94 BOTANY. 



193. The relatives of the beans (i.e., the leguminous 

 plants, or Leguminosse) have been most frequently observed 

 in a sleeping state; but it is very likely that when we study 

 them attentively very few of the higher plants will be 

 found which are wanting in this power. 



194. The familiar closing of certain flowers at night and 

 opening again in the morning, and the exactly reversed 

 action, are to be regarded as of the same nature as the 

 diurnal and nocturnal position of leaves. 



195. The turning of leaves and stems toward the light, 

 as is commonly seen in a plant grown in a window, is re- 

 garded by Mr. Darwin as a modified circumnutation. Here 

 the lateral light controls ordinary nutation, and modifies it 

 so that, instead of describing ellipses, the leaf or stem moves 

 in a zigzag course toward the light. The stronger the 

 light the more nearly will the course approach to a straight 

 line. Some plants or parts of plants when exposed in this 

 way to the light bend away from it : this is well seen in the 

 runners of the so-called strawberry-geranium (Saxifraga 

 sarmentosa), a well-known pretty little basket-plant. This 

 last kind of bending is known as hegative heliotropism, 

 while the bending toward the light is distinguished as posi- 

 tive heliotropism. 



196. Allied to the foregoing is the bending of roots and 

 stems toward or away from the earth, i.e., with or in oppo- 

 sition to the force of gravitation. It is a familiar fact that 

 in the growth of most seedlings the roots grow downward 

 while the stems take an upward direction. Experiments 

 made long ago proved that this was due in some way to 

 the action of gravitation, and Mr. Darwin now considers it 

 to be the result of gravitation acting upon and modifying 

 the circumnutation of root and stem. Geotropism (as this 



