498 A. PIRSON 



photosynthesis and not by the periodicity of dark resjjiration. These 

 last-mentioned results and similar ones have already been published 

 elsewhere (8). 



Discussion 



Tamiya : How do you measure growth? 



Pirson : Only by measuring elongation, but we know that at the time of elonga- 

 tion plasmatic growth takes place also. 



Witt : Can you also change the period by periodically varying carbon dioxide? 



Pirson : This may be possible and it is on our program icf. 2a). 



Witt: Can you change the period, by periodically changing the temperature 

 between 30°C. and 5°C.? 



Pirson : It would be fine if we could do that, but if we go to temperatures of 

 about 5°C. we observe a long time before recoverj\ Then we cannot decide whether 

 the effects are recovery effects or periodical effects. 



Wassink : I would like to make an additional comment on this etiolation ques- 

 tion. It seems dangerous to use the term etiolation, unless specificallj' attached 

 to the absence of light. You may know that a considerable time ago we published 

 evidence that, for instance, in lettuce seedlings j^ou can easily make a light regimen, 

 say 10 hours white light and 14 of darkness, that will produce no etiolation or at 

 least no appreciable elongation in leaves and stems. On the other hand, if you 

 supply this basic light period with, say, 8 hours of weak intensity of either near 

 infrared or the blue-violet region, you will find that the plant will show etiolation 

 characteristics. So it seems that we have to deal here with special photomorpho- 

 genetic phenomena and not only with the absence of light. You might be able 

 to produce conditions similar to ours, in which you really would find etiolation 

 in terms of elongation by supplying your cells with special regions of light; 

 and thus we will now have to say more precisely what we mean by etiolation. 



Pirson: Your criticism of the use of the term etiolation is quite right. I said 

 they grow very little in the dark. 



Rabinowitch: I just want to ask whether your medium was constant; was it a 

 flowing medium, or was there any periodic change in the medium involved? 



Pirson: It was not a flowing medium, but the solution was renewed several 

 times during one period. Hence we can e.xclude the possibility of a change in the 

 medium. It should be stressed that the cells suff'ered tig deficiencies during our 

 experiments. Photosynthesis and respiration were related to dry- weight; for this 

 purpose the samples were changed every 3 hours. The chlorophyll content re- 

 mained constant on a dry-weight basis. This means that there was no change be- 

 tween periods of activity and storage periods. 



References 



1. Neeb, ()., "Hydrodictyon als Objekt einer vergleichendcn Untersuchung 



physiologischer Grossen," Flora, 139, 39-95 (1952). 



2. Schon, W. J., "Periodische Schwankungen der Photosynthese und Atmung 



bei Hydrodictyon," Flora, 142, 347-380 (1955). 



