342 PLANT RESPONSE 



exhibits, under the influence of periodic variations of tem- 

 perature and other factors. An extremely interesting example 

 of such an hourly periodicity is given above (fig. 144), where 

 the mean plane of vibration is itself seen to exhibit a periodic 

 up-and-down oscillation. There is, again, the still larger 

 periodicity of diurnal variation of day and night. We thus 

 see how complex may be these wave-systems, in which, 

 superposed over large waves, are smaller waves, and on the 

 latter still smaller wavelets. 



Summary 



In Desmodium, as in cardiac muscle, rise of temperature 

 produces increased frequency with diminished amplitude of 

 pulsation. This is true within a certain normal range of 

 temperature. When the temperature of Desmodium is reduced 

 to a thermo-tonic minimum — which is about 17 C. but subject 

 to certain individual variations — the amplitude of pulsation, 

 owing to the loss of internal energy, is decreased till there is 

 an arrest. If now the temperature be gradually raised, the 

 pulsations, owing to the absorption of energy, become again 

 increased in a staircase manner, the period remaining approxi- 

 mately constant. 



Under normal tonic conditions, the decrease of amplitude 

 of pulsation with rising temperature — when this is not 

 excessive — is not indicative of loss of excitability. It is due 

 to the increase of internal energy, which hastens recovery and 

 acts antagonistically to the responsive movement of contrac- 

 tion. The same explanation is probably applicable to the 

 diminished amplitude of pulsations with rise of temperature, 

 observable in cardiac response. 



Rise of temperature, by increasing the internal energy 

 and consequent turgor of the plant, causes expansion of the 

 organ, thus bringing about a shifting of the pulsation towards 

 1 diastole.' 



A converse effect, or shifting towards ' systole,' is seen in 

 Desmodium, as the result of cooling. 



