IRRITABILITY. 543 



It will be observed that the primary petiole has a com- 

 plete daily period of this kind, that from dawn to evening 

 it sinks, and rises from evening to dawn. Millardet has 

 shewn that this periodicity of movement of the primary 

 petiole stands in direct relation to the daily periodicity of 

 tension in the plant (p. 406), just as is the case with the daily 

 period of growth in length (see p. 408). He finds, namely, 

 that the maximum tension occurs just before dawn, and the 

 minimum early in the evening, the tension decreasing during 

 the day, and increasing during the night. The time of 

 maximum tension coincides with the most elevated position 

 of the primary petiole ; and as the tension diminishes during 

 the day, so the primary petiole sinks downwards. There 

 is, however, an apparent discrepancy in that whereas the 

 tension begins to increase early in the evening, the primary 

 petiole sinks rapidly during the first hours of the night. 

 This discrepancy is, as Pfeffer has clearly shewn, only ap- 

 parent and not real. The rapid fall of the primary petiole 

 at this period is due to the increased mechanical moment 

 of the secondary petioles and their leaflets when they have 

 assumed their nocturnal position, in consequence of which 

 the primary petiole is bent downwards. When the secondary 

 petioles are removed, the primary petiole does not perform 

 this downward movement, but begins to rise as the tension 

 increases. When the leaf is entire the primary petiole is 

 only able to raise the weight of the secondary petioles and 

 leaflets when the tension has increased considerably above 

 the minimum, and its rise is then assisted by the gradual 

 separation of the secondary petioles which is also a result 

 of the increased tension. This relation between the tension 

 of the tissues and the daily movement of the leaves has only 

 been investigated in the case of the Sensitive Plant, but it 

 doubtless exists in all cases. 



The daily periodicity of movement thus induced by the 

 normal variations in the intensity of light during the twenty- 

 four hours, makes itself apparent for a time after the plant 

 has ceased to be exposed to them, either, by being kept 

 in darkness or by being exposed to continuous light. Thus 



