510 LIFE MOVEMENTS IN PLANTS 



The case becomes simpler after tlie attainment of 

 maximum curvature. Enhanced temperature has a ten- 

 dency to diminish the tropic curvature, as we found in 

 the arrest and reversal of phototropic curvature under the 

 application of warmth (p. 393) : it appears as if rise 

 of temperature induced a relatively greater expansion of 

 tlie contracted side of the orj^an. 



I shall now describe the effect of rising temperature on 

 geotropic curvature in general, including torsion. A horizon- 

 tally laid shoot curves upwards under geotropic action ; 

 a dorsiventral organ, owing to the differential excitabi- 

 lities of its upper and lower sides, places itself in the 

 so-called dia-geotropic position. A dorsiventral organ, more- 

 over, exhibits a torsional movement under lateral stimulus 

 of gravity. 



In the geotropic movements we are able, as stated before, 

 to distinguish three different phases (cf. Fig. 161). In the 

 first, the movement initiated undergoes an increase ; in the 

 second, the rate of movement becomes more or less uni- 

 form ; and in the last phase, a balance takes place 

 between the tropic reaction, and the increasing resistance 

 of the curved or twisted organ to further distortion. 



The question now arises whether this position of 

 geotropic equilibrium is permanent, or whether it undergoes 

 modification in a deflfnite way by variation of temperature. 

 I shall proceed to show that the position of equilibrium 

 undergoes a change in one direction by a rise, and in the 

 opposite direction by a fall of temperature. I shall use 

 the term thermc-yeotropisni as a convenient phrase to 

 indicate the effect of temperature in modification of 

 geotropic curvature and torsion. 



I shall first deal with the effect of variation of tem- 

 perature on geotropic torsion. Under the continued action 

 of stimulus of gravity the torsion increases till it reaches 



