202 TRANSPIRATION AND ASCENT OF SAP ch. 



cm. per minute), while less than 1 per cent, or 0*0035 cal. 

 was used in photosynthesis, leaving more than 0*0965 cal. 

 available for carrying out other processes in the leaf 

 and for raising its temperature above the surroundings. 



External energy of this nature may not be available 

 for secretion when the temperature is high and the sun- 

 shine is veiled ; for then vaporisation lowers the tem- 

 perature of the evaporating surfaces, and these possibly 

 absorb the energy before available. Thus in an experi- 

 ment on a leaf of Helianthus annum under such conditions 

 Brown and Escombe found : 



Total radiation falling on the leaf per sq. cm. per min. 02746 cal. 



absorbed by the leaf ,, ,, 0'1884 ,, 



Energy expended in photosynthesis ,, ,, 0033 ,, 



,, ,, vaporisation ,, ,, 0*3668 ,, 



In this case 0*1817 cal. must have been derived from 

 the surroundings, and hence very probably the cells of 

 the leaves drawing forward the water lost rather than 

 gained energy in the process. 



Energy set free by respiration. With regard to 

 the stored energy set free by respiration and at least 

 partially available for secretory processes, precise figures 

 are not to hand ; but we may infer from certain obser- 

 vations the order of the amounts available. 



Aubert records that 1 gr. of leaves of Hedera helix rises in 

 respiration 252*1 c.mm. of oxygen per hour. Assuming a 

 hexose is oxidised according to the equation 



C 6 H 12 6 + 60 2 = 6CO, + 6H 2 + 677 2 x 10 3 cal. 



we find that the respiration of 1 gr. of these leaves generates 

 1*27 cal. per hour. 1 gr. of leaf has about 45 sq. cm. under 

 surface. Therefore the respiration of 1 sq. cm. of Hedera 

 leaf would generate 0*0282 cal. per hour. The amount 

 of water vapour exhaled from 1 sq. cm. will probably not 

 exceed, under normal conditions, 0*0252 gr. per hour. 

 Consequently for every gramme of water vapour given off 

 there may be as much as a calorie available for raising it 



