Plants are grown on benches and on trucks, which may be readily 

 placed in dark houses whenevc" essential. Ample provision is made 

 for artificial illumination by means of electric lamps. Beneath the 

 glass jars plants may be grown with different amounts of moisture or 

 any desired combination of gat i in the atmosphere. All these factors 

 influence photoperiodic sensiti 1 ty, and it is important that they be 

 studied critically because they represent natural conditions prevailing 

 in some sections of the United S ates or in other agricultural countries. 



Painstaking detail in experimentation constantly increases under- 

 standing. One of the most significant findings of detailed experimen- 

 tation is the fact that in general the leaves of a plant are the organs 

 through which photoperiodic conditions primarily influence it. 

 It is not necessary, for example, to expose an entire Biloxi soybean 

 to short photoperiods to induce it to bloom; it will bloom promptly 

 if only a few of the leaves are subjected to a short photoperiod, even 

 though all the rest of the above-ground parts receive long photo- 

 periods. Moreover, if a single leaf of a variety such as Agate soy- 

 bean, which blooms readily even when photoperiods are long, is 

 grafted to Biloxi, which normally requires short photoperiods for 

 blooming, the Biloxi plant is thereby enabled to bloom on long photo- 

 periods also. This, together with much other evidence based on crit- 

 ical experiments, indicates that under certain conditions the plant 

 forms some substance which may be transported to other parts of a 

 plant or to another plant where the production of flowers occurs. A 

 similar situation, resulting in tuber formation underground, occurs in 

 the Jerusalem-artichoke. 



It is not too much to hope that eventually substances which when 

 applied may aid in the control of growth and flowering may be either 

 obtained directly from plants or manufactured artificially. Two sub- 

 stances, ethylene and acetylene, have already been shown to hasten 

 flowering when properly applied to pineapple plants. They have 

 already been used for this purpose under field conditions. Undoubt- 

 edly others will be discovered and put to use. 



From the instances just cited, it is clear that at present means are 

 available and practices have been established whereby it is possible, 

 commercially, to take advantage of the part which photoperiod plays 

 in the way plants develop or to offset such disadvantages as it may 

 impose. Once the facts and details are established it is not difficult 

 to work out ways and means of turning knowledge of them to imme- 

 diate use. 



22 



U.S. GOVERNMENT PRINTING OFFICE: 1942 



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