CHAP, i Processes of Absorption of Water 267 



early part of the period under review the views definitely 

 stated in 1876 by Wiesner were more or less acquiesced 

 in, it being held that such an enormous quantity of water 

 as passes through a tree is necessary for the supply of 

 a sufficient amount of the mineral constituents of the soil, 

 the solutions absorbed being extremely dilute. Transpira- 

 tion was regarded, therefore, as subordinate to, but con- 

 nected with, the nutritive processes. This view is supported 

 by the fact that such plants as exhibit the most active 

 transpiration are for the most part of considerable bulk, 

 but there is very little direct evidence forthcoming on the 

 point. 



The observations of Hales and others of the older writers 

 that the phenomenon of the bleeding of cut stems cannot be 

 observed in summer time received an explanation in con- 

 nexion with work on transpiration, and a further function 

 of the process came to light. Hales, Meyer, and Sachs all 

 spoke of the evaporation of the water forced by root- 

 pressure into the vessels of the wood, and the last-named 

 writer suggested that such evaporation would lead to the 

 setting up of a negative pressure of the air which gradually 

 replaces the water. Experiment showed Sachs' view to be 

 well-founded, for definite proof was afforded by Von Hohnel 

 in 1876. He cut under mercury the stem of a transpiring 

 plant and found the metal rise at once to a height of several 

 centimetres in the vessels, particularly those most recently 

 formed. Von Hohnel calculated the force of this negative 

 pressure or suction to be from one-half to one-third of an 

 atmosphere. 



This discovery led botanists almost immediately to 

 attribute to the transpiration process a share in the pro- 

 pulsion of water along the axis of the woody plant. 

 Vesque took up the subject in 1880, and showed that 

 such suction materially aids the root-pressure in driving 

 water into the cavities of the vessels. He made the 



