132 THE FLOWERING PROCESS 



of Technology, demonstrated the reality of this concept, calling it 

 thermoperiodism. He grew various species of plants under many 

 combinations of day and night temperatures, and in a majority of 

 cases growth was considerably accelerated when the night temperature 

 was lower than the day temperature. There are exceptions, including 

 many plants which respond equally well to constant temperatures 

 (such as cocklebur) and even plants which grow best when the night 

 temperatures are somewhat above the day temperatures (such as the 

 African violet^). Probably it is not safe to conclude with certainty 

 that thermoperiodism is a demonstration of biological timing, but 

 it does seem likely that this will prove to be true. It may even be 

 a special case of persistent circadian rhythms in sensitivity to 

 temperature. 



4. Celestial Navigation 



This is certainly the most spectacular demonstration of biological 

 timing which has yet been discovered. Certain insects and birds are 

 able to sense direction on the earth's surface by comparing the posi- 

 tion of the sun or stars with their endogenous time-measuring system. 

 The honey bee, which has been studied intensively, flies in a given 

 direction to a feeding place by noting the position of the sun, and 

 correcting for the time of day. The pond skater when placed on land 

 always hops south — or the direction that would be south according 

 to the position of an artificial sun set up by the experimenter! 

 Celestial navigation was demonstrated for a European warbler, the 

 lesser white throat (Sylvia c. curruca) that migrates at night by 

 placing the birds in a planetarium in Freiburg, Germany, where it 

 was possible to shift the projected "stars" in the "sky", setting up 

 star configurations comparable to various seasons and times during 

 the night. Knowing the directions taken by migrating birds in nature, 

 it was possible to predict the positions they should assume in relation 

 to the planetarium dome, with its projected "stars". The birds 

 responded exactly as predicted. Anyone who has navigated by the 

 stars can appreciate what a fantastic feat this really is. The timing 



® Thus Went generalizes that there are two kinds of people in the United States : 

 those who can grow African violets and those who can't. He further concludes 

 that those who can are those who fail to change the thermostat setting when they 

 go to bed at night, leaving the house warm, while those who can't prefer to sleep 

 in cool houses and not only lower the temperature setting but may throw open 

 the windows before retiring. 



