1 1 4 MOVEMENTS OF CURVATURE 



According to Vochting 1 , sufficient cooling causes certain shoots of 

 Mimulus Tilingii and the flower-stalks of Anemone stellata to assume 

 a drooping position ; and, according to Lidforss 2 , the same applies to the 

 shoots of such plants as Lamium purpureum^ Veronica chamaedrys, and 

 Chrysanthemum leucanthemum. In addition, the evening drooping of 

 certain flowers and inflorescences appears in part to be the result of a 

 thermonastic reaction, such as may also be responsible for the drooping of 

 the shoot and leaves of many plants under natural conditions when the 

 temperature falls nearly to the freezing-point. It has, however, yet to be 

 determined whether these latter effects are actually due to a thermonastic 

 reaction, or are merely the result of the tissues being flaccid. The phe- 

 nomenon may indeed be as complex in origin as is the downward curvature 

 and plagiotropic position shown by the shoots of various plants in autumn 

 and winter 3 . It is worthy of note that the changed orientation takes 

 place slowly, and that the daily changes of illumination induce no marked 

 movement in these cases. 



Rapidly reacting thermonastic organs usually pass at first beyond the 

 position which they ultimately assume when the changed temperature is 

 maintained for some time. This is especially well shown by the flowers of 

 Crocus luteus, for the perianth-segments become temporarily partially 

 reflexed when the temperature is suddenly raised (Fig. 32, p. 113), whereas 

 when the temperature is raised slowly they hardly pass beyond the 

 position maintained by them so long as the new temperature remains 

 constant. 



After the removal of all the perianth-segments but one, exactly similar 

 reactions to sudden and to gradual falls of temperatures can be traced for 

 the closing movement as for that of opening. As the result of its thermo- 

 nastic properties the intact flower of Crocus may pass through its entire 

 period of development without ever opening if the temperature is kept 

 below 8 or even I2C. 4 The inflorescences of Leontodon hastilis, Hiera- 

 cium vulgatum and the flowers of Oxalis rosea remain closed at i to 3 C. 

 even in diffuse daylight, partially open at 8 to 10 C., but do not fully 



1 Vochting, Ber. d. bot. Ges., 1898, p. 42 ; Jahrb. f. wiss. Bot., 1890, Bd. xxi, p. 285. 



2 Lidforss, Bot. Centralbl., 1901, Bd. LXXXVI, p. 169; Jahrb. f. wiss. Bot., 1902, Bd. xxxvm, 

 p. 343. According to Vochting (Bot. Ztg., 1902, pp. 90, 107), a fall of temperature also causes 

 young potato-shoots to droop. We are here only dealing with the results of a change of temperature 

 under otherwise constant conditions, and leave it an open question as to how far the results observed 

 are due to induced changes of geotropic irritability. 



3 Cf. Vochting, Ber. d. bot. Ges., 1898, p. 50 ; Warming, Oekologische Pflanzengeographie, 

 a German translation by Knoblauch, 1896, p. 26 ; Krasan, Engler's bot. Jahrb., 1882, p. 185 ; 

 Lidforss, 1. c. 



* Pfeffer, Physiol. Unters., 1873, p. 189; Period. Bewegungen, 1875, p. 131; Jost, Jahrb. f. 

 wiss. Bot., 1898, Bd. xxxi, p. 352. 



