FLOWERING IN ITS BIOLOGICAL FRAMEWORK 5 



2. The Response of an Organism to its Environment 



In our "type" plant, the cocklebur, the fundamental response to 

 environment is a response of the leaf to an uninterrupted dark period 

 which exceeds about 8 hr 20 min. Given such a dark period, the 

 plant flowers; on shorter dark periods the plant remains vegetative. 

 Of course there are other effects of environment : temperature must 

 be right, adequate soil nutrients and water must be available, and if 

 the dark period is to be highly effective, it must be preceded and 

 followed by exposure of the plant to high intensity light. Obviously 

 the response to environment is a matter of physiology, but it can 

 nevertheless be considered in an ecological sense (see Chapter 3). 

 The whole modern study of the flowering process was initiated 

 quite recently (1920) by the discovery that flowering in many plants 

 is an environmentally conditioned response. W. W. Garner and 

 H. A. Allard, working at the United States Department of Agriculture 

 Plant Industry Station at Beltsville, Maryland, wondered about the 

 peculiar flowering habits of two economically important species. A 

 variety of tobacco, called Maryland Mammoth, grew 10 ft tall during 

 the summer months at Beltsville, but failed to flower and set seed. 

 Transplanted as cuttings or root-stocks into the greenhouse, plants 

 would flower in winter when they were less than 5 ft tall. A certain 

 variety of soybean, when planted successively at various times 

 throughout the spring, tended to come into flower on the same 

 summer date regardless of the planting time. This variety (and some 

 others as well) would flower in winter in the greenhouse even when 

 the plants were very small. Obviously there was something about 

 winter greenhouse conditions which seemed to cause flowering in 

 these two species. 



Garner and Allard first tested effects of light intensity, temperature, 

 and available soil moisture and found no definite eff'ect on flowering. 

 Then, almost reluctantly, they tested the effects of day-length, by 

 extending it with artificial light or shortening it by placing plants in 

 cabinets. They were thus able to show that flowering occurred in 

 these two plants when days were shortened — regardless of other 

 environmental conditions (providing it was not too hot, dry or shady 

 for survival). They called the class of plants that responds in this 

 way short-day plants. With other species, long days resulted in 

 flowering (long-day plants), while flowering occurred in some plants 

 on any day-length (day-neutral plants). This basic response is 

 c 



