The Beginning of Photosynthesis and the 
Development of Chlorophyll . 1 
BY 
A. A. IRVING, 
Newnham College, Cavibridge. 
With ten Figures in the Text. 
HIS work deals with the question of how soon the power of photosyn- 
X thesis attains an appreciable magnitude when young leaves are de¬ 
veloping in light, and when leaves that have been etiolated in the dark are 
exposed to light and turn green. 
Several investigators have attributed to etiolated chloroplasts the 
power of assimilating C0 2 in the light, before they develop detectable 
amounts of chlorophyll. The evidence that was brought forward in support 
of this has been in all cases indirect, and based almost entirely upon the re¬ 
action of bacteria in the Engelmann method. 
At Dr. F. F. Blackman’s suggestion I have carried out direct gaso- 
metric investigations on this point. The method adopted has been to 
measure the output of C0 2 in the respiration of etiolated or greening shoots 
alternately in dark and light. The first dawning of the power of photo¬ 
synthesis should thus be made evident by the amount of respiratory C0 2 in 
the light being consistently smaller than that in the dark by just that 
fraction which the shoot was capable of assimilating. 
The power of photosynthesis was expected to augment as the shoot 
gradually became greener, and even if the yellow tissues should show no 
detectable assimilation of C0 2 , the green should do so, and one would have 
been able to say at what tint of greenness the function reached an appreciable 
magnitude. 
1 This paper constitutes Part VII of * Experimental Researches on Vegetable Assimilation and 
Respiration’. The earlier papers of this series carried out at Cambridge under the general direction 
of Dr. F. F. Blackman are :—I and II, Blackman, Phil. Trans. Roy. Soc. B, 1895 ; III, Matthaei.Phil. 
Trans. B, 1904; IV, Blackman and Matthaei, Roy. Soc. Proc. B, vol. 76; V and VI, Thoday, Roy. 
Soc. Proc. B, vol. 82. 
[Annals of Botany, Vol. XXIV. No. XCVI. October, 1910.] 
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