The crops which have been mentioned illustrate clearly some of the 

 various responses which different plants may show. Suggestions 

 for practical application have grown out of past experiments, and 

 others will follow from other experiments still in progress. 



The equipment with which Garner and Allard first worked was rela- 

 tively simple. Similar apparatus is still extremely useful in photo- 

 periodic studies, but it has been essential to supplement it for closer 

 observation and more detailed experimentation. Their plants were 

 grown in pots. Some of them were placed on trucks which could be 

 conveniently moved into dark houses nearby so that the plants could 

 be exposed to any determined photoperiod. 



Garner and Allard also used a small chamber. This was so arranged 

 that boxes containing living plants could be fitted into the side of it, 

 and the whole was left open or darkened at will. The illustration on 

 page 18 shows clearly how a single plant may exhibit two responses 

 to photoperiod at the same time. The shorter, more bushy plant is 

 a species of Sedum, or liveforever, and the other is a species of Rud- 

 beckia. The half of each plant outside the box has been exposed each 

 day to the natural period of illumination prevailing during the sum- 

 mer in Washington, which reaches a maximum of approximately 15 

 hours. As a result of exposure to this relatively long photoperiod the 

 stem has elongated and the plant has come into flower. The other 

 half of each plant, inside the box, has been exposed to photoperiods 

 of but 10 hours each day. Although there has been some growth, 

 the rosette stage still persists and there are no evidences of flower 

 buds or flowers. This is a particularly striking demonstration of how 

 the effects of photoperiod may at times be very precisely localized. 



Such localization of photoperiodic response does not always occur, 

 for in many instances such responses may be shown by portions of the 

 plant at some distance from the part directly exposed to light. Such 

 an instance is especially well illustrated by the Jerusalem-artichoke, 

 which produces tubers in darkness below ground when the leaves are 

 subjected to conditions of short photoperiod. If the leaves are exposed 

 to long photoperiod, no tubers are formed. The response to photo- 

 period is indicated mainly by the extent of tuber formation rather 

 than by any readily observable effects on the leaves. 



The necessity for closer observation and more detailed experimen- 

 tation has increased as understanding of the problem has progressed, 

 and its greater complexity has become more clearly recognized. This 

 is particularly true in attempts to study and evaluate the relative 

 effects of photoperiod and temperature and their possible interrela- 

 tionship. As a result, it is necessary to devise ways and means by 

 which most environmental conditions, especially temperature and 

 light, can be much more precisely controlled. A greenhouse well 

 equipped for making such studies on photoperiodism is illustrated. 



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