94 BOTANY. 
193. The relatives of the beans (ie., the leguminous 
plants, or Leguminos) 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 negative 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 isa 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 
