THE INFLUENCE OF THE EXTERNAL CONDITIONS 29 



amount of drooping curvature 1 . The existence of such mechanical curvatures is 

 easily shown by hanging the apex downwards or by laying it on a horizontal sheet 

 of paper and noting the remaining curvature. The straightening of the apex on a 

 klinostat shows that this curvature results from the antagonism of klinogeotropism 

 and orthotropism. The somewhat irregular character of the curve described is due 

 to the action of disturbing factors. 



SECTION 6. The Influence of the External Conditions. 



Autonomic movements like all vital phenomena are dependent upon 

 the external conditions, and are accelerated or retarded according to the 

 temperature and the supply of food, oxygen, or water. The stimulating 

 action of light or gravity may also form an essential or favouring condition 

 for movement. The effects of a change in the external conditions may 

 persist for a long time, and since a local or general stimulus may modify 

 or awaken various correlated activities, it is not always easy to distinguish 

 between autogenic and aitiogenic movements. Nor can any general rules 

 be laid down. For instance an increased rate of growth, or in general 

 a greater demand, results in a diminution of the autogenic movements in 

 some cases, whereas in others they increase 2 . It is, however, commonly 

 observed that any serious general disturbances are reflected in the autonomic 

 movements, with the result that when these are normally feeble they are 

 excited to greater activity. 



A certain temperature is necessary for all autogenic movements, 

 the optimum in the case of the leaflets of Desmodium gyrans being as high 

 as 35 C., and the movements becoming slower as the temperature falls. 

 Dutrochet 3 observed that the tendrils of the Pea circumnutated once in 

 9 to ii hours at 5 to 6 C, but in i hour 20 minutes at 24 C. Darwin 4 

 observed that the pronounced circumnutation of the internodes and tendrils 

 of Eccremocarpus scaber ceased in a cool house, in which slow growth 

 continued. Fritzsche 5 found that any rise of temperature below the 

 optimum increased the rapidity and amplitude of the feeble nutation- 

 movements of the stems of seedlings. It is, however, also possible that 

 the rapidity of movement might increase while the amplitude decreased, 

 and in fact Darwin 6 observed a result of this kind when the leaflets of 

 Averrhoa bilimbi were subjected to rising temperatures. 



1 Baranetzsky, 1. c., pp. 19, 48. A drooping shoot curves geotropically upwards at the apex, 

 where the static moment is least, and hence attains a p^ shape. 



2 Askenasy (Ber. d. hot. Ges., 1890, p. 77) states that the nutation of roots decreases when 

 growth is active, whereas Fritzsche (1. c.) obtained in some cases exactly opposite results. 



3 Dutrochet, Ann. d. sci. nat, 1843, 3 sen, T. XX, p. 312. Cf. also Simons and MacKenney, 

 Bot. Jahrb., 1898, 1, p. 594. 



4 Darwin, Climbing Plants, 1875, pp. 72, 103. 



5 Fritzsche, Die Beeinflussung der Circumnutation durch verschiedene Factoren, 1899, p. 23. 



6 Darwin, The Power of Movement in Plants, 1880, pp. 331-5. A rapid oscillation is also 



