THE METHOD OF MOVEMENT 151 



concentration of the receiving cell and the suction tension 

 (turgor deficit) of the solution in the xylem. This latter 

 is likely to approach zero; therefore solution must be 

 forced into the receiving cell with sufficient pressure to 

 develop an internal hydrostatic pressure in this cell 

 greater than the osmotic value of the cell contents on the 

 face next to the water-conducting tracts. To maintain 

 this excess pressure, the turgor in the sieve tubes must be 

 exceedingly high and the turgor in the supplying leaf cells 

 must be still higher ; for it is at this point that the high 

 pressure gradient is initiated and maintained. Since the 

 receiving cell is assumed to have a low osmotic concentra- 

 tion, either because the materials are used up in respiration 

 and formation of cell parts or because their osmotic con- 

 centration is reduced by condensation into storage prod- 

 ucts, the pressure necessary to force water out of this cell 

 into xylem is supposed not to be great. Experimental 

 evidence presented under (3), however, demonstrates high 

 osmotic concentrations in receiving cells. 



c. For transport, according to the hypothesis of Miinch, 

 pressures at the source must increase directly with the 

 distance to which the material is to be transported. Pres- 

 sures may seem great enough to force phloem contents a 

 few centimeters or even decimeters, but in order to bring 

 about flow over great distances, as in our tallest trees or in 

 vines where distances up to 100 m. or more are concerned, 

 excessive pressures would seem to be necessary. Although 

 in trees there seems to be a tendency for an increase in 

 osmotic concentration of leaves at greater heights, this 

 increase is not uniform, is at best not very great, and seems 

 to bear no relation to the distance through which foods 

 must move. MacDaniels and I have obtained a few data 

 bearing on this point. By use of the freezing-point method 

 the osmotic concentrations of grape leaves on shoots arising 

 5 to 8 m. from a short trunk were compared with those on 

 shoots arising directly from the trunks. The data are 

 presented in Table 18. Although in all but one case the 

 leaves at a greater distance had somewhat higher concen- 



