THE WATER-RELATION 



107 



that very high tensions are necessary to break down the cohesion 

 of such a column of water. 



In consequence of its cohesion, a column of water could be pulled 

 up the tube supporting it if suction were to be applied to the top 

 of the column ; just as a vertical steel rod can be lifted by its upper 

 end. In the plant, the vessels of the xylem provide a series of rigid 

 tubes, through which continuous columns or threads of water may 

 stretch from the leaves down to the roots. While it is true that 

 individual vessels are of limited length only, yet the walls separating 

 one vessel from its neighbours are thoroughly per- 

 meated with water, and this provides for continuity 

 of the water columns. When, as described above, 

 the leaf-cell abstracts water from the xylem of the 

 vein, the abstracted molecules draw up more behind 

 them by cohesion. It is suggested that the wholesale 

 abstraction of water from the leaf -veins during trans- 

 piration pulls the water of the transpiration stream 

 up the trunk or stem. That at least is how the 

 Cohesion Theory explains the ascent of the water. 

 Summarising, we may say that the ascent of the water 

 is regarded as being mainly initiated by an osmotic 

 suction arising in the leaf- cells as the result of trans- 

 piration, and this pulls the water up by virtue of the Fkj 

 cohesive properties of water enclosed in rigid tubes. Experiment to show 



. , c cohesion of water. 



There is considerable evidence in favour 01 (After Dixon.) 

 this theory. Calculation shows that the leaf-cells, 

 with an osmotic pressure of 10 to 20 atmospheres, are in a position 

 to exert an osmotic suction sufficient to account for the ascent of 

 the transpiration stream to the top of the tallest tree, while the 

 cohesive power of water is ample to withstand the resulting tension. 

 The theory implies that the water in the vessels is in a state of 

 tension, not of pressure as would be the case if the water were being 

 forced up from below. A simple experiment confirms this. If a 

 seedling is placed with its roots and lower part of the stem in eosin 

 solution, and a cut be made across the stem below the level of the 

 dye, it will be observed that the eosin ascends the plant very quickly, 

 and in less than a minute may appear in the veins of the leaf. 

 Evidently the opening of the conducting strands by cutting relieves 

 a state of tension and leads to a rapid absorption of the dye. 



The Cohesion Theory requires the existence of continuous columns 

 of water stretching right through the plant. These must be free of 



