396 The Ascent of Water in Trees, 



the vessels, these are always longer, and sometimes considerably longer, than 

 the piece of stem in which they occur. 



Experiments on the suction and exudation of trees at different levels, and 

 upon the influence of the entry of air and water under pressure, showed that 

 no continuous suspended water columns, or high internal tensions, existed in 

 the conducting elements of the trees experimented on (Maple and Poplar) 

 during active transpiration, or, indeed, at any period of the year. 



The same was shown by direct measurements of the pressure in intact 

 vessels of Wistaria during active transpiration. This fact, coupled with the 

 high total resistance to flow, shows that this resistance is overcome locally 

 from point to point, and not by any enormous tension from above or pressure 

 from below, neither of which exists, nor could be maintained to a sufficient 

 extent to carry on the elevation of water in a tall tree. A high tension from 

 above leads to rapid blocking with air ; a high pressure from below leads to 

 great loss by lateral exudation from the vessels. 



The apparent differences of osmotic pressure previously observed between 

 leaves at the base and apex of a tall tree are not evidence of the existence of 

 any greater osmotic suction at the higher level, but are the result of the 

 different ages and sizes of the cells, and of the different conditions to which 

 they have been exposed. Equally great differences may exist between leaf- 

 cells at the same level. 



The surface adsorption of dissolved solids in the vessels plays a very 

 important part in their function as translocatory channels, causing a delay 

 in the ascent of dissolved solids, such as sugar and salts, and an accumulation 

 of them along the outer walls of the vessels. The latter facilitates their 

 outward diffusion, but at the same time renders the transference of 

 small quantities of material between widely removed organs difficult or 

 impossible. 



The tallest trees in Australia do not appreciably exceed 300 feet in height, 

 so that the values previously given for the maximal total resistance to the 

 upward flow of sap in actively transpiring trees must be reduced to between 

 30 and 50 atmospheres. 



In a Maple tree whose younger wood had been killed by formalin the 

 transpiration current turned to the older, partially blocked, but still living 

 wood, and none of Strasburger's experiments show definitely that efficient 

 conduction is possible in the dead wood of tall trees. The ascent of water 

 is, therefore, a vital problem, in so far as it depends upon conditions which 

 hitherto can only be maintained in living wood. 



