272 The Physiology of Plants bookiii 



to flow upwards. He denied that the root-pressure is a 

 contributing factor, calling attention to the fact that when 

 the current is at its maximum owing to vigorous transpira- 

 tion the vessels of the root contain air at a negative pressure. 

 In this particular, however, his reasoning is clearly falla- 

 cious ; though the negative pressure in the vessels exists, 

 it does not interfere with the osmotic activity of the cortex 

 of the root and the hydrostatic pressure resulting from it, 

 which we have seen to cause the filtration into the vessels. 

 The suction of the negative pressure no doubt aids the 

 filtration when it is once established, but the root-pressure 

 remains at work, as may be seen by cutting off the top 

 of the stem or branch, when the exudation of water com- 

 mences after a short time, during which the negative 

 pressure in the vessels of the root-stock becomes gradually 

 extinguished. The force of the root-pressure cannot there- 

 fore be ignored, as Sachs supposed, in considering the forces 

 at work in causing the ascent of the stream. His own 

 view, that nothing but evaporation of the water at the 

 head of the columns is necessary, apparently predisposed 

 him to regard it as superfluous when transpiration is 

 established. 



The hypothesis of Sachs that the water moves upwards 

 in the walls of the cells in consequence of the evaporation 

 from the leaves, did not long meet with general acceptance. 

 The presence of water in the lumina of the vessels in 

 varying quantity and mixed with air-bubbles of different 

 dimensions forming columns known as Jamin's chains, 

 which had been first determined by Hofmeister in 1857, 

 opened up probabilities of other forces at work and other 

 paths than those he postulated. Both Von Hohnel and 

 Boehm, in 1878, confirmed Hofmeister's observation that 

 the lumina of the vessels always contain water. Elfving 

 showed in 1882 that if the lumina of the vessels of a cut 

 branch are occluded by drawing or forcing cacao butter 



