58 CHAP. VII. THE PHYSIOLOGICAL FACTOR 



response of Mimosa and the autonomous response of 

 Desmodium undergo marked diminution after the develop- 

 ment of the flowers. 



(c) Effect of season. — The spring season is obviously 

 more favourable to physiological activity than winter ; and 

 the records which I obtained of the mechanical response of 

 Mimosa, of the autonomous pulsation of Desmodium, and 

 of the movement of growth exhibit similar differences due 

 to change of season. The greater irritability in spring- 

 specimens of Mimosa is exhibited in another way, namely 

 by the low intensity of minimal stimulus which is effective 

 in evoking response. In spring the intensity of minimally 

 effective stimulus is considerably lower than in winter. 



In photosynthesis the effect of season has been shown 

 to be in every way similar. Photosynthetic activity is rela- 

 tively higher in spring than in winter, and the minimal 

 intensity of light which initiates photosynthetic activity is 

 very much lower in spring. 



(d) Effect of unfavourable environment. — If the plant be 

 maintained under unfavourable environment, its tonic con- 

 dition falls below par, with resulting depression of physio- 

 logical activity. Prolonged maintenance of the plant in 

 darkness, for example, causes a depression in the mechan- 

 ical response of Mimosa, in the pulsations of Desmodium. 

 and in the rate of growth in the growing organs. In the 

 sub-tonic specimens, previous stimulation is generally 

 found to confer an enhanced activity to the organism, so 

 that the formerly ineffective stimulus now becomes effective. 



The characteristic modifications of response described 

 above will also be found in photosynthesis, proving the 

 importance of the physiological factor in the process. 



Summary 



The photosynthetic curve is similar to the phototropic 

 curve ; the maximum response is not absolute but increases 

 with the intensity of the stimulus. 



