126 



PHYSIOLOGY. 

 The lateral growth of secondary roots is dia- 



is progeotropism. 

 geotropism. 



The stem, on the other hand, whicli was placed in a horizontal 

 position has become again erect. This turning of the stem in 



Fig, 112. Fig, 113. 



Germinating pea placed in a hori- 

 zontal position. 



Figs. 112, 113. — Progeotropism of the pea root 



In 24 hours gravity has caused the root to 

 turn downward. 



the upward direction takes place in the dark as well as in the 

 light, as we can see if we start the experiment at nightfall, or 

 place the plant in the dark. This up- 

 ward growth of the stem is also influ- 

 p'f enced by the earth, and therefore is a 

 case of geotropism. The special desig- 

 nation in the ca,se of upright stems is 

 nega/ive gentropism, or apogeotropism, or 

 the stems are said 

 to be apogeotropic. 



Fig, lr4. 

 Pumpkin seedling sliowing apogeotro]nstn. Seedling at the left ]>laced In 

 zuntally, in 24 hours the stem has 



If we place a rapidl)- gro\. ing jiolted plant in a horizontal 



position by la)'ing the pot on its side, the ends of the shoots 



will soon turn upward again «]icn placed in a horizontal 



position. Young bean |ilants growing in a jiot began within 

 two hours to turn the ends of the shoots ujiward. 



