196 TRANSPIRATION AND ASCENT OF SAP ch. 



Experiments 80 and 81 were made on sap from the leaves 

 of an old Wistaria trained on a low wall. One sample of 

 leaves was gathered from short branches near the base of 

 the main stem. The leaves were about three feet over 

 the ground. The second sample of leaves was taken from 

 the terminal branches of a stem running 65 feet approxi- 

 mately horizontally along the wall at a level of about 

 three feet. Here again we find a slight difference in 

 pressure in favour of the distal leaves. 



Table 33. 

 Ulmus campestris : sap from untreated leaves. 



The numbers in Table 33 show the real meaning of the 

 results which had apparently indicated that the level was 

 the controlling factor in determining the osmotic pressure. 

 If the hydrostatic head defined the pressure of the leaves, 

 it is evident that the pressure in the leaves examined 

 in Experiment 21 should have been the greatest ; if the 

 resistance of the water-tracts were the controlling factor, 

 those of Experiment 23 should have had the maximum 

 pressure, which should have been much greater than those of 

 22 and 24. The actual order is 24, 23, 21, 22. From this 

 it is clear that the resistance of the water-tracts was not 

 the controlling factor of the pressure ; accordingly some 

 other cause for its variation must be sought. This cause 

 seems to be principally the fluctuations in the sugar- 

 content of the leaves due to difference in illumination. 



