6 THE FLOWERING PROCESS 



illustrated, with some plants commonly used in such studies, in the 

 photographs of Fig. 1-1. Garner and Allard called their newly 

 discovered phenomenon photoperiodism. In later experiments of 

 these and other workers, it was found that many plants respond more 

 to the night-length than to the day-length; thus the term photo- 

 periodism is not entirely accurate, but usage has made it secure. 



It was known by nineteenth century farmers in the United States 

 that winter wheat, which usually flowers in the spring after being 

 planted the previous fall, will flower even if it is planted in the spring, 

 providing that moist seeds have been exposed to low temperatures 

 for a few weeks. This flowering response was also studied intensively 

 in the years following 1920, primarily in Europe and Russia, but to 

 some extent in the United States (see Chapter 4). 



Thus it became clear that the change from the vegetative to the 

 reproductive condition in higher plants may often be initiated by 

 some change in the environment. The changing length of day seems 

 to be such an obvious aspect of this environment that it is indeed 

 quite surprising that the discovery was made virtually within our 

 own generation. 



3. Biological Timing 



Perhaps the most impressive thing about the phenomenon of 

 photoperiodism is the implication that the organism is measuring 

 time (see Chapter 8). Thus the cocklebur requires at least S^hr 

 of uninterrupted darkness (the so-called critical dark period or 

 critical night) to initiate flowers. Most amazing of all, essentially the 

 same critical dark period is required over at least the temperature 

 range of 15 to 30°C. We can easily visualize time measurement by 

 thinking of the time required for completion of a chemical reaction, 

 but that this should be independent of temperature is not easy to 

 understand. 



It is probably safe to say that the formal study of biological timing 

 was first initiated by Garner and Allard in 1920, although previous 

 work was closely related, and man has always had a sense of time and 

 probably suspected that other animals, at least, shared this. In the 

 late 1920's zoologists observed that bees could be trained to feed at 

 certain times of day. About this time rhythmical movements of 

 leaves and other organs were clearly shown to continue under 

 constant environmental conditions. In spite of this early work, the 



