ASCENT OF SAP 215 



Cucurbitaceae. Bryonia and Cucurbita are found to show a rate 

 of 6 meters per hour. The rate is probably between one and two 

 meters per hour in ordinary broad-leaved trees. In other species 

 the rate may be as low as . 1 8 meter per hour. 



286. Mechanism by which Water is Conducted Upward through 

 Stems. Information concerning the factors operative in the con- 

 duction of sap from the roots to the crowns of tall plants is most 

 incomplete and unsatisfactory, and no adequate explanation may 

 be given of the phenomena involved. 



Only two sources of energy are known by which water may 

 be lifted from the soil to the crown of a tree : the force set free 

 in evaporation, and the osmotic action of the cells, from which 

 the transpiration has withdrawn water. The evaporation of water 

 from the walls of cells in leaves might be replaced by water of 

 imbibition drawn up the entire length of the plant in the wall ; 

 it would be utterly impossible to carry upward the enormous 

 amount of water actually thrown off by the plant in this manner, 

 however. Evaporation from the walls of a turgid cell in a leaf 

 would be replaced by liquid withdrawn from the cell sap. The 

 cell sap would thus be rendered more concentrated and would 

 set up a chain of osmotic attractions reaching to the tracheids. 

 The osmotic attraction of the sap of transpiring cells is capable 

 of exerting a force of 6 to 8 atmospheres, which would be suffi- 

 cient to lift water to the tops of tall trees if the liquid were 

 in the form of a solid vertical column. The tensile strength 

 of the solid column would prevent it from breaking if it were 

 continuous, but the column passes through the cavities of the 

 box-shaped tracheids, and exists in the cross walls as water of 

 imbibition. Furthermore the cavities of the tracheids are partly 

 filled with air bubbles of varying size, which would tend to weaken 

 the cohesive power of the column. The physical properties of 

 the column under such conditions are not easily predicated. The 

 theory of Westermaier that water is forced upward from one level 

 to another by the action of the living cells of the medullary rays 

 is found to be entirely unsupported, but interference with the as- 



