CHAPTER 13 



Light 



Light is a powerful factor in determining the course of development in 

 plants and has a much more important morphogenetic effect on them 

 than it does on animals. This is to be expected, since light is necessary 

 for photosynthesis and thus for the production of food. Experiment has 

 made it clear, however, that the morphogenetic influence of light is 

 much more subtle and indirect than this and results from its control not 

 only of food production but of various physiological activities in the 

 plant by which this food is distributed in the processes of growth and 

 differentiation. The role of light in plant development has been studied 

 actively for many years and is the basis of an extensive literature. Among 

 the more inclusive reviews of this field are those by MacDougal ( 1903<7 ) , 

 Burkholder (1936), and Parker and Borthwick (1950). 



Many of the early results are invalid because of the impossibility in 

 those days of exact control of light, as to its intensity and quality, in 

 experimental work, but most of these difficulties have now been over- 

 come, and light in a plant's environment can be manipulated with 

 relative ease. 



It is a matter of common observation that plants reach their greatest 

 size and vigor in good light and that insufficient illumination results 

 in weak and spindly growth even if water, soil nutrients, and tempera- 

 ture are at their optimum levels. Most of the experimental work with 

 light has involved not merely differences between light and darkness 

 but measured differences in the light stimulus itself. Three of these are of 

 chief importance: the intensity, the quality, and the duration of the 

 light. Intensity is the brightness of the illumination, the actual energy 

 of the radiation. Quality concerns the wave length of the light. Dura- 

 tion refers to the relative lengths of the alternating periods of light and 

 darkness to which the plant is exposed. These differences are not always 

 sharply separable, and one often modifies the effects of another. 



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