130 MOVEMENTS OF CURVATURE 



per hour. The simultaneous measurement of the under side gave a shorten- 

 ing of 0-53 per cent., so that the growth of the middle lamella was 4-07 per 

 cent., or half the algebraic sum of the growth on the two sides. Similarly 

 in the flower of Crocus a sudden fall of temperature from 17 to 7 C. may 

 cause the average growth of the middle lamella of the active zone of the 

 perianth to increase transitorily from seven to ten times in rapidity, although 

 ultimately growth is strongly retarded at 7 C. Observations on Tnlipa 

 also showed an increase of growth to eighteen times its previous rapidity 

 when the temperature was suddenly raised from 11 to j8C. and, even 

 allowing for the permanent increase at the higher temperature, the transitory 

 rise is eight times greater. 



As in the case of the curvature of tendrils, during these photonastic or 

 thermonastic responses the concave side retains the same length or ex- 

 periences a very slight shortening during curvature. During the return move- 

 ment by which the leaf of Impatiens nearly regains the day position after 

 being darkened, the previously accelerated side grows but little or not at all. 

 The recent researches of Wiedersheim carried out at Leipzig under PfefTer's 

 direction show that the return movement is accompanied by a secondary 

 feebler acceleration of the growth of the middle lamella, as in the case of 

 tendrils. This secondary acceleration is shown by the flowers of Crocus and 

 Tnlipa, but is comparatively feeble, since the return movement only takes 

 place to an extent sufficient to remove the excess of curvature. 



As in the case of tendrils, a transitory change only produces a temporary 

 curvature, the organ returning to its original position when the previous 

 conditions of temperature or illumination are restored. In such cases the 

 secondary acceleration of growth during the return movement naturally 

 becomes more pronounced. Although the curvature of tendrils results 

 from a tropic stimulus, and those of thermonastic and photonastic organs 

 from diffuse stimulation, the growth-mechanisms involved are the same in 

 both cases. The entire active zone on both sides of the organ experiences 

 an acceleration of growth, which begins at a later time on the side which 

 becomes concave, but which, whenever the organ straightens again, ultimately 

 produces the same total growth in spite of its originally slower rate on the 

 concave side. It follows, therefore, that the production of a permanent 

 curvature involves either a partial inhibition of the slower but more 

 prolonged growth response on the concave side or the prolongation of the 

 growth period on the convex side. 



Among the factors responsible for these reactions the stimulating effects 

 of shock and of the realized movement are to be included. It is not, how- 

 ever, certain whether the latter is directly responsible for the return move- 

 ment by which the original position may be partially or entirely restored. 

 Wiedersheim has, however, found that when a fixed leaf of Impatiens parvi- 

 flora is darkened two opposed successive accelerations of growth ensue just 



