33§ PLANT RESPONSE 



is to say, by increase of amplitude or by increase of frequency 

 of pulsation. 



Increased internal energy, shown by : (a) Increase of 

 amplitude, period remaining cotistant. — And we shall, as the 

 simpler of the two, consider that case in which the latent energy, 

 or tonic condition of the plant, is below par, that is to say, the 

 case in which it is near the thermo-tonic minimum. The effect 

 of increased absorption of energy with rising temperature 

 would here be indicated by increasing amplitude of pulsation, 

 the period remaining constant. Conversely the reduction of 

 latent energy with falling temperature would be indicated, 

 when the period is constant, by the fall of amplitude. This 

 we find fully illustrated in records obtained with BiopJiytum 

 and Desmodium. In the former, when nearing the thermo-tonic 

 minimum, it is found that while the period remains approxi- 

 mately constant, that is to say, two and a half minutes, the 

 amplitude of pulsation falls from 8 divisions at 35 C. to 5*5 

 divisions at 32 C. (fig. 124). In Desmodium, again, we find 

 a converse case. Here, while the plant is rising from the 

 thermo-tonic minimum to the normal condition, the amplitude 

 of pulsation is seen to increase progressively, while the period 

 of 2 '4 minutes remains constant (fig. 141). 



{b) Increase of frequency. — Taking next the case of a plant 

 in the ordinary tonic condition, we find that the increase of 

 internal activity, due to the greater absorption of energy 

 during a rise of temperature, is exhibited by a higher frequency 

 of vibration. . The reason why, with this increase of frequency, 

 there is a diminution of amplitude of pulsation, is now to be 

 explained. 



We have seen that an increase of internal energy, as 

 caused by rise of temperature, brings about an increase of 

 turgor, and that this increased turgor hastens the process 

 of recovery, and by acting antagonistically to the con- 

 tractile phase of responses diminishes its amplitude. In- 

 creased internal pressure also, generally speaking, increases 

 the frequency of vibration. If, then, the rise of temperature 

 increases the turgor of Desmodium, as we have found it to do 



