152 



TRANSLOCATION IN PLANTS 



trations than those close to the trunk, the differences were 

 slight. On the average the leaves at the greater distances 

 could have a maximum pressure of only 1.15 atmospheres 

 above those close to the trunk and yet they were from 5 to 

 8 m. farther from the trunk or roots to which they were 

 sending sugar. This would indicate a gradient of less than 

 0.2 atmosphere per meter, which seems inadequate. 



Table 18. — Freezing-point Depressions of Sap of Grape Leaves from 



Shoots Growing on Canes 5 to 8 M. from a Short Trunk and 



PROM Those Growing Directly on the Short Trunk 



Leaves from long canes . . 

 Leaves from short canes. 



Diff. long-short -0.040 +0.150 +0.245 +0.065 +0.120 +0.030 +0.095 av. 



Max. difF. in pressure, 



atmosphere -0.48 1.81 2.95 0.78 1.45 0.36 1.15av. 



Mlinch's own observations and those of Haller (1931), 

 that apple fruits on defoliated branches at a distance of 

 5 ft. or more from leaves grew as well as other fruits close 

 to the leaves, would be difficult of explanation on a pressure 

 gradient hypothesis. 



3. a. Disregarding the seriousness of the criticism that 

 the pressure gradients that might be available may not be 

 adequate to overcome the resistance to be met in causing a 

 flow through the phloem and into the receiving cells, there 

 still remains the added difficulty that the naturally existing 

 pressure gradients, or concentration gradients, may not 

 lead in the same direction as the movement of materials. 

 The Miinch hypothesis demands a higher turgor pressure 

 in the supplying cells than in the receiving cells so that 

 there will be an actual pressure gradient in the proper 

 direction. To maintain such a pressure gradient, the 

 supplying cells must ordinarily have an osmotic concentra- 

 tion greater than that of the receiving cells. Possible 

 exceptions to this rule will be discussed shortly. Experi- 

 mental evidence, however, demonstrates that receiving 

 cells frequently, and, when storage tissues are supplying 

 growing tissues, may consistently have higher osmotic 



