IRRITABILITY. 541 



The accompanying tracing illustrates the downward 

 sleep-movement of a leaflet of Averrhoa bilimbi. It will be 

 seen that the fall is not continuous, but that it is interrupted 

 by rises, so that the lines traced is a zig-zag. These secondary 

 rises are due to the spontaneous movements of the leaflet. 



Inasmuch as the movements which depend upon the 

 alternation of day and night have been more especially 

 studied in the Sensitive Plant (Mimosa pndica], it will be 

 well to describe them as they occur in this plant. The leaf 

 is bipinnate : the primary petiole is articulated to the stem 

 by a pulvinus, as are also the secondary petioles to the 

 primary, and the leaflets to the secondary petioles. Begin- 

 ning, now, the observation of the changes in position of the 

 leaf, we find that in the middle of the day the primary 

 petiole of a vigorous and fully developed leaf, makes an 

 acute angle, not much smaller than a right angle, with the 

 internode above its insertion ; at the same time the secondary 

 petioles are widely divaricated and the leaflets are fully 

 expanded. In the course of the afternoon the main petiole 

 slowly sinks, so that the angle which it makes with the 

 internode above its insertion increases to a right angle or 

 even to an obtuse angle. As darkness comes on the leaflets 

 fold upwards and forwards in pairs, and the secondary petioles 

 approach each other so as to become nearly parallel ; this 

 is followed by a sudden fall of the main petiole so that 

 it now makes a large obtuse angle with the internode above 

 its insertion. This is the nocturnal position. It was thought 

 that the leaf retained this position throughout the hours of 

 darkness, but the researches of Bert, Millardet, and Pfeffer 

 have shewn that this is not the case. After having sunk 

 during the first hours of the night, the primary petiole slowly 

 rises until it forms a highly acute angle with the internode 

 above its insertion ; at the same time the secondary petioles 

 gradually separate and expand. With the morning light, 

 the leaflets open, and the main petiole sinks down during 

 some hours until it gains the nearly horizontal direction in 

 which it remains during the middle of the day, and which 

 is characteristic of the diurnal position of the leaf. 



