112 MOVEMENTS OF CURVATURE 



changes of illumination or of temperature, or whether it was the result of the 

 regulation of a hereditary periodicity. De Candolle at first inclined to the former 

 view, but later appears to have assumed that the periodicity was hereditary 1 . 

 Dutrochet 2 , Sachs 3 , and Hofmeister 4 all adopted the same view, and apparently 

 considered that the periodic illumination regulated the rhythm, but did not induce 

 it, while the continuance of the movements in darkness was ascribed either to a 

 hereditary periodicity or to the incomplete absence of light. Pfeffer then showed, 

 in 1876, the induced character of the periodicity, and pointed out that the daily 

 movements might be produced by thermonastic responses as well as by photo- 

 nastic ones, or by a combination of the two. Royer B went, however, too far in 

 ascribing all sleep-movements of flowers to changes of temperature, while it is 

 evident that all daily movements are not the result of circumnutation as Darwin 

 supposed, nor is the daily periodicity capable of hereditary transmission. 



Methods. Pfeffer employed the light from a couple of Argand burners, which was 

 passed through cold water to diminish the heating effect 6 . Nowadays, incandescent 

 burners, arc lights, or Nernst lamps might be used in preference. The incandescent 

 electric light is less suitable for the reinduction of the photonastic periodicity, since 

 it contains relatively fewer of the blue rays, which exercise the greatest photonastic 

 action 7 . It has been observed that certain plants cease to perform sleep-movements 

 during the continuous summer day of high northern latitudes, as in the north of 

 Norway 8 . By the aid of artificial illumination, the periodicity may be reversed, 

 so that the sleep-movements take place in the daytime, or it may be lengthened or 

 shortened 9 . Experiments in darkness are only decisive when the absence of light 

 does not appreciably affect the power of reaction. 



SECTION 24. Thermonastic Curvatures. 



Apart from the general influence of temperature on growth, a special 

 power of thermonastic response has been developed by various flowers, in 

 which low temperatures produce closing movements and high temperatures 

 opening ones. The flowers of Crocus vernus and Crocus luteus are especially 

 responsive, as are also those of Tulipa Gesneriana, for these flowers per- 

 ceptibly respond to a change of temperature of half a degree centigrade, 



1 A. P. de Candolle, Physiologic des Plantes, a German translation by Roper, 1835, Bd. n, 

 p. 640. 



2 Dutrochet, Memoires p. serv. a 1'histoire etc., Bruxelles, 1837, P- 2 ^7- 



3 Sachs, Flora, 1863, p. 469. 



* Hofmeister, Pflanzenzelle, 1867, p. 331. 



5 Royer, Ann. de sci. nat., 1868, v e sen, T. IX, p. 355. Cf. Pfeffer, 1. c., p. 170. 



6 Pfeffer, 1. c., p. 31. The experiments of other authors are discussed here. 



7 Cf. Pfeffer, 1. c., p. 67. 



8 Cf. Schiibler, Die Pflanzenwelt Norwegens, 1873, p. 88 ; Bot. Jahresb., 1880, p. 262. 



9 Pfeffer, I.e., pp. 40, 55. On the registration of the movement see Baranetzsky, Ber. d. bot. 

 Ges., 1899, p. 190. 



