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"-BIOLOGY FOR BEGINNERS 



with an opening at one side for light to enter. The other is placed 

 under the same conditions of heat and moisture, but is given light 

 from all sides. 



The plant in the box will be found to turn toward the light and 

 to grow rapidly in that direction. However, its stem will be weaker 



FIG. 28. 



Coleus leaf showing green and Similar leaf treated with iodine, the 



white areas, before treatment with starch reaction only showing where 

 iodine. the leaf was green. From Atkinson. 



and slenderer, its leaves smaller and paler than the one with 

 uniform lighting. 



This experiment shows the response that plants make to light, 

 and also the effect of a limited supply of light on their growth. 

 Every time we see the leaves of house plants turning toward the 

 window, we have a similar experiment in heliotropism. The 

 plant kept outside the dark box was used as a check for this ex- 

 periment. 



