Daily movement di/eS t6 TtfEliMO-^EOTROPlsM 561 



ence being due to the physiological depression consequent 

 on prolonged maintenance of the plants in a closed chamber. 



MODIFICATION OF THE DIURNAL CURVE. 



I shall now proceed to explain the modifications that 

 may occur in the standard thermo-geotropic curve. 



Turning points. — In the bulky Palm, the reversal of 

 movement from fall to rise or vice versa takes place about 

 an hour after the thermal inversion. This lag is partly 

 due to the time taken by a mass of tissue to assume the 

 temperature of the surrounding air. There is, moreover, 

 the question of physiological inertia which delays the 

 reaction. In leaves this lag may be considerably less or even 

 absent. In certain cases the reversal of movement may 

 take place a little earlier than the temperature inversion. 

 It should be remembered in this connection, that in 

 response to temperature change, the leaf is often displaced 

 to a considerable extent from its ' mean position of equili- 

 brium' ; moreover the force of recovery is greatest at the 

 two extreme positions. These considerations probably explain 

 the quick return of the leaf to equilibrium position. The 

 slow aatonomous movement of the leaf may sometimes 

 prove to be a contributory factor. 



Effect of irregular fluctuation of temperature. — In settled 

 weather the diurnal rise and fall of temperature is very 

 regular. But under less settled condition, owing to the 

 change of direction of the wind, the temperature curve shows 

 one or more fluctuations, specially in the forenoon. It was a 

 matter of surprise to me to find the plant-record repeating 

 the fluctuations of thermal record with astonishing fidelity. 

 This common twitch in the two records is seen in the 

 record of the Sijberia Palm (Fig. 199). Certain plants are 

 extremely sensitive to variation of temperature ; so 

 much so that these physiological indicators of thermal 



