CHAP, i Processes of Absorption of Water 273 



into them, thereby making them impassable by water, the 

 leaves wither very rapidly. Vesque, during the next 

 year, compressed the vessels of a cut branch by a clamp 

 so strongly as to occlude them. They were then unable 

 to conduct water upwards, but regained the power when 

 the clamp was removed and the lumina became re-opened. 

 This observation was repeated by Kohl in 1885 and by 

 Strasburger in 1891. 



Sachs' suggestion of the part played by imbibition soon 

 met with vehement opposition, though for some time 

 nothing at all satisfactory took its place. Boehm, in 1878, 

 restated the older physical ideas ; he said that the move- 

 ment of water set up by transpiration is a function of the 

 elasticity of the cell walls and the atmospheric pressure in 

 parenchymatous tissues filled with sap ; in cells with rigid 

 walls the elasticity of the latter is replaced by that of air 

 shut up in the cells. In another place he said, ' the move- 

 ment of the water set up in plants by transpiration is 

 a phenomenon of filtration depending on the pressure 

 differences in neighbouring cells.' Though many botanists 

 took part in the controversy, particularly Elfving, Von 

 Hohnel, Russow, and Vesque, no uniformity of opinion 

 was arrived at ; indeed we find Vesque writing that of all 

 the theories devised to explain the movement of water in 

 the plant, that of Boehm is the least in agreement with 

 the facts observed. In 1889 Boehm came to regard capil- 

 larity as a sufficient explanation. 



Nothing material was contributed to the elucidation of the 

 problem till about the years 1883-4, when Westermaier again 

 brought forward the view of a special mechanism set up by 

 the plant, and argued against mere physical agencies being 

 solely concerned in the matter. His view of the nature of 

 the mechanism differed from that of Sachs, inasmuch as he 

 attributed the prominent part in the process to the pumping 

 action of living cells. At the outset he criticized the theory 



GREEN 



