DIURNAL THERMONASTIC MOVEMENT 547 



ami there is no essential difference in the production of 

 such curvatures in pulvinated, growing, and fully grown 

 organs. The exhibition of nyctitropic movement by the 

 fully grown, and rigid ' Praying Palm ' is a striking de- 

 monstration of the unity of response of all plant organs 



As regards the distinction between the tropic anil nastic 

 niovements, it will be fouml that thei'e is no sharp line 

 of demarcation between the two. A movement is said to 

 be tropic, when unilateral stimulus acts on an organ and 

 induces in it u directive movement. Curvature induced by 

 diffused stimulus on a dorsiventral or anisotropic organ 

 (with differential excitabilities of the two halves) is termed 

 nastic. Daylight is supposed to act diffusely i^i.e., equally 

 on all sides) on leaves ; this is, however, not strictly true? 

 since the light from sky above is stronger than from 

 ground below. Moreover, the tropic action c^f unilateral 

 light may become nascic by internal diffusion of excitation. 

 This is seen in the response of the pulvinus of Mimosa 

 to light acting from above. The leaf at first moves 

 upwards towards the stimulus, the response being positive- 

 ly phototropic. But under the continued action of lights 

 excitation becomes internally diffused, and the leaf under- 

 goes a fall by the greater contraction of the more excit- 

 able lower half of the organ (p. 331). No sharp distinction 

 can therefore be made between the movements of growth 

 and of variation, between tropic and nastic curvatures. 



The employment of the term 'nastic' is, however, 

 convenient when used in a well-defined and restricted 

 sense. "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".* It will sometimes 

 be necessary, in the course of this paper, to discriminate 



* Strasburger — " Text-book of Botany " (1912), p 30U. 



