THE MECHANICS OF NUTATION MOVEMENTS 129 



is reached. This depends upon the growth tendencies of the different 

 tissues, coupled with the mechanical and physiological reactions due to the 

 realized curvature. , 



It depends upon the properties of the organ, and upon the rapidity 

 of the change of temperature or illumination, whether the new position 

 is assumed directly, or after a number of oscillations. These may arise 

 either owing to the fact that the different tissues assume rates of growth 

 proportionate to the new conditions with unequal rapidity, or they may be 

 due to the fact that the shock-stimulus produces a transitory and unequal 

 acceleration or retardation of growth. These transitory oscillations must 

 be reduced and finally eliminated when the change of temperature or 

 other condition is brought about sufficiently slowly. Their production 

 has, however, no influence upon the ultimate position, which when once 

 attained is maintained so long as no internal or external change occurs. 

 An organ may, however, react in such a way that the change produces 

 pronounced oscillation, but no permanent alteration of the original 

 position. 



It is impossible, therefore, to say whether any shock-effect comes into 

 play. The new rapidity of growth corresponding to changed conditions 

 of temperature or illumination is, however, usually assumed without any 

 perceptible transitory disturbances being shown, and hence more especially 

 the slower photonastic and thermonastic curvatures, and possibly also 

 certain typical sleep-movements, may be produced without any transitory 

 acceleration or retardation of growth due to the effect of shock. A shock- 

 stimulation is, however, exercised in many cases 1 in which a fall or rise of 

 temperature or illumination produces a certain transitory acceleration 

 of growth. As in the case of tendrils this renders the reaction more rapid, 

 and enables a flower of Crocus to close rapidly when subjected to a fall of 

 temperature at which growth ultimately almost ceases. 



This acceleration of growth is as pronounced in highly photonastic, or 

 thermonastic plants as in the case of tendrils. Thus the growth of the 

 middle lamella of the petiole of Impatiens noli-me-tangere may temporarily 

 attain about twenty times its previous rapidity when an energetic photo- 

 nastic reaction is produced by sudden darkening 2 . In one experiment the 

 marks on the petiole covered 1 83-5 of the micrometer divisions after four 

 hours instead of the original 183, which indicated a growth in length of 

 O'2i per cent, per hour. After darkening the leaf curved strongly down- 

 wards in half an hour, and since the marks extended on the upper side from 

 1 84 to 192 micrometer divisions the growth in length was 8-68 per cent. 



1 Pfeffer, Period. Bewegungen, 1875, PP- I 3> I22 i J? 1 * The -* ex ^ * s based mainly on these 

 researches and upon those of Jost. 

 3 Pfeffer, 1. c., p. 21. 



PFEFFER. Ill 



