THE MECHANICS OF NUTATION MOVEMENTS 131 



as in the case of stimulated tendrils. That a secondary acceleration of 

 growth actually occurs on the concave side is shown by the tendency to 

 a return curvature in a fixed leaf, as well as by direct measurement. Fixed 

 perianth-segments of Crocus and Tulipa show a feebler secondary accelera- 

 tion of growth, owing to the fact that a permanent change of temperature 

 alters the position of equilibrium in such manner as to lessen the return 

 movement. It is, however, possible that the return movement, although 

 excited in the absence of any realized curvature, may result from the altered 

 tensions in the tissues. However this may be, there can be no doubt that, 

 in the case of variation movements, correlative influences, as apart from 

 mechanical ones, do travel between the closely related halves of motile 

 pulvini. 



A direct or indirect regulation of the growth in the different parts is 

 essential to produce a definite reaction. The fact that the concave side may 

 retain approximately its original length during the curvature of tendrils as 

 well as of photonastic and thermonastic organs simply shows that the growth 

 acceleration lessens towards the concave side, for in the middle lamella 

 of this side the growth will be ten times accelerated when the growth of the 

 middle lamella of the entire organ is accelerated twenty times. The slight 

 shortening sometimes shown on the concave side is probably the result of 

 compression, and would be greater during curvature were it not for the 

 simultaneous awakening of an increased tendency to growth 1 . 



Jost 2 erroneously supposed that the thermonastic or photonastic 

 stimulation directly accelerated the growth on one side and retarded it on 

 the other, and does not sufficiently distinguish between the transitory and 

 stationary reactions and their results. It is not, however, impossible that 

 in isolated cases some such antagonistic action may be exercised, or that as 

 the result of shock-stimulation particular cells may experience a temporary 

 retardation of growth followed by the usual acceleration. The new constant 

 conditions of temperature or illumination always, however, produce the 

 same qualitative effect on growth although not always the same quantitative 

 effect, and special peculiarities may be shown when the temperature 

 or the illumination rises above the optimal values. Apart from this the 

 formal effect of a rise of temperature or decrease of illumination is an 

 acceleration of growth, while a permanent fall of temperature or increase 

 of illumination produces a retardation of growth. As the result of shock, 

 however, a sudden rise or fall of temperature or illumination may produce 

 either a transitory acceleration or retardation of growth according to the 

 nature of the plant. True 3 observed that a sudden rise or fall of tempera- 

 ture produced a transitory retardation of growth in the radicle, but it is 



1 Pfeffer, Period. Bewegungen, 1875, p. 17. 



a Jost, Jahrb. f. wiss. Bot., 1898, Bd. xxxi, p. 368. 



3 True, Annals of Botany, 1895, Vol. ix, p. 365. 



K 2 



