108 TRANSLOCATION IN PLANTS 



force solutions through the phloem in appreciable quantities and because 

 the narrowness of the sieve tubes, their frequent cross walls, and their high 

 content of viscous material seem to preclude the possibility of movement 

 through them at adequate rates. On the other hand, solutions can be 

 easily caused to flow with great rapidity in either direction through the 

 xylem. These tissues have also apparently been found to contain appre- 

 ciable quantities of sugar. Although the phloem seems inadequate as a 

 path for transport of appreciable amounts of sugar, evidence cited in Chap. II, 

 where it was conclusively demonstrated that it can carry materials at 

 adequate rates, even though the mechanism may not be understood, removes 

 the major objection to transport through the phloem. Although it has 

 been claimed that sugar is present in xylem vessels during midsummer, this 

 claim is not well substantiated. There is no evidence that solutions or 

 water normally move backward through the xylem, and even if such flow 

 should occur, no mechanism has been proposed whereby the backward 

 flowing solution can be directed to the tissues using the foods. 



