4 OPENING ADDRESS 



I did not believe he would show up— too many of our meetings during 

 past years had to be held without him. But to my surprise, and to 

 everybody's pleasure, he arrived yesterday evening. 



Our program promises us the description of quite a number of new 

 observations, experimental techniques and contributions for or against 

 certain hypotheses aimed at explaining the particular kind of metabol- 

 ism that sets the purple bacteria apart from the green plant. 



Hardly any one of us who were around twenty years ago would have 

 believed that van Niel's idea of a photolysis of water as the core of 

 the photosynthesis problem could still elicit a vivid discussion today. 

 For the green plants it had been proven as correct by Hill's reaction. 

 And as a reasonable interpretation also for the anaerobic photo- 

 metabolism of purple bacteria there was the indirect evidence of the 

 adaptable hydrogenase- containing algae. 



Purple bacteria furnished van Niel the key to the first generally 

 convincing picture of the photosynthetic process in terms of modern 

 metabolic ideas. And purple bacteria are now believed to provide 

 clear evidence that a photolysis of water— water as an intermediate 

 hydrogen donor— should not be accepted as part of the hypothetical 

 picture for bacterial photosynthesis. That is, van Niel's generalization 

 of 1935 is disallowed. 



It is about this question mainly that I would like to speak to you. 

 Usually after thirty years a theory ought to have been transformed into 

 fact or replaced by a better one. With van Niel's theory it so happened 

 that after ten years there were no doubts left that the oxygen of photo- 

 synthesis originates from water. This we have accepted as fact. 



I propose toshowthat, like any good scientific theory which managed 

 to live in these hectic times for thirty years, van Niel's extended 

 version is still useful. A truly good theory never dies— it only becomes 

 more refined. This may makeit more difficult to explain and to teach- 

 but it does not render the simpler version wrong. 



It is often repeated that one new fact which does not fit destroys a 

 hypothesis. This is not true. As long as this new observation does not 

 give birth to a better theory— and better is by definition the more 

 encompassing view— it should be noted but treated as if with a little 

 more thought and patience it may soon find its place within the existing 

 order. 



We have accepted the proposition that light will split, oxidize, de- 

 hydrogenate, or photolyze water in green plants, because on the face 

 of so much evidence we cannot explain from where else the oxygen 

 could originate. On the other hand, purple bacteria do not evolve oxy- 

 gen. Why should we assume that water is involved in the photochemical 

 process, even as an intermediate and incomplete process, when there 

 is as yet no incontrovertible evidence that the assumption is warranted? 

 How sound a viewpoint— and what a dull one. As I pointed out recently 

 somewhere else, the mechanism to release oxygen from water with 



