SECT. II 



PHYSIOLOGY 



323 



(c) Seismonasty Q'^^') 



In Dionaea the two halves of the leaf-blade (p. 228) close together 

 not only as a result of chemical stimuli but also owing to a mechanical 

 stimulus. In contrast to the thigmotropic movements of tendrils or 

 of Drosera resulting from contact with solid bodies, in the case under 

 consideration every disturbance resulting from a mechanical shock 

 acts as a stimulus ; the movement can thus be brought about by rain- 

 drops. These movements are termed seismonastic. 



The most familiar example of this irritability to mechanical stimuli is furnished 

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



Pig. 240. — Mimosa pudica, with leaves in normal, diurnal position; to the right, in tlu' position 

 assumed on stimulation ; B, inflorescences. 



sensitive plant to its extreme sensitiveness to contact. The leaves of this plant are 

 doubly compound (Fig. 240). The four secondary leaf-stalks, to which closely 

 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 diff'erent parts are 

 capable of independent movement, and the appearance of the entire leaf becomes, in 

 consequence, greatly modified. In their unirritated, light position (Fig. 240, 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 vibi'ation of the leaf, in 

 favourable conditions of temperature (25°-30° C.) and moisture, all its parts perform 

 rapid movements. The leaflets fold together, and, at the same time, move forward, 

 the secondary petioles lay themselves laterally together, while the primary leaf-stalk 

 sinks downwards (Fig. 240, on the right). Leaves thus aifected, if left undisturbed, 

 soon resume their former position. 



The behaviom' of the leaves is still more remarkable when only a few of the 



