1982 EPILOGUE CHAP. 38 



but in the de\-elopmeiit of new strains of more produoti\'e plants and of 

 new methods of their cultivation (of which the now extensively studied 

 large-scale growth of microscopic algae is a first example). 



This does not mean that the achievement of photosynthesis outside 

 the living cell would not })e an important milestone in man's mastery of 

 natural processes; but the importance of this achievement will not lie in 

 immediate practical applications. 



With the increasing understanding of the different aspects of the photo- 

 synthetic process in nature, the very definition of what is to be considered 

 as "imitation of photosynthesis outside the living cell" becomes difficult. 

 Which of the many facets of natural photosynthesis have to be faithfully 

 reproduced for the experiment to be called "repetition of photosynthesis 

 in vitro''! Certainly not every reduction of carbon dioxide to carbohy- 

 drate level by the action of light, can be called so (as has often been done 

 in the past). At the very least, simultaneous evolution of an equivalent 

 amount of oxygen is to be required. Beyond this, a certain similarity of 

 the energy conversion yields is essential, as well as a capacity for continuing 

 the process steadily for a reasonable length of time. No experiment per- 

 formed as of January 1, 1955 can claim to have satisfied all these require- 

 ments; we can thus say that, at the termination of this monograph, photo- 

 synthesis outside the living cell still remains unachieved. This is tme 

 not only of artificially compounded chemical systems, but even of prepara- 

 tions derived from cells and containing various intact cell structures and 

 cell ingredients. 



Considerable progress has been achieved, since the writing of this mon- 

 ograph began, in the separation of some of the most important partial 

 reactions in photosynthesis from their former bondage to the intact cell 

 structure. In Chapter 4, the discovery of the "Hill reaction" in chloro- 

 plast preparations could just be announced; while in Chapter 35, a long 

 series of studies could be described, revealing the analogy and the differences 

 between this reaction and the true photosynthesis. These studies cli- 

 maxed in the coupling of the Hill reaction with the reduction of certain 

 carbonyl and — with the help of high-energy phosphate — also of some 

 carboxyl compounds: the nearest approach to date to true photosynthesis 

 in cell-free preparations. After Chapter 35 was completed, the photo- 

 chemical incorporation of tagged carbon dioxide into organic compounds in 

 light could be demonstrated by Tolbert on squeezed-out cell contents, 

 and by Arnon on whole chloroplasts (c/. p. 1535). It now seems more 

 likely than ever that the complete enzymatic system of photosynthesis is 

 contained in chloroplasts. (Certain early experiments with the isotope 



