84 Dr. H. T. Brown and Mr. F. Escombe. [Jan. 9, 
(w). Energy used for photosynthesis ............... 0°66 
(W). { Frans pia bloMiy.se-ne-e) 2 eee 48°39 
(W+w). Total energy expended in internal work ...... 49-05 
R—Ra. Solar radiant energy transmitted by leaf ... 31°40 
(7). Energy lost by “ thermal emission ”............ 19°55 
100-00 
The “ economic coefficient ” of the leaf in the above instance was 49:05 per 
cent. if we include the whole of the internal work done in the leaf. If, on 
the other hand, as is more convenient, we only take into account the photo- 
synthetic work on which the plant relies for the production of new formative 
material, then the “economic coefficient” was only 0°66 per cent.* 
We will now consider a case in which the facilities for the performance of 
the internal work of vaporization, such as are afforded by fully open stomata, 
and high temperature and low humidity of the surrounding air, are more than 
sufficient to utilise the whole of the direct solar radiation absorbed by the leaf. 
Under these conditions (W+7) will exceed Ra, and the balance of energy 
required for the internal work will be derived from the environment, the 
temperature of the leaf falling below that of its surroundings. 
The following experiment on leaves of Helianthus annuus on a warm dry 
day in July serves to illustrate the point. The transpiring leaf was placed 
under the revolving sectors so arranged as to cut off exactly one-half of the 
total solar radiation which would otherwise have fallen upon it. 
R. Total direct solar radiation falling on the 
leaf per square centimetre per minute.. 0°2746 calorie. 
ta. Solar radiation absorbed per square centi- 
metre per minute (coefficient of absorp- 
TOU = HOO SO) Owe cess. cae Ney ies cls eet O13S84 ee 
w. Energy expended in assimilation.......... . ) O:0033eae 
W. 2 2 transpiration ......... 03668) = 
W+w. Total energy used for internal work ...... 0:3 0a ele 
Hence the energy (7) which the leaf has derived from its surroundings 
= (W+w)—Ra=01817 calorie per square centimetre of leaf-lamina per 
minute. 
Notwithstanding the fact that the leaf was receiving an incidence of solar 
radiation equivalent to 0°2746 calorie per square centimetre per minute, its 
* In this restricted sense the “economic coefficient’ of the leaf is the ratio of the energy 
utilised by photosynthesis to the total radiation falling on the leaf. 
