THE METHOD OF MOVEMENT 



197 



seems that there must be some mechanism by which 

 phloem can accumulate sucrose against a steep reverse 

 gradient. They suggest that the companion cells and the 

 enlarged companion-hke cells that surround the sieve tubes 

 in the fine veins may be responsible for the accumulation 

 of sucrose in the sieve tubes. Schimper (1885) gives 

 evidence indicating that these cells are capable of absorb- 

 ing sugar against a concentration gradient. After entering 

 the phloem it seems that sucrose moves along a positive 

 gradient. This is shown in Table 22. 



Table 22. — Concentrations in Various Parts of Leaf 

 Grams per 100 cc sap 



(From Phillis and Mason) 



In order to reverse the sugar movement they darkened 

 10 per cent of the leaves, then a day later ringed the bases 

 of the plants, and followed by taking, for a period of 4 

 days, daily samples of those leaves that were darkened and 

 those that were illuminated. The changes in composition 

 over the 4-day period are shown in Table 23. It is indi- 

 cated by these data, as well as by others presented in the 

 original, that sugar has moved rather freely into the 

 petioles and veins of the darkened leaves but only very 

 sUghtly into the mesophyll. In fact, the small gains 

 recorded may have been mostly in the finer veins. This 

 evidence also supports the suggestion that there is some 

 mechanism effective in moving sugar into the sieve tubes 

 against a concentration gradient, and in preventing its 

 movement back into the mesophyll. 



In material not as yet published Scofield and I have 

 obtained considerable evidence that growing tissues receiv- 



