,^ FOOD MAKING 85 



become giants or they may flower while very young and only 

 a small fraction of their natural size. 



This is one of the methods used by greenhouse people to 

 regulate the time of flowering of those plants that would 

 otherwise bloom in certain seasons. It has always governed 

 the time of planting such crops as lettuce, corn, wheat, etc., 

 but the reason for the time requirement was not known. By 

 regulating the length of day, greenhouse people control the 

 time of flowering of those plants that would otherwise have 

 seasonal bloom and plant breeders force plants that would 

 ordinarily flower at different times to flower at the same time 

 for cross-pollination. Such plants as some varieties of po- 

 tatoes and Jerusalem artichokes respond to length of day by 

 storage of more food in their tubers with a short day. Bulb 

 and stem growth in some plants responds in a similar way. 



The reason for length of day phenomena is not known. 

 It is not a matter of quantity of light since the light necessary 

 is far below the minimum amount for photosynthesis, but 

 rather a matter of the number of hours. For this reason the 

 discussion may not belong in a chapter on food making, but 

 it is possible that some unknown substance is made which 

 can be considered a food. A difference of less than half an 

 hour in day length at the critical point will determine the 

 growth into flowering or continued increase in size.^ Much 

 work has been done on the problem, studying such fields as 

 nitrogen assimilation, temperature, stored food, and growth 

 hormones, but at present the problem remains unsolved. 

 The fact that a whole plant responds to an increase in the 

 light period for a few of its growing leaves, strongly suggests 

 it to be a hormone reaction. 



1 At the present time experiments are in progress at the United States Department 

 of Agriculture's experiment station at Beltsville, Md., to perfect the discovery they made 

 by which the short-day fall-blooming chrysanthemums are prevented from blooming by 

 giving them a short period of light in the middle of the night. It is possible that ten 

 minutes of a low light intensity each midnight on these October-blooming plants will 

 delay them until January or February. But the interesting part is that they can be 

 brought into flower any time during this period by discontinuing the artificial light. 



