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chapter five t Floral Hormones 



and the Induced State 



Even before the effects of light and temperature— the major 

 natural environmental influences on flowering— were known, the 

 question of what internal changes lead to flowering was of obvious 

 importance; photoperiodism and, to a lesser extent, vernalization 

 made experimental approaches to it more feasible. The next three 

 chapters are largely concerned with this question in one way or 

 another; the present will examine the nature and origin of sub- 

 stances controlling flowering and transmissible from one part of a 

 plant to another or from plant to plant by grafting. 



DEFINITIONS AND BACKGROUND: AUXINS AS 



PLANT HORMONES 



Hormones can be defined as substances produced in one part 

 of an organism and acting in another, and active in very low con- 

 centrations. Action at a distance from the site of production is the 

 most crucial characteristic of a hormone; activity in low concen- 

 trations simply serves to distinguish it from substances furnishing 

 energy or structural materials and used in large quantities. Sugars, 

 for example, are produced in aerial parts of the plant and used 

 in the roots (as well as elsewhere) but cannot be considered hor- 

 mones. 



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