xi ENERGY AVAILABLE FOR RAISING SAP 203 



through the water tracts and secreting it on the outside 

 of the leaf -cells. 



No energy need be spent in separating the water from 

 the solution in the cells, inasmuch as there is a constant 

 supply of water on the inner side bordering on the tracheae, 

 and the water passing out to the seat of evaporation is 

 immediately replaced by that coming in from the vascular 

 bundles. 



Hence, taking into account only respiratory energy, we 

 have available for raising the water in a tree and exposing 

 it for evaporation something approximating to one calorie 

 for every gramme of water given off. 



It has previously been shown that the resistance to be 

 overcome in moving the transpiration current through the 

 stems of trees is not much more than equivalent to a head 

 of water equal to the length of the stem. Hence, as each 

 cubic centimetre of water given ofr from the leaves of a 

 tree 100 m. high requires an expenditure of work to the 

 extent 100 x 100 gr. cm. to lift it, we must add the same 

 quantity of work to overcome the resistance of the con- 

 ducting tracts ; and the total work for raising a cubic 

 centimetre in the tree will be about 2 x 10 4 gr. cm., or in 



2 x 10 4 

 calories 77^ 77^, i.e., about 0'5 calorie. 

 428 x 10" 



As we have seen, the respiratory energy of the leaf 

 supplies something of the order of one calorie for each 

 cubic centimetre of water given off and hence would be 

 quite adequate to do the raising of the sap from the root 

 to the leaves. 



Raising of sap by evaporation. We have already 

 seen that under certain conditions, e.g., when evapora- 

 tion from the transpiring cells removes water faster than 

 their secretory powers can provide it, the menisci formed 

 in the substance of their walls must support the 

 tensile columns of water in the plant. Evaporation from 

 these menisci must provide the traction to raise the 



