560 LIGHT AND LIFE 



whose discoveries marked the beginning of this field of inquiry. He 

 wrote in 1777: "This is a new and a wide field of experiment and 

 speculation, and a premature attachment to hypothesis is the greatest 

 obstruction we are likely to meet with in our progress through it . . . 

 Speculation is a cheap commodity. Neiu and important facts are most 

 wanted, and therefore of most value" (122) . 



The new facts which led to the concept of photosynthesis presented 

 here are the following: (a) Isolated chloroplasts use the energy of 

 visible light for converting CO.j to sugars and starch with a simul- 

 taneous evolution of oxygen, (b) Isolated chloroplasts use light energy 

 for a simultaneous reduction of TPN, formation of ATP, and evolu- 

 tion of oxygen (non-cyclic photophosphorylation) . (c) Isolated 

 chloroplasts and bacterial chromatophores use light energy for an 

 anaerobic formation of ATP as the sole product of the light reac- 

 tion (cyclic photophosphorylation) . (d) By fractionating chloro- 

 plasts and Chromatium cells, the light-dependent phase of photosyn- 

 thesis has been separated, in time and space, from the dark phase 

 that is concerned with carbon assimilation. Variants of these reac- 

 tions and the supporting evidence have been discussed above. 



In closing, I wish to borrow these words from Priestley, which ac- 

 curately express the spirit in which this survey was undertaken. "I 

 have been most particularly careful to distinguish jacts from hypo- 

 thesis; and whenever I have occasionally attemj)ted anything of this 

 latter kind, it has been with diffidence, and in such a manner as it 

 would give me no pain to relinquish my own opinions, and adopt 

 those of any other person that should appear to me more naturally 

 to arise from the facts" (122). 



REFERENCES 



1. Allen, M. B., Arnon, D. 1., Capindale, }. B.. Whatlev, F. R.. and Durham, L. J.. 



J. Am. Chem. Soc, 77, 4149-4155 (1955). 



2. Allen, M. B.. Whatley, F. R.. and Arnon, D. I., Rinchim. el Riophys. Acta, 27, 



16-23 (1958). 



3. Anderson, I. C, and Fuller, R. C. Arch. Riochem. Riophys.. 76, 168-179 (1958). 



4. Arnold. W., and Clavton, R. K., Pmc. Nnl. Acad. Sci. U. S., 46, 769-776 (1960). 



5. Arnon, D. 1., A'fla/re, 167, 1008-1010 (1951). 



6. Arnon, D. I., paper presented at the Cell Symposium, American Association 



for the Advancement of Science, Berkeley Meeting (1954); Science, 122, 

 9-16 (1955). 



7. Arnon, D. I., Ann. Rev. Plant Physiol., 7, 325-354 (1956). 



8. .\rnon, D. I., in Enz\me.s: Units of Riohx^ical Structure and Function. (O. H. 



Gacbler, ed.), pp. 279-313, Academic Press, New York (1956). 



9. Arnon, D. T., Nature, 184, 10-21 (19.59). 



10. Arnon. D. I., in Handbuch der Pflanzenfihysiologie, Vol. 5, (A. Pirson, ed.). 

 pp. 773-829, Springer. Heidelberg (1960). 



