130 PHOTOSYNTHESIS 



gree of depression depending upon the temperature and length of time 

 the plants were exposed to the higher temperatures. 



The methods of studying photosynthesis in the field have not been 

 perfected so that reliable results can be obtained from plants growing 

 under natural conditions. As has been repeatedly emphasized the inter- 

 action of so many factors makes it impractical to determine all of these 

 simultaneously, and even if this were possible, to attain a rational inter- 

 pretation of the results of photosynthesis determinations under constantly 

 varying conditions leads only to spurious results and false conclusions. 

 So that, highly desirable as it undoubtedly is to obtain determinations of 

 the influence of various external conditions, as existing in the field, on 

 the photosynthetic activity of different plants, we are, nevertheless, con- 

 fined to laboratory experimentation in order to attain results approaching 

 scientific accuracy. 



One of the most difficult problems in the determination of the influ- 

 ence of temperature on photosynthesis is the ascertaining of the tempera- 

 ture of the plant itself. In aquatic plants the problem is somewhat 

 simplified because with plants of small volume their temperature during 

 illumination probably does not dift'er greatly from that of the surrounding 

 water. But with land plants the problem is much more difficult. An 

 illuminated leaf absorbs radiant energy, only a small portion of which 

 is converted into chemical energy ; the greater proportion of the absorbed 

 energy is converted into heat which results in the evaporation of water 

 from the leaf, in alterations in the size of the stomatal openings and in 

 other more obscure internal changes. In working with land plants it 

 is therefore essential to know the internal temperature of the leaf and 

 to obtain this, several methods have been employed.^-* 



Brown and Escombe ^-^ recognized that a certain amount of the radiant 

 energy absorbed by a leaf is dissipated through the evaporation of water 

 and they endeavored to calculate this amount. The following physical 

 and chemical changes taking place within the leaf are of influence on 

 the thermal relations of the leaf and its environments : 1— the evaporation 

 of water, 2— photosynthesis, 3— respiration. Of these 1 and 2 are 

 endothermic while 3 is exothermic. The actual determination of each of 

 these factors and the calculation of their energy relations is associated 

 with considerable difficulty. It is, first of all. necessary to know the 

 thermal emissivity of the leaf. This term includes the gain or loss of 

 heat due to radiation, convection and conduction of a unit area of leaf 

 in unit time with unit difference of temperature between the leaf and 

 its surroundings. The simplest condition is that of a leaf in the dark 

 in an atmosphere saturated with water-vapor. Owing to the oxidative 

 processes taking place within the leaf there is a tendency of the leaf to 

 rise in temperature. This thermal disturliance due to respiration is, 



"'Shreve, Edith, Plant World. 22, 100 (1919). Miller and Saunders, Jour. Agric. 

 Res.. 24, 15 (1923). 



"•Brown and Escombe, Proc. Rov. Sac. London, B 76, 69 (1905). 



