CHAPTER XX 



THERMONASTIC MOVEMENT OF NYMPHAEA 



The term ' nastic ' is convenient when employed in the 

 restricted sense as defined by Strasburger. * We speak of 

 tropism when the organ takes up a resting position definitely 

 related to the effective stimulus. Nastic movements, on the 

 other hand, are curvatures which bring about a particular 

 position in relation to the plant, and not to the direction of the 

 stimulus.' ^ 



In describing the direction of responsive movements, 

 confusion is likely to arise unless the observer's point of 

 view be carefully defined. An up-movement of the petal 

 in a flower means approach towards the growing-point of 

 the axis. This may be variously described as movement of 

 closure or of folding. A down-movement may, on the other 

 hand, be described as a movement of opening or of unfolding. 

 If the movement be nastic, then the closure or the opening 

 movement wdll remain the same, whether the organ be held 

 in normal position or upside down. If, on the other hand, 

 the direction of the movement be determined by the para- 

 tonic effect of an external stimulus, gravity for example, 

 then the responsive movement in relation to the plant will 

 be different. The closure-movement in the normal position 

 will, on inversion, be reversed into a movement of opening. 

 The reversal of closure into an opening movement or vice 

 versa will thus be a test of the paratonic effect of geotropic 

 stimulus. 



Typical examples of thermonasty are afforded by the 



1 strasburger, Text-Book of Botany (1912), p. 300. 



