io6 MOVEMENTS OF CURVATURE 



the light is withdrawn all the leaves capable of growth gradually assume 

 a more erect position. On the other hand, darkness favours the epinastic 

 growth of the leaves of Impatiens, Helianthus annmis, Ceratophyllum, and 

 MyriophyttuWi and hence causes a more or less pronounced downward 

 curvature of the leaves 1 . 



Certain flowers which perform no evident sleep-movements respond to 

 the presence or absence of light. Thus the flowers of Gagea lutea, Gentiana 

 campestris, Stellaria media, Holosteum umbellatum, Veronica alpina, and 

 Drosera longifolia develop and fade without ever opening in darkness 2 , and 

 are therefore ' photo-cleistogamic.' The flowers of Stellaria media require 

 a considerable intensity of light to induce their expansion, and hence 

 remain closed when grown behind a window facing north. 



All these considerations apply only for moderate intensities of light, 

 and leave it an open question whether under sufficiently intense diffuse 

 illumination the reaction would be reversed. The cases in which movements 

 have been observed in the leaves of Acacia, Mimosa, Robinia, &c., in response 

 to strong sunlight falling on one side give no satisfactory answer, since these 

 are heliotropic curvatures towards the light performed by the motile 

 pulvini. In this way the blades of the leaflets are placed parallel to the 

 incident rays. This reaction, sometimes termed midday sleep, was called 

 paraheliotropism by Darwin 3 , and is due to the unilateral illumination pro- 

 ducing a greater fall of turgor in the more strongly illuminated half of the 

 pulvinus than in the less strongly illuminated one. In this position the 

 chloroplastids are protected, and the transpiration is usually diminished 4 . The 

 leaflets of Cassia montana, however, assume positions which tend to increase 

 transpiration, the stomatic ventral surfaces facing outwards or upwards, so 

 that the plant apparently risks a fatal loss of water in order to keep down 

 the insolation temperature 5 . That the response in not due to the localized 

 warming of the exposed side of the pulvinus is shown by the fact that it 

 takes place when the pulvini are submerged under water, and, as in the case 



1 Frank, Die natiirl. wagerechte Richtung von Pflanzentheilen, 1870, p. 46; Detmer, Bot. Ztg., 

 1882, p. 787 ; Wiesner, Bot. Ztg., 1884, p. 677 ; Vines, Annals of Botany, 1889, Vol. ill, p. 421 ; 

 Czapek, Jahrb. f. wiss. Bot., 1898, Bd. xxxn, p. 272 ; Mobius, Biolog. Centralbl., 1894, Bd. XV, 

 pp. 8, 14. 



2 Vochting, Jahrb. f. wiss. Bot., 1893, Bd. xxv, p. 180; Hansgirg, Physiol. u. PhycophytoL, 

 Unters., 1893, pp. 27, 45, 53 ; Beihefte z. bot. Centralbl., 1902, Bd. XII, p. 271 ; Oltmanns, Bot. Ztg., 

 l8 95> P- 3 1 5 Leclerc du Sablon, Rev. ge"n. de bot, 1900, T. xii, p. 305. 



8 The Power of Movement in Plants, 1880, p. 445. Cf. also Pfeffer, Periodische Bewegungen, 

 1875, p. 62; Hansgirg, 1893, 1. c., p. 134; Oltmanns, Flora, 1892, p. 238; Wilson, Contributions 

 from the Botanical Laboratory of the University of Pennsylvania, 1892, Vol. I, p. 66; Ewart, Annals 

 of Botany, 1897, Vol. xi, p. 447; Jost, Jahrb. f. wiss. Bot., 1898, Bd. xxxi, p. 385. 



* Wiesner, Die natiirl. Einrichtungen zum Schutze des Chlorophylls, 1875, p. 62 ; Stahl, 

 Bot. Ztg., 1897, p. 91. 



5 Ewart, 1. c., p. 456. 



