THE PHOTOTROPIC OURVE AND TTft CHARACTERISTICS 349 



Different organs of plants exhibit unequal suscepti- 

 bilities ; some undergo excitation under feeble stimulus, 

 while others require more intense stimulus to induce 

 excitation. But even in an identical organ the 

 susceptibility undergoes, as we have seen, a characteristic 

 variation, being feeble at the beginning of the excitation 

 curve, considerable in the middle, and becoming feeble 

 once more towards the end of the curve. The most 

 difficult problem that faces us is an explanation of this 

 characteristic difference in different parts of the tropic 

 curve. 



GENERAL CONSIDERATIONS. 



Before entering into the fuller consideration of the 

 subject, it will be helpful to form some mental picture of 

 the phenomena of excitation, however inadequate it may 

 be. The excitation is admitted to be due to the molecular 

 upset induced by the shock of stimulus* ; the increased 

 excitation results from increasing molecular upset brought 

 on by enhanced stimulus. The condition of molecular 

 upset or excitation may be detected from the record of 

 any one of the several concomitant changes, such as the 

 change of form, (contraction or expansion) or change of 

 electric condition (galvanometric negativity or positivity). 

 These means of investigation are not in principle different 

 from those we employ in the detection of molecular dis- 

 tortion in inorganic matter under increasing intensities of 

 an external force. 



THE CHARACTERISTIC CURVE. 



Thus the molecular upset and rearrangement, in a 

 magnetic substance under increasing magnetising force are 



* I shall use the term stimulus in preference to stitnulation, for the latter 

 19 often taken in the senae of the resulting excitation. 



