THE ORIGIN OF THE DAILY PHOTONASTIC PERIODICITY 115 



expand until the temperature is still more favourable l . In addition, the 

 flowers of Spergula salina, as well as those of Hordettm distickum, and of 

 a few other grasses, remain closed at low temperatures, while there are 

 presumably numerous plants whose flowers perform no pronounced opening 

 and closing movements but remain closed at low temperatures 2 . 



Many organs are capable of both thermonastic and photonastic move- 

 ment, although usually those organs which are highly thermonastic are 

 only feebly photonastic, and the converse is also true 3 . Thus the daily 

 opening and closing of the feebly photonastic flowers of Crocus and Tulipa 

 are mainly determined by the changes of temperature ; and the rapid 

 opening usually produced by insolation is mainly the result of the heating 

 effect of the sun's rays. Even a small fall of temperature is sufficient to 

 produce the closure of the flower in spite of the feeble opposed photonastic 

 action produced by exposure to diffuse daylight. 



The daily temperature-curve, and hence also that of the resulting 

 thermonastic movements, are much more irregular than the periodicity 

 dependent upon the changes of illumination. Hence the absence or 

 feebleness of any induced periodicity in the flowers of Crocus and Tulipa^ 

 and in other thermonastic organs, enables them to assume positions directly 

 corresponding to the prevailing temperatures. In this way spring flowers, 

 among which most strongly thermonastic flowers are included, are able 

 to remain closed on cold days a fact of considerable biological importance. 

 When an organ is capable of both thermonastic and photonastic 

 response, increases of temperature and of illumination usually produce 

 similarly directed movements, and the same applies to decreases. In general, 

 therefore, the changes of illumination and of temperature co-operate in pro- 

 ducing the sleep-movements. Curvature can be induced in dorsiventral 

 organs in various ways even when the general rate of growth is accelerated 

 by a moderate rise of temperature but slightly retarded by a concomitant 

 increase of illumination. In addition, a transitory acceleration of growth 

 may result from the shock due to a sudden change of temperature or of 

 illumination. Nor is it surprising that in certain cases the thermonastic 

 and photonastic responses should be dissimilar in character. Thus Vochting* 

 found that a decrease of illumination produced an upward curvature in 

 certain shoots of Mimulus Tilingii, and, according to Jost 5 , decreases of 

 temperature and of illumination produce opposed movements in the case 

 of the leaflets of Mimosa pudica. It is, however, not certain whether in 

 all cases a rise of temperature above the optimum might produce a reversal 



1 Pfeffer, 1873, 1. c., p. 189. 



2 For facts see Hansgirg, Physiolog. u. Phycophytologische Unters., 1893, pp. 30, 46, 64; 

 Fritsch, Bot. Ztg., 1852, p. 897. Further critical investigation of these facts is, however, requisite. 



3 Pfeffer, Period. Bewegungen, 1875, P- I22 



4 Ber. d. bot. Ges., 1898, pp. 39, 45. 5 Bot. Ztg., 1897, p. 35. 



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