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PHYSICAL FACTORS: LIGHT 



The light period most favorable for the production of reserve food 

 in such structures as tubers, bulbs, and thickened roots, however, 

 may differ from both that which is most favorable for vegetative 



Fig. 58.^Wild lettuce {Lactuca spicata). Plants at left, exposed to a ten- 

 hour day beginning March 29, produced much larger, coarser leaves than the 

 control plants (at right) exposed to the full summer length of day. Under 

 the short-day exposure both leaf and stem were far more hairy than under the 

 long-day conditions. The stature was greatly reduced, but the time of flowering 

 was not affected. (After Garner and Allard.) 



growth and that which induces reproduction. These facts have great 

 practical value because with an adequate knowledge of the responses 

 of plants to length of day it should be possible to plant field and 



