THE MECHANICS OF AUTONOMIC MOVEMENT 31 



the circumnutating apices of many climbers become partially erect after 

 shaking \ Slight injuries do not affect the power of curvature, and this 

 may even be retained when the root-tip is cut off in such fashion as not 

 to induce any traumatic curvature 2 . Since severe injury to the root- 

 system does not perceptibly affect the nutation of the shoot, the lessened 

 circumnutation of the cut shoots of twiners is either due to the manipulative 

 disturbance, or to a diminution of turgidity 3 . 



SECTION 7. The Mechanics of Autonomic Movement. 



It is not definitely known in a single case whether the average rate of 

 growth alters or not during spontaneous movement. Presumably, however, 

 the nutation curvatures of Oedogonium and of Zygnemaceae are attended by 

 an acceleration of growth, and it is not unlikely that the feebler nutation 

 movements are connected with the continual variations in the activity of 

 growth in length. It is, however, uncertain whether the circumnutation 

 of twiners involves an increase in the average rapidity of growth. In 

 any case it does not follow that the maximum curvature should take 

 place at the period of most active growth, so that during elliptical nutation 

 growth would be most active during the passage of the extremities of 

 the major axis of the ellipse of curvature 4 . 



Autonomic growth curvatures are certainly not always produced in 

 the same way. Plastic growth takes place in Oedogonium^ but it is not 

 known in a single case whether rises of turgor come into play, de Vries' 

 researches being inconclusive in this respect. The curvatures of shoots, 

 and of filaments of Spirogyra persist when the plant is suddenly killed 

 by immersion in hot water, so that the growth responsible for the curvature 

 takes place without any perceptible preparatory elastic stretching 5 . 



The movements of the leaflets of Trifolium pratense and of Oxalis 

 acetosella are produced by the expansion of one half of the pulvinus coupled 

 with a corresponding contraction in the other half 6 . This is shown by 

 the fact that the rigidity of the leaflet remains constant even during active 

 movement. If the tendency to expansion increased in both halves of the 

 pulvinus, but in one more than the other, the rigidity of the leaflet would 



1 Baranetzsky, Die kreisformige Nutation und das Winden der Stengel, 1883, p. 20. 



2 Darwin, 1. c., 1880, p. 540; Prantl, Arb. d. Bot. Inst. in Wiirzburg, 1874, Bd. I, pp. 548, 554; 

 Czapek, Jahrb. f. wiss. Bot., 1895, Bd. xxvn, p. 292 ; Fritzsche, 1. c., p. 31. 



8 Baranetzsky, I.e., p. 6l. 



* Cf. Wiesner, Die undulirende Nutation d. Internodien, 1878, p. 26 (Repr. from Sitzungsb. d. 

 Wien. Akad., Bd. LXXVII, Abth. i). 



5 Frank (Beitrage zur Pflanzenphysiol., 1868, p. 62) showed that the nutation of peduncles was 

 due to growth. Hofmeister (Pflanzenzelle, 1867, p. 324) also concluded that growth was in part 

 responsible for the movements. Cf. also Winkler, Kriimmungsbewegungen bei Spirogyra, 1902. 



6 Pfeffer, Periodische Bewegungen, 1875, pp. 88, 156. 



