HELIOTROPES, OR SUNFLOWERS. 183 



movement takes place, for the leaflets rise upwards 

 and present their edges to the light. This is the 

 movement called " paraheliotropism," and is made in 

 the reverse direction to the nocturnal motions of the 

 same organs. 



The other example is also that of a plant which 

 has been alluded to as going to sleep at night, viz., 

 the wood-sorrel {Oxalis acetosella). In this plant the 

 leaves are depressed under the influence of a strong 

 light, just in a similar manner to their nocturnal 

 movements when deprived of light. The motion in 

 one case is not distinguishable from that in the other. 

 It is stated on the authority of Professor Batelin that 

 the leaflets of this plant may be exposed to the sun- 

 light daily for many weeks, and not suffer from the 

 exposure if they are allowed to depress themselves. 

 But if the depression of the leaflets is prevented, by 

 any mechanical means, they lose their colour, and 

 wither in two or three days. Yet the duration of the 

 leaves naturally is about two months, when subjected 

 only to diffused light. Under such latter conditions 

 they do not become depressed during the day. 



That the leaves of a tree, or a plant, should inherit 

 the faculty of self-preservation by this simple ex- 

 pedient, is one of the " curiosities of vegetation.',' 

 That it may be something more, we leave to the 

 ■conviction which it may present to thinking minds. 



