THERMO-GBOTROPISM 517 



Rise of temperature is thus shown to diininish geotropic 

 torsion of dorsiventral organs, and the apo-geotropic 

 curvature of radial organs. We have next to study the 

 effect of temperature variation on the dia-geotropic equili- 

 brium of leaves. 



EFFECT OF VARIATION OF TEMPERATURE ON 

 DIA-GBOTROPIC EQUILIBRIUM. 



In the normal position of the plant, the leaf of Mimosa 

 assumes, under geotropic action, an equilibrium position 

 which is approximately horizontal. I shall proceed to show 

 that this position of equilibrium also undergoes appropriate 

 variation under changing temperature, the leaf under- 

 going a fall during rise, and an erection during fall of 

 temperature. 



I stated that the torsional response is one of the means 

 of recording geotropic eifect and its variations. In the 

 ordinary position of the plant, the geotropic variation will 

 be indicated by the responsive up or down movement of 

 the leaf in a vertical plane. Taking th*^ leaf of Mimosa. 

 we have thus the means of studying the effect of varia- 

 tion of temperature by two independent means of inquiry, 

 namely, by record of ordinary responsive movement in a 

 vertical plane, and also by record of torsional response. 

 The variation of temperature which induces these move- 

 ments may be simultaneously recorded by means of a 

 differential metallic thermometer. The Multiplex Recorder 

 employed for this research consists of three recording 

 levers. A photographic reproduction of the apparatus will be 

 found in a subsequent chapter (see Fig. 190). The first lever 

 is attached to the leaf of Mimosa placed in the normal 

 position ; the second lever records the torsional response 

 of Mimosa leaf, the plant being placed on its side ; the 

 third lever attached to the differential metallic thermo- 

 meter gives a continuous record of variation of temperature. 



