132 MOVEMENTS OF CURVATURE 



also possible, thpugh hardly probable, that a decrease of temperature or 

 illumination might transitorily affect growth but not an increase. It is also 

 possible that in thermonastic flowers a temporary retardation of growth 

 may precede its acceleration, but may be too transient to be capable of 

 detection, or may merely antagonize the first tendency to increased growth, 

 thus increasing the latent period of response. 



In any case when we remember the influence of the specific properties 

 and its variable tone upon its power of response, it is not surprising to find 

 that the results obtained do not in all cases precisely agree. Thus Pfeffer 1 

 found that a fall of temperature produced a very pronounced acceleration 

 of growth in the perianth-segments of Crocus, but that a sudden rise 

 produced no perceptible acceleration in the growth of the middle lamella, 

 whereas Jost 2 observed in both cases a strong acceleration of growth in the 

 perianth of the Tulip. 



It is uncertain to what extent sudden changes of illumination may 

 exercise shock-effects upon photonastic organs. During the daily move- 

 ments of the flowers of Leontodon hastilis 3 and of Taraxacum officinale 4 

 the average growth is accelerated, but here the effect of the direct stimula- 

 tion is coupled with the induced periodicity. Since this periodicity and 

 also the daily periodicity of growth in length are induced by periodic 

 changes of illumination, we may assume that every photonastic reaction is 

 coupled with a temporary acceleration of growth. Even when the opening 

 and closing movements assume a more rapid rhythm in constant darkness, 

 each periodic reaction involves a temporary acceleration of the average 

 growth. 



A very pronounced movement and acceleration of growth is produced 

 by darkening the leaves of Impatiens noli-me-tangere and /. parviflora. 

 Illumination only produces a feeble movement, but it is not certain whether 

 the acceleration of the average growth is also feebler than when the leaf is 

 suddenly placed in darkness. 



A transitory acceleration of growth may enable more rapid curvature, 

 but it is not essential, and probably is either absent or feeble in many thermo- 

 nastic and photonastic movements. In the latter case it is easily overlooked, 

 since the activity of growth is always liable to spontaneous fluctuations, and 

 since it assumes a different stationary value in response to the new con- 

 ditions. Pfeffer 5 was, however, overcautious in refusing to accept the 

 general acceleration of growth shown by his measurements as being the 

 result of the shock-stimulus. 



Special instances. The following results have been obtained by micrometric 



1 Pfeffer, Period. Bewegungen, 1875, p. 122. 



a Jost, Jahrb. f.vwiss. Bot., 1898, Bd. xxxi, p. 346. 



3 Cf. Table 4, p. 134, and Pfeffer, 1. c., p. 26. 



* Jost, 1. c., p. 354. 5 Pfeffer, 1. c. 



