898 LIGHT AND LIFE 



species, even when EDTA is added. Within the pH range 3.6 to 5.0, 

 the action of EDTA is greater at higher pW. In the case of L. per- 

 pusilla, the effect of EDTA is imitated by a variety of other chelating 

 agents; in that of L. gibba, it can be dupHcated by using "old" medium 

 which has previously supported the growth of plants under a long- 

 day regimen. Although EDTA also affects vegetative growth, it seems 

 that in respect to flowering it must affect the photoperiodic mechanism 

 itself — but not be a simple, general inhibition or promotion of flower- 

 ing. What it appears to do is to potentiate the inherent, and different, 

 response of each species to a long-day photoperiod. 



Colin S. Pittendrigh presented a stimulating review of short-term 

 photoperiodisms in animals. These phenomena have more recently 

 been termed "circadian" rhythms because they commonly have a 

 period of approximately a day, and more popularly they are called 

 "biological clocks." The paper was not submitted for publication. 



Vision 



The Intensity Factor 



Since the studies of Selig Hecht and his associates, in 1942, on the 

 number of quanta which must be absorbed by the rhodopsin of the 

 rods of the vertebrate eye in order to see, physiologists have been 

 greatly interested in the intensity factor in vision. Hecht and his 

 group demonstrated, by two independent methods, that it requires 

 absorption of about 6 quanta in order to see, and that each quantum 

 is absorbed in a separate rod cell. W. A. H. Rushton and his 

 colleagues have pursued this type of investigation with fruitful, if 

 still incomplete, results. It is now possible to relate the nerve signal 

 to the light intensity, to state where light adaptation and dark adap- 

 tation occur, to relate the light threshold to the amount of pigment 

 bleached, and to exjjress all of this information in a single mathema- 

 tical expression — one which a comprehensive theory of vision must 

 satisfy. 



As to adaptation, the reduction of the visual threshold during dark 

 adaptation by lOO-fold or more is certainly not localized in the brain, 

 since records taken from the gangUon cells of an excised eye and elec- 

 troretinograms themselves show the same change of sensitivity as the 

 intact system. Records taken with a micropipette from a single nerve 

 cell of a single onnnatidium of the horseshoe crab, Limulus, demon- 

 strate that the same conclusion may be drawn with respect to light 



