112 DIFFUSION AND OSMOTIC PRESSURE 



of which dipped in mercury, was sufficient to lift a column of 

 the latter liquid higher than the barometer column at the 

 time of the experiment. Askenasy was able to construct 

 apparatus whereby he could demonstrate a pressure of 90 cm. 

 of mercury arising from evaporation of water from a satu- 

 rated plaster of Paris plate. Copeland constructed a column 

 of plaster of Paris 3mm. in diameter and 12.4m. high, 

 which terminated below in a mercury manometer and above 

 in a Cu 2 Fe(CN) 6 membrane. The whole column was as 

 nearly saturated with water as might be (there were many 

 air-bubbles, however), and the membrane above was covered 

 by an exposed solution of CuSO 4 . Evaporation from the 

 surface of this solution caused, in five days, a suction on the 

 manometer below of 301 mm. of mercury. Essentially this 

 apparatus is a bundle of minute water columns held in the 

 pores of the plaster, but broken here and there by air bubbles. 

 There was certainly enough water in the column to give a 

 pressure of more than 459mm. of mercury (1 atmosphere 

 minus 301 mm.). If this be true, the suction set up by evap- 

 oration above surpassed the pressure of an atmosphere. 



This theory of sap ascent seems to be gaining ground, and 

 it is quite probable that the idea of Godlewski and Janse 

 will eventually be put entirely aside. The experiments of 

 Strasburger and Dixon show that osmotic pressure must be 

 active in the leaves in order that sap may ascend at its usual 

 rate. Another proof that living protoplasm is necessary in 

 leaves lies in the fact that transpiration can be influenced by 

 ansesthetics. 1 Such reagents act in a similar manner to the 



1 For a discussion of the relation of anaesthetics to transpiration see H. H. 

 DIXON, "On the Effects of Stimulation and Anaesthetic Gases on Transpiration," 

 Proceed. Roy. Irish Acad., Ser. Ill, Vol. IV (1898) ,pp.618-26 ; H. JUMELLE, "Influence des 

 anesth6tiques sur la transpiration chlorophyllienne," Rev. gen. bot., Vol. II (1890), 

 pp. 417-32; IDEM, "Nouvelles recherches sur I'assimilation et la transpiration 

 chlorophylliennes," ibid., Vol. Ill (1891), pp. 241-88, 293-305 ; A. SCHNEIDEB, "The 

 Influence of Anaesthetics on Plant Transpiration," Bot. Gaz., Vol. XVIII (1893), pp. 

 57-69; A. WOODS, "Some Recent Investigations on the Evaporation of Water from 

 Plants," ibid., Vol. XVIII (1893), p. 304-10. 



