1314 INDUCTION PHENOMENA CHAP. 33 



ciilations. In these studies, variations of gas exchange of unexpected 

 extent, variety and duration have been uncovered. As the proofs of this 

 chapter are being revised, it seems well nigh impossible to combine these 

 new observations \yiih. the previously digested ones, in a single logical pres- 

 entation. As an interim solution, the original organization of the chapter 

 has been left unchanged, and the new results interpolated where this seemed 

 most appropriate. The resulting picture is bewildering, although a few, 

 repeatedly noted, regularities begin to emerge from the confusion. 



A. Gas Exchange during the Induction Period* 

 1. "Long" and "Short" Induction 



The changes that occur when a plant, after having been kept in darkness 

 for a certain period of time, is brought into light are complex. In recent 

 years, it has become increasingly clear that different investigators of this 

 so-called "induction" have dealt with different phenomena. Even now, 

 after the extensive studies of McAlister, Aufdemgarten, Blinks and Skow, 

 Kautsky, Gaffron, Steemann-Nielsen, Emerson, Franck, van der Veen, 

 Warburg, Osterlind, Brackett, Hill, Whittingham and others, it is still dif- 

 ficult to correlate all observations, much less to explain them in terms 

 of one simple mechanism. What is clear is that the induction phenomena 

 are caused by several changes that the photosynthetic apparatus undergoes 

 in the dark, and which are reversed by several reactions of different velocity 

 after the plant had been returned into light. The relative prominence of 

 the different induction phenomena depends on the nature and state of the 

 plant, the duration of the dark period and the conditions that prevailed 

 during this period. 



In trying to bring some order into the multitude of observed phenomena, 

 we will distinguish between "short" and "long" induction. The first 

 usually consists of a sharp drop of the rate of photosynthesis in the first 

 second or two of illumination, followed by a gradual rise to a steady level 

 (often interrupted by a secondary depression), which comes to an end in 

 from 2 to 5 minutes (at room temperature). These features may be fully 

 developed after only a few minutes of "incubation" in the dark at room 

 temperature. 



"Long induction," on the other hand, is fully developed only after sev- 

 eral hours of darkness, and is characterized by a slow change in rate, which 

 may last for several hours. However, this simple relation between the 

 lengths of incubation and induction is not always found. In the measure- 

 ments of Steemann-Nielsen (1942) with Fucus, for example, no prolonged 

 induction of oxygen liberation appeared even after dark periods of 15 to 16 

 hours. On the other hand, the graph showing the induction losses incurred 



* Bibliography, page 1429. 



