USDA's experience in photoperiodic research in 40 years has at- 

 tracted a considerable nunnber of staff scientists to join teams making 

 light -growth experiments. Three, in particular, have been leaders since 

 the Garner and Allard era : 



Dr. M. W. Parker, plant physiologist, worked on nnany light experi- 

 ments from 1936 to 1952, and is now director of the Crops Research 

 Division in the Agricultural Research Service. Dr. H. A. Borthwick, 

 another plant physiologist, joined in the photoperiodic research in 1936. 

 He now leads the group specializing in this work in a pioneering labora- 

 tory designated for the study of "light as an environmental influence on 

 plant life." Dr. Sterling B. Hendricks came to the USDA as a soil chemiist, 

 and by 1944 began working with Borthwick. Hendricks is currently leader 

 of a pioneering group studying the rnineral nutrition of plants, but he 

 continues to work actively also with the photoperiodic team. He has been 

 the key man in discovering and separating fronn plants the vital phyto- 

 chrome that triggers growth changes. 



THE DISCOVERY ROAD 



The straightest short-cut to catching up with photoperiodic dis- 

 coveries is to start at 1918 and think forward with scientists through 

 nnajor advances. 



A few selected experimental projects by ARS plant pioneers working 

 as teams, and in some cases with cooperating agencies, can suffice to 

 show how clues and their meaning have led forward from one landmark 

 to the next on the discovery road. 



I ^ 



Four Milestones 



Major milestones thus far are these: 



1. The take-off pc^int, that photoperiodism is a natural law of growth. 



2. Finding that plants use one part of sunlight, red, in launching 

 growth changes. 



3. Detecting a hidden substance that works reversibly, operating 

 go -stop signals under red and far red. 



4. Bringing from hiding this chemical substance, and proving that it 

 is a plant's means of sensing time and responding to season. 



- HOW PHOTOPERIODISM WAS DISCOVERED 



Three Tobacco Plants and a Few Soybeans 



Like some other laws of nature, photoperiodism was discovered in 

 simple circumstances. On July 10, 1918, while armies in France were 

 readying for the Second Battle of the Marne, two scientists in Virginia 

 at 4 o'clock in the afternoon took three tobacco plants and a box of soybean 

 seedlings into a small, crude shack. They had placed the shack in a field 

 at the USDA experimental farm, which was then across the Potomac from 

 Washington, D. C. They needed a dark room to try the latest of their 

 uncounted attacks on one of nature's obstinately held secrets. This day 

 they were making scientific history. 



