196 



TRANSLOCATION IN PLANTS 



present in the relatively quiet parts, perhaps in the vacuole 

 where they may be present as temporary storage materials. 

 Phillis and Mason (1933) have presented evidence that 

 is of considerable importance in its bearing on concen- 

 tration gradients and transport. They analyzed saps 

 expressed from various parts of cotton leaves which were 

 divided into lamina (excluding midribs), veins, and petiole. 

 The petiole was also further subdivided into "outer bark," 

 "inner bark" (principally phloem), and "wood." In their 



1.0 



0.8 



0.6- 



o0.4 

 CI 



E 

 ^0.2 



C.G-l 





a.m. fxm. a.m. p.m. cum. p.m. am. p.m. a.m. pm. nxn. p.m. 



1 



2 ' 3 ■ 4 ' 5 ■ 6 • 



Days 



Fig. 12. — Daily variations in sugars (grams per 40 leaves) of lamina and petiole. 

 (From Phillis and Mason.) 



analyses they distinguished between sucrose, glucose, 

 fructose, and an unidentified polyglucoside. As shown in 

 Fig. 12, they found marked daily fluctuations in sucrose 

 and but shght fluctuations in the other constituents. They 

 present strong evidence that sucrose is the transport 

 sugar, and, what is of special interest to the present sub- 

 ject of discussion, they found what seems conclusive 

 evidence that the sucrose concentration of the veins is 

 much higher than that of the mesophyll from which sugar 

 is received. There is therefore a reverse gradient, and it 



