446 Prof. C. Timiriazeff. [Apr. 30, 



der Sauerstoffabscheidung im gelben Licht stattfindet. Alles zusani- 

 mengefasst, haben die Untersuchungen tiber das Chlorophyllspectrum 

 bisher auch nicht die geringste physiologisch werthvolle Thatsacbe 

 ergeben, d. h. wir warden von der physiologischen Function des 

 Chlorophylls genau dasselbe wissen, wenn uns auch das Spectrum 

 desselben vollstandig unbekannt ware." 



It is also a well-known fact that for a long while Professor Pfeffer 

 was my opponent and the staunchest adherent of the old theory. In 

 the first edition of his Handbook he treated my researches in general 

 as an experimental blunder not worthy of further notice. But shortly 

 after the publication of his work a sudden change of scientific opinion 

 was observed in favour of my point of view, thanks to M. Van Tieghem 

 in France, and to Professor Sydney Vines in this country. At the same 

 time Engelmann and Reinke though using methods far less exact 

 than those I had used obtained the same results. It was impossible 

 for Professor Pfeffer to hold his position any longer, and in the second 

 edition of his book he practically adopts my views ; but this time the 

 conclusions, which were subject to doubt so long as they were associated 

 with my name, are simply attributed to the two German investigators. 

 More than that, a new theory is advanced in order to conciliate his 

 former and present points of view. Professor Pfeffer admits the 

 existence of two different assimilation curves : the one corresponding 

 to the chlorophyll absorption spectrum the primary as he calls it, and 

 another, the secondary, having its maximum in the yellow rays. But 

 nothing can be more easy than to prove the non-existence of this latter. 

 In fact, if Professor Pfeffer is right, I ought to have obtained his 

 secondary curve. But, as we have seen, I was the first to obtain the 

 primary the only real one and that is probably the chief reason 

 why Professor Pfeffer was obliged to pass over my researches in silence. 

 His secondary curve is brought forward in order to cover a retreat 

 imposed on him by the evidence of facts. 



Having thus stated the position taken towards me, with rare excep- 

 tions, by the German botanists, I feel that I am the more bound to 

 express my gratitude to the late Edmond Becquerel and M. Berthelot, 

 to whom I am indebted for the appearance of my researches in the 

 pages of the ' Comptes Kendus ' and the * Annales de Chimie et de 

 Physique.' 



Up to this point we have been considering the qualitative aspect of 

 the problem, we have studied the photolysis or the reduction of the 

 carbon dioxide and the photosynthesis or the production of starch, in 

 their relation to the chemical and optical properties of chlorophyll and 

 to the energy of the active rays. We will now consider the same 

 phenomenon in its quantitative relation to the total amount of 

 radiant energy incident on the green leaf, and we will try to find the 

 law which regulates its dependence on the intensity of the radiation. 



