THE ORIGIN OF THE DAILY PHOTONASTIC PERIODICITY 113 



and which pass from the closed to the fully expanded condition in a few 

 minutes when the temperature rises from 12 to 22 C. (Fig. 32). The 

 flowers of Adonis vernalis, Ornithogalum umbcllatum, and Colchictim 

 autumnale react more slowly and less strongly, while those of Ranunculus 

 Ficaria^ Anemone nemorosa, and Malope trifida are still less sensitive, 

 although they respond to changes of temperature of 5 to 10 C. by a 

 distinct movement. On the 

 other hand, such changes of 

 temperature induce only a 

 slight thermonastic movement 

 in the flowers of Oxalis rvsea, 

 Nymphaea alba, Leontodon, 

 and Taraxacum \ and flowers 

 which open once only show 

 no distinct thermonastic re- 

 sponses, although their open- 

 ing is hastened by moderately 

 high temperatures and re- 

 tarded by low ones. 



Foliage-leaves usually re- 

 act but feebly to changes of 



temperature, although these induce perceptible thermonastic responses in 

 the pulvini of the leaflets of Oxalis acetosella, Desmodium gyrans, Averrhoa 

 Bilimbi, and Mimosa pudica 2 . In addition, the bilobed leaf of Aldrovanda 

 only opens when the temperature is raised sufficiently, while either a rise 

 or a pronounced fall of temperature may produce a thermonastic curvature 

 in dorsiventral tendrils and this is similar in character to the thigmotropic 

 curvature 3 . 



FIG. 32. Flower of Crocus luteus. A closed, B expanded 

 owing to a rise of temperature. 



1 Pfeffer, Physiol. Unters., 1873, p. 194; Periodische Bewegungen, 1875, p. 122. Crocus and 

 Tulipa react so rapidly that the movement may be demonstrated to a large audience by means of 

 a projection lantern. Cf. Pfeffer, Jahrb. f. wiss. Bot., 1900, Bd. xxxv, p. 731. The simple observa- 

 tion that warming hastened the opening of the flowers of an anemone was made by Cornutus (quoted 

 by Ray, Historia plantarum, 1686, Vol. I, p. 2). Hofmeister (Flora, 1862, p. 516) found that varia- 

 tions of temperature produced opening and closing movements in the flower of the garden tulip ;^and 

 Royer (Ann. sci. nat., 1868, v e ser., T. IX, p. 355) regarded changes of humidity and temperature 

 as being responsible for the opening and closing of flowers. The true relationships were 'then 

 established by Pfeffer. Additional instances of thermonastic flowers are given by Hansgirg, 

 Physiolog. u. Phycophytolog. Unters., 1893, pp. 27, 64. According to Mikosch (Bot. Jahrb., 1878, 

 p. 219), changes of temperature induce the opening and closing of the anthers of Bulbocodium 

 vernum and of certain species of Alchemilla, but it is not certain whether the reaction is a true 

 thermonastic one. 



* Pfeffer, Physiol. Unters., 1873, pp. 65, 78 ; Periodische Bewegungen, 1875, p. 135 ; Darwin, 

 The Power of Movement in Plants; Jost, Jahrb. f. wiss. Bot., 1898, Bd. xxxi, p. 376 ; Bot. Ztg., 



iS97> P- 35- 



3 Correns, Bot. Ztg., 1896, p. 2 ; Macdougal, Bot. Centralbl., 1896, Bd. LXVI, p. 145; Fitting, 

 Jahrb. f. wiss. Bot., 1903, Bd. xxxvm, p. 562. 



PFEFFER. Ill 



