Garner and Allard as a crop research team had puzzled for years 

 over the question: Why did Mamnnoth tobacco grown in Maryland continue 

 vegetative growth so long in sumnner that it bore giant leaves and bloonned 

 too late to form seed before frost? Maryland growers who planted Mam- 

 moth variety had to grow seed in a winter greenhouse. Biloxi soybeans 

 similarly failed to form seed in this area. 



Testing probable causes for this seeding habit, the two scientists 

 had eliminated temperature, moisture, fertilization, and light intensity. 

 Now, they were trying day leng.th. They would give test plants 17 hours 

 of darkness --from 4 P.M. to 9 A. M.--and then put thenn outdoors for a 

 7 -hour day in July, like the days in late autumn and winter when Mammoth 

 tobacco bore greenhouse seed crops. After a few days, the test plants 

 bloomed and then formed seed. 



Armed with a new concept- -that some plants are short -day plants -- 

 the two scientists tested more plants, including cabbage, carrots, lettuce, 

 ragweed, and wild violets. They found that plants differ in the proportions 

 of day and night that lead to seed formation, and that these proportions 

 also govern flowering, stem lengthening, and other growth transitions. 



Accounting for Earth's Plant Distribution 



The initial report by Garner and Allard in 1920 marshaled evidence 

 that temperature, water, and light intensity could no longer end the list 

 of nnain external influences governing where and how widely the earth's 

 plants can grow. Scientists were asked to consider a new underlying 

 cause --the relative length of days during a growing period. 



The report indicated a 4-way grouping of plants that explained for 

 the first tinne why vegetation differs with latitudes and seasons: 



• Plants that normally flower in late spring or in summer respond 

 to long days and may be called long -day plants. 



• Conversely, plants that nornmally flower in the autumn or winter 

 respond to short days and nnay be called short -day plants. 



• A third and large group can flower and fruit under a wide range of 

 day lengths. Garner and Allard later called these indeternainate 

 plants. 



• Plants tested and assigned to each group had varying day length 

 requirements. Some were so near to a 12-12 hour response that 

 they might be separated into a fourth group and were later given 

 the name intermediate. 



Some close plant relatives, such as varieties in the same species, 

 fitted into different photoperiodic groups. 



Photoperiodism as a Life Principle 



The Garner and Allard report introduced a life principle so broad 

 that it operates extensively over our sunlighted world; so basic that 

 without it plant and animal life as we know it could not exist; and so 

 complex that the related parts of the operating mechanism are just 

 beginning to be fitted into place. 



