MOVEMENT IN PLANTS. 



247 



the movements of the pulviiii at the bases of the leaflets 

 cause the latter to move upwards (so as to bring their 

 upper surfaces in contact) or to spread out horizontally. 

 The leaves of Mimosa perform movements as the result of 

 (1) shock or contact, (2) changes of temperature and 

 illumination. 



342. Day and Night Positions (Fig. 55). Note 

 that during the day the main stalk is directed upwards, 

 making with the stem an angle of about 60 ; the second- 

 ary stalks diverge, the two lower standing at right angles 

 to the main stalk, the two 



upper forming an angle of 

 about 60 with each other ; 

 and the leaflets spread out 

 horizontally, forming 

 angles of about 90 with 

 the secondary stalks in the 

 same plane. At night the 

 primary stalk bends down- 

 wards through about 90 ; 

 the four secondary stalks 

 bend forwards, so as to 

 place themselves almost 

 parallel with the axis of 

 the main stalk ; the leaf- 

 lets bend upwards, coming 

 together in pairs with their 

 upper faces and also twisting slightly so as to form an 

 acute angle forwards with the secondary stalk, the basal 

 leaflets overlapping the apical ones like tiles on a roof. 



343, Effects of General Mechanical Stimulation. 



Shake a Mimosa plant : the leaves rapidly assume the 

 "night" position. After a short time they regain the 

 normal " day " position ; in fact, as soon as the main stalk 

 reaches the position of maximum depression, it begins to 

 rise again, and in 10 to 15 minutes the original position 

 is regained. Shake the plant continuously for several 

 minutes : the leaves become insensible to shock, and resume 



b' 



5. Leaves of Mimosa Pudica (the 



Sensitive Plant). 



a, Expanded day position ; b, Drooping 



folded night position. 



