158 PHYSIOLOGICAL, BOTANY. PAST II 



be attributed to the operation of the vital force : these 

 are, the elastic and hygroscopic powers of some vege- 

 table tissues. 



(142.) El'ixticity <;/' 7V,w//r. This property is 

 eminently conspicuous when the tissue is distended with 

 fluid ; and, unless its effects be duly appreciated, we 

 might be misled, and inclined to consider certain phe- 

 nomena as the direct result of an irritability residing in 

 the plant, whilst, in fact, they may be easily accounted 

 for by the action of elasticity alone. Thus, in the flower* 

 of the common nettle (fiy. 147- )' ^ u ' filaments are at 



first curved inwards, and the anthers meet in the centre. 

 When the flower is completely expanded, the filaments 

 have become highly elastic ; but are still retained in 

 their original curved position by the mutual pressure 

 which they exert upon 'each other. If this state of 

 equilibrium be disturbed, either by slightly displacing 

 the anthers with the point of a pin, or by the further 

 progress made in vegetation, the stamens are suddenly- 

 thrown back by the elasticity of their filaments, the 

 anthers burst and the pollen is scattered by the shock 

 (6). This appearance is very like that of some other 

 sudden motions, which, as we shall hereafter show, 

 must be referred to the direct influence of some stimulus 

 upon the vital principle. Many seed-vessels when 

 fully ripe, burst as it were spontaneously, by the in- 

 creased elasticity of their tissue, and the seeds are often 

 scattered to a considerable distance by this means ; but 

 although all the organs of plants when replete with 

 fluid, are generally elastic, a remarkable exception oc- 

 curs in the pedicels of Dracocephalum moldavicum. 



