52 THE SOIL AND THE SAP 



The simpler plants which live in water have no special 

 apparatus for the absorption or for the transpiration of the 

 water with its dissolved salts. This passes through the general 

 surface into the body of sea- weeds and other aquatic plants. 

 Even the higher vascular plants such as water-lilies, which live 

 in water, are to a great extent devoid of woody tissue, as the 

 water has no need to be conveyed from the root to the leaves. 

 It is in terrestrial plants where the absorption of water is 

 almost confined to the root that the conducting cells leading 

 upwards to the leaf are most highly developed. 



The capillary action plays but a small part in the ascent of 

 sap. A more important part is played by what is called root 

 pressure. If you cut across a growing stem of a vigorous 

 plant in the early part of the year sap may be seen exuding 

 from the cut surface of the lower half, and this cannot be 

 due to any "pull." It is attributed to pressure in the root. 

 But here again root pressure does not account for more than 

 a certain "lift" in the sap. It varies also from time to time, 

 and is not constant in all plants. Great fir trees have a very 

 small root pressure and yet their sap must be raised to very 

 considerable heights, and root-pressure frequently seems to 

 disappear in the height of summer when the demand for water 

 by transpiration from the leaves is at its greatest. 



Thus of the three forces which act in the ascent of sap, the 

 only two appreciable ones are root pressure, or the "push" 

 from behind, which is variable in amount and ceases at times, 

 and does not at best raise very many ounces. Many more 

 ounces are lifted by the "pull" from above, due to the viscous 

 nature of a column of water. The Avhole thing recalls the 

 poem: 



The Temple of Fame is open wide, 



Its Halls are always full. 

 Many get through by the door marked "Push," 

 But more by the door marked "Pull." 



The rate of ascent of sap varies under varying conditions. 

 The force which is available for moving the sap is about 

 800 atmospheres. The resistance maj^ be great or small 

 according to the supplies of water in the soil and the height 

 of the plant. 



