THERMO-GEOTROPISM 513 



intensity of li^'ht was varied by means of a focussing lens, 

 wliich gave a parallel, a convergent or a divergent beam, 

 with corresponding increase or diminution of intensity of 

 light. Light was in each case continued till equilibrium 

 was reached. Inspection of figure 185 shows that the 

 position of equilibrium depends on the intensity of stimu- 

 lation ; the balance is ' raised ' under increased and ' lowered ' 

 under decreased intensity. 



In the case of geotropism the stimulus is constant, 

 but its tropic eflfect, we shall presently see, undeigoes 

 variation with changing temperature. 



EFFECT OP VARIATION OF TEMPERATURE ON GEOTROPIC 



TORSIGJf. 



JIncfification of (/entropic loi'shm : Experiment 199. — The 

 Mimofia plant was placed on its side, so that the pul- 

 vinus was subjected to lateral geotropic action. In response 

 to this it underwent torsion, the upper half of the pulviiius 

 tending to place itself so as to face the vertical lines of 

 gravity. This torsional response was recorded as an up- 

 movement ; on the attainment of equilibrium the record 

 l)eeame horizontal. The plant was now subjected to a 

 cyclic variation of temperature, and the resulting variation 

 of torsion recorded at the same time. The temperature of 

 the plant chamber Was gradually raised from the normal 

 1)0^ to .*>4° V. and then allowed to return to the normal : 

 finally the temperature was lowered to 2(i*^C. Rise of 

 temperature was effected by means of an electrical heater 

 ])laced inside the chamber with a vessel of water placed 

 above it. Care has to be taken that the rise of temper- 

 ature is gradual, since a sudden variation often acts as a 

 stimulus. The water in the vessel not only keeps the 

 chamber in a humid condition but also prevents sudden 

 fluctuation of temperature. After the temperature hail been 

 raised to ?A (\, the heating current was stoppetl and the 



