2 Mr. D. Thoday. Experimental Researches on [June 11, 



photosynthesis in leaves was called in question in 1905 by Brown and 

 Escombe. In their paper " On the Physiological Processes of Green Leaves,"* 

 they published an account of four experiments in which they had determined 

 for the same individual leaves both the increase of dry weight, by Sachs' 

 method, and the amount of carbon dioxide actually absorbed, by their own 

 method. They sum up the results by saying : " If we take the mean of all 

 four experiments we find that the Sachs method gives an estimate of the 

 assimilation rate between two and three times greater than that deduced 

 from the intake of carbon dioxide."t 



In attempting to explain this discrepancy they made some determinations 

 of the degree of symmetry existing between opposite sides of various leaves, 

 and of the amount by which half-leaves changed in area under experimental 

 conditions. They concluded from these that the errors to which the method 

 is liable are of the same order of magnitiide as the quantities to be measured, 

 and that therefore " the Sachs method cannot be trusted for anything like 

 exact quantitative estimation of the photosynthetic work which is going on 

 in an assimilating leaf. As ordinarily applied, its general tendency is. to give 

 far too high an estimate of the rate of assimilation.''^ 



Since- Sachs' method is the only one available for measuring the total 

 photosynthetic activity of leaves under natural conditions, it is of the utmost 

 importance that it should not lightly be abandoned. Its loss would be felt 

 in pure physiology, and perhaps still more in ecology. I therefore undertook, 

 at Dr. ¥. F. Blackman's suggestion, a detailed iuvestigation of the Sachs 

 method, in order to discover whether it was completely untrustworthy, or 

 whether its weaknesses could be overcome. 



Since the present paper is concerned principally with the results of this 

 examination of the method, much is included that is technical and of 

 imjjortance only to those who wish to use the method ;§ but the nature of 

 the changes occurring in the leaf lamina during insolation (Section III) and 

 the phenomena of shrinkage and expansion in area which follow variations 

 in the intensity of illumination (Section V) are subjects of more general 

 iiit(;iest. Some api)lications of the method will be described in a later paper. 



The work was carried out in the Cambridge University liotany School 

 during my tenure of tlie ('ambridge University Frank Smart Studentship. 

 My thanks are due to ])r. F. F. I'.lackman for liis kind interest in tlie woi'k 

 throughout its course and for many hi^lpful suggestions. 



* 'Roy. Soc. Pi-oc.,' B, vol. 76, 190.'), p. 2!). 

 t Loc. cit., ji. .58. 

 I Loc. cit., p. 01. 



Sj 'J'lie iiioHt (crlinical portions are in Hniallor type. 



