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TEXTBOOK OF PLANT PHYSIOLOGY 



hold that this stimulus is a special growth-checking hormone 

 diffusing from the top downward. Others think that light delays 

 the manufacture of the growth-promoting substance necessary for 

 the elongation of the cells (Art. 86). 



The dwarfing effect of light on the aerial organs is particularly 

 marked in high mountains, where, because of greater transparency 



Fig. 121. — Seedlings of peas grown for 



an equal period (a) in white, (b) blue, 



and (c) orange light (after Duggar). 



Fig. 122. — Betonica plants, P, grown in 



the valley, and M, upon a mountain 



(after Bonnier). 



of the atmosphere, solar light is considerably richer in ultra-violet 

 rays than in valleys. Alpine plants, as the investigations of 

 Bonnier have shown, are always of short stature. And when 

 common plants of the valley, for instance the dandelion, or 

 Betonica, are grown in high mountains, they assume the charac- 

 teristics of alpine plants (Fig. 122). The retarding influence of 



