98 MOVEMENTS OF CURVATURE 



to darkness may produce a pronounced temporary but no permanent 

 curvature, since the leaf gradually returns to approximately the same 

 position that it occupied when illuminated. 



The ultimate position is naturally independent of the transitory 

 oscillations, which are due to the fact that the antagonistic tissues attain 

 their new positions of equilibrium in different ways, or at least with 

 unequal rapidity. Hence oscillations are absent when the change in the 

 external conditions takes place gradually, as also are the temporary 

 curvatures shown only when the temperature or illumination is suddenly 

 altered. These considerations have been shown experimentally to apply 

 to the thermonastically-reacting flowers of Crocus and Tulipa, and 

 to the photonastically-reacting leaves of Impaticns and Robinia. Nor 

 is it surprising that slowly reacting or comparatively insensitive organs 

 should gradually assume a new position in response to sudden and pro- 

 nounced changes of temperature or illumination without exhibiting any 

 perceptible transitory oscillations. A good analogy is afforded by two 

 metal rods riveted together, and one of them being surrounded by a non- 

 conductor, for when the system is suddenly warmed a transitory curvature 

 will be produced independently of whether the rods have the same or 

 dissimilar coefficients of expansion, that is independently of whether the 

 rods ultimately straighten again or remain permanently curved. 



Obviously the relationships are not quite so simple in a living organism 

 as in this instance, for although the curvatures are primarily due to the 

 unequal growth or expansion of the opposed tissues, the causes which 

 induce these variations of growth, or which cause the tendency to elastic 

 expansion or contraction, are extremely complex in origin. In addition, the 

 realized curvature, like vital reactions in general, excites regulatory stimuli 

 and counteractions, so that the progress of the response and the ultimate 

 position assumed depend upon the conjoint action of these factors with the 

 original stimulus. It has already been mentioned that special conditions 

 may be introduced by accommodation, by changes of excitability, and 

 by alterations in the power of response during excitation, even when the 

 organ remains excitable during response. 



Transitory disturbances may frequently be produced by sudden 

 changes as the result of shock. For instance, a sudden change of temperature 

 induces an acceleration of growth in the perianth-segments of Crocus and 

 Tulipa, while a sudden change of illumination has the same effect upon 

 foliage and floral leaves capable of photonastic reaction. This is of 

 importance in so far as it increases the power and rapidity of response. In 

 addition, owing to the unequal responses of the inner and outer sides of the 

 perianth-segments, a sudden fall of temperature produces a rapid closure of 

 the flower of Crocus even when the temperature is so low that growth 

 ultimately almost entirely ceases. A similar transitory acceleration of 



