xi ENERGY AVAILABLE FOR RAISING SAP 209 



drawn to the menisci, and the tension is generated and 

 maintained in the liquid. It is clear that the capillary 

 forces of the menisci must be sufficiently great to support 

 the tension needed to raise the water ; hence the fine 

 grain of the walls of the transpiring cells forms an essential 

 link in the mechanism which utilises the energy entering 

 at the evaporating surfaces in the leaves. 



Effect of tension in sap on evaporation. 

 When the heat entering at the evaporating surfaces is, 

 as just described, the source of the energy which raises 

 the transpiration current, it is evident that less water 

 will be evaporated from these surfaces than from similar 

 surfaces under similar conditions, but relieved of the 

 work of drawing the water through the conducting 

 channels. Accordingly it is of interest to inquire what 

 proportion the work of raising the sap bears to that of 

 evaporation, or, in other words, how will the tension in 

 the sap retard evaporation from the transpiring cells. 



As was shown previously, each cubic centimetre of 

 water given off from the leaves of a tree 100 m. high 

 requires an expenditure of work amounting to 0'5 cal. 

 for transporting in the conducting tracts one cubic centi- 

 metre from the roots to the leaves. 



To evaporate a cubic centimetre of water at 20 C. 

 requires 592'5 cal. Therefore the work done in transport- 

 ing the water from the roots to the leaves of a 100 m. 

 tree will not require more than the one-thousandth part 

 of the energy required for the evaporation of the water. 

 Hence to obtain the energy needed to raise the water 

 in a 100 m. tree the amount evaporated will only be 

 diminished by one-thousandth part. Even taking the 

 highest and certainly excessive estimate of the resistance, 

 the amount evaporated will only be reduced by one-thirtieth, 

 if, in addition to evaporation, the energy absorbed by the 

 leaf has to do the work of transporting the water from 

 the roots. 



