EXTERNAL STIMULUS AND INTERNAL ENERGY 73 



I shall next give examples in which the opposite effects 

 of external stimulus and internal energy are exhibited in 

 growth response, motile response,' and electrical response. 

 To take first the response of growth : we have seen that 

 a steady rise of temperature brings about an increase of 

 internal energy, while a sudden variation of temperature 

 acts as a stimulus. Thus, if we effect a sudden augmenta- 

 tion of temperature, this will act on the organ, during the 

 period of variation, as a stimulus; but afterwards, when 

 the temperature itself, or its rate of rise, has become 



F"iG. 50. Longitudinal Contraction and Retardation of Growth under 

 Light in Hypocotyl of Sinapis nigra 



The first part of the curve shows the normal rate of growth. Arrow ( \ ) 

 indicates moment of application of diffuse light, which is seen not only 

 to retard growth, but also to induce a marked contraction. The 

 second arrow indicates moment of withdrawal of light, and dotted 

 portion of the curve shows recovery. 



steady, the condition will act by increasing the internal 

 energy of the organ. These opposite results are seen to 

 be strikingly illustrated by growth response in the case of 

 the following record (fig. 51), 



The normal rate of growth at 34 C. was here '015 mm. 

 per two minutes. By a sudden application of heat, raising 

 the temperature of the chamber ultimately by iC, a respon- 

 sive contraction was caused, as is seen in the record. But, 

 on the attainment of a steady augmented temperature of 

 35 C., an increased rate of growth, which now amounted to 

 '024 mm. per two minutes, was observed, owing to the 

 increase of internal energy. 



