. The evidence is fairly 

 clear, however (Curtis, 19206), that the developing shoots 

 are not dependent on food stored in the trunk, and the 

 ringing experiments also indicate that the food is not 

 carried longitudinally through the xylem. It would seem 

 more likely that the food stored in the xylem is moved out 

 radially to the phloem or cambium, and that it largely 

 disappears from the xylem vessels before the transpiration 

 stream is actively moving. 



The available evidence points rather clearly to the sug- 

 gestion that the occasional high sugar content of the xylem 

 is less related to transport, and is more related to conditions 

 influencing the ability of the storage cells to retain their 



