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PLANT PHYSIOLOGY 



influence of light, plants pass more rapidly through the grand 

 period of growth, the period of elongation terminating very 

 early, even before the growing organs have had time to elongate. 

 Consequently, shoots that have developed in strong light are 

 always shorter than those grown in shade. 



Fig. 34. — Seedlings of peas grown Fig. 35. — Betonica plants, P, grown 



for an equal period (a) in white, (6) in the valley, and M, upon a mountain 

 blue, and (c) orange light {after Duggar) . {after Bonnier) . 



The behavior of the seedlings at the moment they have pushed 

 their way through the earth and emerge into light is very inter- 

 esting: their growth is instantly checked. This afifects equally 

 all parts remaining in the soil. Under the influence of the light, 

 there evidently arises in the tip some growth-inhibiting stimulus 

 that is transmitted to the underground parts. Some authors 

 hold that this stimulus is a special growth-inhibiting hormone 



