IRRITABILITY. 



83 



is progeotropism. The lateral growth of secondary roots is dia- 

 geotropism. 



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- In 24 hours gravity has caused the root to 



zontal position. turn downward. 



Figs. 68, 6g. — Progeotropism of the pea root. 



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 geotropisvi, or apogeoiropism, or 

 the stems are said 

 to be apogeotropic. 



Fig. 70. 

 Pumplcin 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 wdthin 

 two hours to turn the ends of the shoots upward. 



