114 THE BIOLOGY OF FLOWERING PLANTS 



absorbed 68*6 per cent. Young leaves absorbed rather less 

 than old ones, as might be expected. The leaves investi- 

 gated, however, are all of much the same type, so that the 

 relatively small range observed does not really give a good 

 idea of that likely to be found in a more varied selection. 

 The absorption in diffused light has not been specially 

 studied. 



It would be natural to suppose the main part of this 

 absorbed light to be taken up by the green pigment ; but 

 this is not the case. Comparison of the absorption by white 

 and green portions of variegated leaves of Acer Negundo 

 showed that the former absorbed 74*5 per cent, of the 

 incident sunlight and the latter 787 per cent., the chloro- 

 phyll being therefore responsible for absorbing only 4*2 

 per cent, of the incident or 5*35 per cent, of the absorbed 

 light. 



But even this small fraction of the available energy is 

 not all used in assimilation. Browne and Escombe worked 

 out complete balance-sheets for the disposal of the incident 

 energy by a number of leaves under different conditions. 

 To take one example of their results, they found that Sefiecw 

 grandiflorus, assimilating in bright sunshine, absorbed 

 65*49 P^^ cent, of the Hght and transmitted 34*5 1 per cent. 

 Of the absorbed energy 64 per cent, was expended in 

 vaporising water in transpiration, and i"22 per cent, in 

 carrying on photosynthesis, the remaining 34*78 per cent, 

 being lost by radiation and convection from the heated leaf 

 surface. Thus, of the total energy incident on the leaf, only 

 0*8 per cent, was used in photosynthesis. In light of half 

 this strength, i'59 per cent, was used. These figures are 

 typical ; in a long series of twenty-four experiments, under 

 varying light conditions, the amount of available energy 

 used in photosynthesis never reached 5 per cent., and on 

 the average it was about i per cent. ; about 99 per cent, 

 was always lost — so far as the building of organic matter was 

 concerned — by transmission, re-radiation, and transpiration. 

 These experiments were carried out in ordinary conditions 

 of illumination and at favourable temperatures, with the 



