MOVEMENTS. 115 



morning and evening, or upon a change of weather. The 

 leaves of various species of Oxalis, Mimosa, etc., are sensi- 

 tive to contact and concussion. A violent or repeated 

 concussion, in some cases even gentle contact, causes the 

 parts to assume the position of sleep. The stamens of the 

 Barberry, in contact with the corolla when at rest, curve 

 inwards when lightly touched near the base, bringing 

 the anther in contact with the stigma. The stamens of 

 the Centaurea and other Compositw are sensitive to irrita- 

 tion. When at rest, their free filaments, bearing synge- 

 uesious anthers, curve concavely outwards; on contact 

 or concussion, they contract and straighten, lengthening 

 again after some minutes, and resuming their curved form. 

 " This phenomenon occurs only while the style is growing 

 through the anther-tube, and the pollen is being emptied 

 into the tube. The motion of the filaments efiected by 

 insects causes the anther-tube to be drawn downwards, and 

 a portion of the pollen thus to escape above it, which is 

 then carried away by insects to other flowers and capitula 

 when the stigmas are already unfolded." 



147. It often happens that growth takes place more 

 rapidly first on one side of an organ, and then on the other 

 side; and then by the alternating rate of elongation on the 

 two sides, a movement of nutation in one plane will 

 take place. Many leaves furnish a good example ; in the 

 bud the greater growth is on the under or outer side of the 

 leaf, the latter bending upwards ; but on the opening of the 

 bud the greater growth takes place on the upper side. 

 Floral leaves often exhibit such nutations, as do also many 

 stamens and styles. If the parts of unequal growth, 

 instead of alternating from side to side, pass regularly 

 around the organ, a revolving nutation will be the 



