270 CHAP. XXV. DIURNAL MOVEMENT OF MIMOSA 



multiple activity of the pulvinus. I found that the very 

 young leaves in spring exhibited automatic pulsation 

 throughout day and night ; in older leaves, tuned to diurnal 

 periodic movements, these natural pulsations are more or 

 less suppressed, but several pulsations may be exhibited in 

 the forenoon even by mature leaves (fig. 156). 



3. The immediate effect of light. — This is not constant, 

 but will be shown to undergo a definite variation with the 



Fig. 156. Record of automatic pulsation of Mimosa leaf in the 

 forenoon. Average period 25 minutes. 



Successive dots at intervals of a minute. 



intensity and duration of light. The great difficulty of 

 recording the change of intensity of light was overcome 

 by the construction of the Radiograph already described 

 (P- 197)- I reproduce a record (fig. 157) obtained in my 

 greenhouse on March 5, 1919, which gives a general idea 

 of the variation of light from morning to evening ; the 

 record shows that light began to be perceptible at 5.30 a.m., 

 and that the intensity increased rapidly and continuously 

 till it reached a climax at noon, after which it began to 

 decline slowly. The fall of intensity was very abrupt after 

 5 P.M., the effect being reduced to zero at 6.30 p.m. 



4. The after-effect of light. — The after-effect is greatly 

 modified by the intensity and duration of the illumination. 



