99 



stimulus. Every body must have observed that plants 

 bend towards the direction from whence its brightest 

 influence proceeds. M. Bonnet, the French botanist, 

 demonstrated this in some very satisfactory experi- 

 ments, by which he shewed that plants grown in a 

 dark cellar all extended themselves towards a small 

 orifice admitting a few rays of light. Every flower 

 almost has a particular degree of light requisite for 

 its full expansion. The blossoms of the pea, and of 

 other papilionaceous plants, spread out their wings in 

 fine weather, to admit the solar rays, and again close 

 them at the approach of night. Plants requiring a 

 powerful stimulus do not expand their flowers until 

 noon, whilst some would be destroyed if compelled 

 to open in the meridian sun. The night-blooming 

 cereus unfolds its flowers only at night. Heat also 

 acts as a stimulus upon plants. M. Duhamel ob- 

 served, that during moderately fine weather the foot- 

 stalk of a leaf of the sensitive plant {Mimosa pudica) 

 stood in the morning at an angle with the lower part 

 of the stem of 100 degs. ; at noon, the angle had in- 

 creased to 112, but at night had fallen to 90. If a 

 leaflet of this plant be but slightly touched, it imme- 

 diately shrinks avvay ; and the impulse being commu- 

 nicated, each pair of leaflets on the branch collapse 

 in succession ; and if the imipulse be strong, the very 

 branch itself will sink down by the side of the stem. 

 If an insect alight upon the upper surface of tlie 



