'V\ 



This birch tree in Tiibingen, Germany, illustrates well the response to day-length which 

 is the principle topic of this book. The street lamp at the lower right has a reflector on 

 top, so that it casts its light only downward and outward. The leaves illuminated by the 

 light at night have responded to this artificially extended day by not going into dormancy, 

 even though it is the first week of November, and Tubingen has already had some severe 

 frosts. Most of the leaves that were in the dark have fallen from the tree some weeks ago. 

 The number and green color of the remaining leaves are closely proportional to the light 

 intensity they receive from the lamp, illustrating a quantitative aspect to this response 

 to a time factor (the length of day). 



