316 PHOTOSYNTHESIS 



energy relations of the leaves of land plants are more complicated than 

 those of aquatic plants. In the former the situation becomes complicated 

 through the transpiratory activity, the opening and closing of the stomata 

 and the exchange of heat between the leaf and its surroundings due to 

 a difference in temperature between the two. In aquatic plants where 

 there is no factor of transpiration and consequent cooling of the plant, 

 the energy relations are somewhat simpler. Warburg and Negelein have 

 taken advantage of this fact in determining the photosynthetic efficiency 

 of a unicellular alga. Determinations of the energy used in photosyn- 

 thesis show a considerable variation. As these results have been arrived 

 at by widely different methods it will be necessary to describe the experi- 

 ments in some detail in order to be able to compare their relative worth. 

 The experiments of Brown and Escombe ^ constitute the first thorough 

 attempt to construct a balance sheet for the energy relations of an illumi- 

 nated leaf. Not the least of the contributions of these authors has been 

 an analysis of the factors which must be considered in attempting such 

 a calculation. It is doubtful whether they are justified in drawing con- 

 clusions as to some of the fundamental physical properties of leaves 

 from the relatively limited data, especially as to the constancy of such 

 properties as thermal emission under dift'erent experimental conditions. 

 Nevertheless, their experiments are of considerable value from the view- 

 point of a general approach to the problem and their results still con- 

 stitute practically the only data which have been obtained by a method 

 which endeavors to account for all the energy taken in and expended by 

 the leaf. 



The absorption of radiant energy by a leaf may result in 1, the 

 vaporization of water and 2, in photosynthesis; both of these are endo- 

 thermic in character. Also in the leaf, 3, the process of respiration is 

 continually going on; the end result of the chemical reactions compris- 

 ing respiration is exothermic. The values of these three processes, vapor- 

 ization of water, photosynthesis and respiration, as measured in units 

 of heat absorbed or evolved per unit of time, are very dissimilar and 

 vary with different external conditions to which the leaf is exposed. The 

 loss or gain of heat, due to each of these three processes, can be deter- 

 mined and referred to the total amount of energy received by the leaf, 

 either by direct radiation or through the convective and conductive action 

 of the air. In order to accomplish this, it is, however, essential to estab- 

 lish the absorptive power of the leaf for the different forms of radiant 

 energy to which it is exposed, as well as the gain or loss of heat due to 

 radiation and air convection and conduction due to differences in tempera- 

 ture between the leaf and its surroundings. 



The coefficient of absorption of a leaf was taken by Brown and Escombe 

 as "the difference between the solar radiant energy falling on the leaf in 

 full sunshine and the amount transmitted" through the leaf. The measure- 

 ments were made by means of a Callendar radiometer; a measurement 

 ■"'Brown and Escombe, Proc. Roy. Soc, 76 B, 69 (1905). 



