IRRITABILITY. 



pism. So the lateral growth of the secondary roots is termed 

 diageotropism. 



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

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



Fig. 68. Fig. 69. 



Germinating pea placed in a hori- 

 zontal position. 



Figs. 68, 69. 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- 

 enced by the earth, and therefore is a 

 case of geotropism. The special desig- 

 nation in the case of upright stems is 

 negative geotropism, or apogeotropism, or 

 the stems are said 

 to be apogeotropic. 



Fig. 70. 



Pumpkin seedling showing apogeotropism. Seedling at the left placed hori- 

 zontally, in 24 hours the stem has become erect. 



If we place a rapidly growing potted plant in a horizontal 



position by laying the pot on its side, the ends of the shoots 



will soon turn upward again when placed in a horizontal 



position. Young bean plants growing in a pot began within 

 two hours to turn the ends of the shoots upward. 



