296 RADIATION BIOLOGY 



the connection of the oxygen evolution with the green parts of plants. 

 At that time a very active study of photosynthesis was taking place, 

 issuing chiefly from Priestley's preliminary observations on the purifying 

 action of green plants on the surrounding air. The essential role of light 

 in performing this effect, for the discovery of which Ingen-Housz is gener- 

 ally credited, was independently discovered by van Barneveld (1781), as 

 was pointed out by Rauwenhoff (1853). A year before Ingen-Housz's 

 well-known study (1779) appeared, van Barneveld concluded that light 

 has a beneficial effect upon the oxygen production of plants. His obser- 

 vations, however, were less profound and less profuse than those of Ingen- 

 Housz, and moreover, for various reasons, his paper appeared later than 

 that of Ingen-Housz. Also, in popular-science treatises of those days 

 suggestions are found that plants derive their nourishment chiefly from 

 the air and that fight is essential for their existence (cf. Wassink, in 

 press). 



The "leaf green," for which Pelletier and Caventou as early as 1818 

 created the name "chlorophyll," in the higher plants is contained in 

 chloroplasts. These are by no means homogeneous but contain smaller 

 green entities, grana, which, in their turn, again are complicated struc- 

 tures. It is very probable that chlorophyll and also other pigments 

 related to it or associated with it are active in photosynthesis only when 

 intimately linked to a protein bearer. This view is relatively young; 

 it was initiated especially by Lubimenko and followed in later years 

 by Mestre, Stoll, Baas Becking, and many others (for references, see 

 Wassink, 1948b). Willstatter and Stoll (1918) have been making 

 attempts to carry out the photosynthetic process in vitro, starting from 

 aqueous suspensions of highly purified chlorophyll in contact with carbon 

 dioxide. The general development of enzymology, together with these 

 facts, would lead one now to consider these trials as pioneer attempts 

 with ' little foundation in nature. * Nevertheless the study of chloro- 

 phyll in solutions or in aqueous suspensions may yield very valuable 

 information concerning certain features of the photosynthetic process, 

 especially light absorption, energy transfer, and photosensitization (cf. 

 Livingston, 1949). But the complete photosynthetic process so far has 

 been found only in intact cells. Parts of the process, especially the evo- 

 lution of oxygen with the simultaneous reduction of certain special hydro- 

 gen acceptors, e.g., quinone, have been realized with preparations con- 

 taining chloroplasts or parts thereof. 



Photosynthesis, like probably all important biological processes, is not 

 such a simple reaction as Eq. (5-1) might suggest but is a sequence of 

 partial processes in which the state given by the right-hand side of the 

 equation is gradually approached. Chiefly since the work of F. F. Black- 

 man (1905) and his collaborators, it is known that besides a specific photo- 

 chemical reaction a dark reaction can be distinguished in the sequence 



