198 TRANSLOCATION IN PLANTS 



Table 23. — Percentage Increase of Final Dry Weight of Mesophyll 

 AND Vein on Initial Dry Weight 



(From Phillis and Mason) 



Illuminated mesophyll 47 . 15 



Illuminated vein 30 . 89 



Darkened mesophyll ■ 4 . 53 



Darkened vein • ■ 10 . 44 



ing sugar from storage tissues may have concentrations 

 of both sucrose and reducing sugars distinctly in excess 

 of the supplying storage tissues. Whether the concen- 

 tration in these tissues is higher or lower than in the 

 phloem or transporting tissue we have no evidence. It 

 would seem, therefore, that meristematic tissues, whether 

 they are elongating shoots or roots or growing storage 

 tissues, can receive sugars from supplying storage tissues 

 when the latter have lower concentrations. This ability 

 seems independent of whether the receiving tissue is basal 

 or apical to the storage tissue and the movement therefore 

 seems fundamentally distinct from the polar movement of 

 growth substance. If direction of flow is determined by 

 abiUty of receiving cells to concentrate against a diffusion 

 gradient — and this seems to be indicated by the available 

 evidence — actual concentration gradients, except within 

 the sieve tubes themselves, may have little to do with 

 direction of movement. It seems probable that the direc- 

 tion of movements within the phloem is actually deter- 

 mined by the concentration gradient in this tissue, as has 

 been so ably demonstrated by Mason and Maskell. On 

 the other hand, the actual concentration within receiving 

 and supplying cells outside of the conducting system proper 

 may not determine the direction of movement, while the 

 ability of the receiving cells to accumulate against a gra- 

 dient or to prevent loss may be of major importance. If 

 living cells other than those of the phloem differ in their 

 abilities to absorb sugars against a gradient, it is con- 

 ceivable that the living phloem cells themselves might 

 differ also, in which case they also may be able to transport 

 against a diffusion gradient. According to this hne of 



