272 



BOTANY 



tion still continues for some time to give rise to visible after-effects 

 (pp. 256, 272), until finally, from the abnormal conditions, an abnormal 

 state of rigour (light rigour, dark rigour) and symptoms of disease are 

 manifested. 



Only a few plants respond with pronounced variation movements to 

 mechanical irritation (shock, friction, injury). Formerly, these alone 

 were considered irritable plants, as in the vegetable kingdom only the 

 apparent mechanical irritations, from which visible movements resulted, 

 were then regarded as stimuli. 



Of irritable plants in this sense, mention has already been made of 

 Dionaea musapula (p. 215), whose leaves when touched on the upper 

 side, especially if the bristles are disturbed, fold together. The most 



Fig. 213.— Mimosa pudica, with leaves in normal, diurnal position ; to the right, in the position 

 assumed on stimulation : B. flowers. 



familiar example of this irritability to mechanical stimuli is furnished 

 by Mimosa pudica, a tropical leguminous shrubby plant, which owes its 

 name of sensitive plant to its extreme sensitiveness to contact. The 

 leaves of this plant are doubly compound (Fig. 213). The four 

 secondary leaf-stalks, to which thickly-crowded leaflets are attached 

 left and right, are articulated by well-developed pulvini with the 

 primary leaf-stalks ; while they, in turn, as well as the leaflets, are 

 similarly provided with motile organs. Thus all these different parts 

 are capable of independent movement, and the appearance of the 

 entire leaf becomes, in consequence, greatly modified. In their 

 unirritated, light position (Fig. 213, on the left) the leaf-stalk is 

 directed obliquely upwards, while the secondary petioles with their 

 leaflets are extended almost in one plane. Upon any vibration of the 



