ioo MOVEMENTS OF CURVATURE 



Flowers which periodically open and close behave like ephemeral ones under these 

 conditions and open once only. The duration of both ephemeral and periodic flowers 

 may vary considerably *, and in fact at low temperatures the life of an ephemeral 

 flower may be so prolonged that it is able to perform daily movements. 



The flowers of Crocus do not open when the temperature is kept low, nor those 

 of Stellaria media when the illumination is feeble. This is owing to the fact that at 

 no period of development does the growth of the inner surface of the perianth-seg- 

 ments become active enough, as compared with that on the outer surfaces, to produce 

 a separation of the closely applied leaves. An opening movement is, however, in 

 part attempted during development, as is shown by the fact that if all the perianth 

 segments are removed but one, this may curve at first nearly at right angles to the 

 stalk, but subsequently straightens more or less. At still higher temperatures the 

 segment expands outwards, but the opening of the flower is slower and less pro- 

 nounced than at the optimal temperature. A sudden rise of temperature produces an 

 opening movement which is temporarily in excess of the ultimate position for this 

 temperature, and this may cause the temporary opening of a flower, when raised to 

 a temperature at which it finally closes again. The same general considerations also 

 apply to photonastic and hydronastic movements. 



The uses of the movements. When feeble they are probably accessory reactions 

 without any special biological importance. Moth-pollinated flowers which close in 

 the daytime avoid the visits of useless insects, and economize scent, nectar, and 

 pollen. Flowers which close at night keep the sexual organs protected from dew, 

 and to a certain extent from injurious cooling 2 . The drooping of flower- and 

 inflorescence-stalks, which causes many flowers to be inverted during the night, may 

 be of use in the same way. 



The sleep-movements of leaves and leaflets reduce the amount of surface exposed, 

 and hence lessen the radiation of heat during clear nights. Darwin 3 showed that 

 less dew formed on such leaves than on ones which had been fixed in the expanded 

 condition. The latter suffered more than the normally sleeping leaves, and hence 

 Darwin concluded that the nyctinastic movements were for the purpose of lessening 

 temporary cooling during night as far as possible. Stahl 4 , however, considers the 

 utility of these movements to lie in the fact that the lessened formation of dew avoids 

 the blocking of the stomata and the consequent hindrance to transpiration. If Stahl's 



1 Cf. Oltmanns, Bot. Ztg., 1895, pp. 32, 52 ; Hansgirg, Physiologische u. Phycophytolog. 

 Unters., 1893, p. 15 ; Kerner (Natural History of Plants, 1895, Vol. n, p. 211). [Hansgirg (1. c., 

 p. 10) suggests the terms thermo-, photo-, and hydrocleistogamy to indicate the main causes which 

 keep a facultatively cleistogamic flower permanently closed. Since the causation may vary at 

 different times, these terms are as unnecessary and superfluous, as it would be to use special terms 

 (mechano-cleistogamy, plaster-of-paris-cleistogamy) for the cleistogamy produced by tying-up a 

 flower or embedding it in plaster-of-paris.] 



2 Hansgirg, I.e., p. 175; Kerner, 1. c., Bd. II, p. 112. Die Schutzmittel des Pollens, 1873. 

 Sprengel (Das entdeckte Geheimniss der Natur im Bau u. in d. Befruchtung d. Blumen, 1793, p. 13) 

 considers the closing movements to be for the protection of the nectar. 



3 Darwin, The Power of Movement in Plants, 1880, pp. 286, 413; Bot. Centralbl., 1881, 

 Bd. vni, p. 77. 



4 Stahl, Bot. Ztg., 1897, p. 81. A detailed discussion of the biological utility of these move- 

 ments is given by Stahl. 



