30 MOVEMENTS OF CURVATURE 



LIGHT and DARKNESS exercise effects dependent largely upon the 

 duration of the exposure. The autonomic x variation and nutation 2 

 movements of plants in a condition of phototonus continue at first unaltered 

 in darkness. In the prolonged absence of light, however, the variation- 

 movements gradually decrease, and cease with the onset of dark-rigor 3 . 

 Nutation-movements, on the other hand, continue as long as growth does, 

 becoming actually more pronounced in some plants, but decreasing in 

 others. For instance etiolated plants of Tropaeolum and Polygonum show 

 pronounced circumnutation, whereas circumnutation decreases so much 

 in etiolated shoots of Dioscorea Batatas and Mandevillea suaveolens 4 that 

 the plants are no longer able to twine. Etiolated shoots of Phaseolns 

 and Ipovwea purpurca, however, circumnutate actively and twine readily 

 in darkness 5 . Other special peculiarities have without doubt yet to be 

 discovered, and it is highly probable that changes of photonasty involve 

 alterations in the power of autonomic movement 6 . 



Autonomic movements are affected by the conditions of turgidity, by 

 the supply of food and by various chemical stimuli 7 . Darwin found, for 

 example, that the absorption of a little ammonium carbonate excited active 

 oscillating movement in the two leaf-segments of Dionaea mnscipnla 

 and in the leaf-tentacles of Drosera 8 . The action of shaking, in retarding 

 growth and in equalizing the tissue-strains, enables us to understand why 



excited in the leaflets of the frond of Aspknium trie ho manes, according to Asa Gray and Loomis, 

 Bot. Gazette, 1880, pp. 27, 43 (quoted by Darwin, I.e., 1880, p. 257). Fritzsche (I.e., p. 15) con- 

 siders this to be due to the changes of temperature influencing the transpiration and hence the position 

 of the leaflets. 



1 Pfeffer, Periodische Bewegungen. 1875, p. 155. 



2 Darwin, Climbing Plants; The Power of Movement in Plants (Twiners); Fritzsche, I.e., 

 p. 14 (Seedlings); Dewevre et Bordage, Revue gen. de Bot., 1892, T. IV, p. 73 (Coloured Light). 

 Rothert (Cohn's Beitrage z. Biologic, 1894, Bd. xxvi) states that the cotyledons of Avena and 

 Phalaris nutate somewhat more actively in darkness. 



3 Pfeffer, I.e., p. 155. According to Maige (Ann. d. sci. nat., 1900, 8 e se"r., T. XI, p. 331) 

 strong light diminishes the movements. 



4 Duchartre, Compt. rend., 1865, T. LXi, p. 1142. The torsion is also absent from these plants 

 in darkness. Stems of Dioscorea developed in light are able to twine in darkness, according to 

 de Vries, Arb. d. Bot. Inst. in \Viirzburg, 1873, Bd. I, p. 328. 



5 Mohl, Ranken- u. Schlingpflanzen, 1827, pp. 122, 150; Sachs, Bot. Ztg., 1865, p. 119; 

 Fritzsche, 1. c. 



6 Heckel (Du rnouvement vegetal, 1875, p. 551) finds the movements of the stamens of Ritta 

 and Saxifraga to be slower in darkness. Carlet (Compt. rend., 1873, T. LXXVII, p. 538) states 

 that the stamens of Ruta do not move at all in darkness. Organs pressed against one another or 

 against a support may not be able to move (cf. Pfeffer, 1. c., p. 48). Stahl (Bot. Ztg., 1898, p. 103) 

 concludes that the autonomic movements decrease in darkness, in order not to disturb the night- 

 position, but Ideological conclusions are valueless in comparison with empirical facts. 



7 Cf. Fritzsche, Die Beeinflussung d. Circumnutation durch verschiedene Factoren, 1899. The 

 statement that weak electrical currents increase the movements of the leaflets of Desmodium gyraiis 

 requires further proof. Cf. Kabsch, Bot. Ztg., 1861, p. 358 ; Meyen, Pflanzenphysiol., 1839, Bd - U r > 

 P- 557- 



' Darwin, The Power of Movement in Plants, 1880, pp. 237-9. 



