12 PHILOSOPHY OF ZOOLOGY. 



touches the inner part of each filament, near the bottom. 

 The irritability of that part is such, that the filament imme- 

 diately contracts there, and consequently strikes its anther, 

 full of pollen, against the stigma. Any other part of the 

 filament may be touched without this elfect, provided no 

 concussion be given to the whole. After a while, the fila- 

 ment retires gradually, and may again be stimulated ; and 

 when each petal, with its annexed filament, is fallen to the 

 ground, the latter, on being touched, shews as much sen- 

 sibility as ever *." 



In another British plant, the Cistus helianthemum, which 

 is very common on dry rocky ground, the filaments, when 

 touched, execute a motion the reverse of that of the Bar- 

 berry, They retire from the style, and lie down in a 

 spreading form upon die petals. This curious example of 

 vegetable irritability was first pointed out by Dr HOPE, 

 Professor of Chemistry at Edinburgh (. 



In animals, this power is displayed by all muscles, in 

 different degrees of intensity, producing directly, the va- 

 rious Involuntary motions ; and indirectly, those which are 

 termed Voluntary. In the former of these, an action takes 

 place upon the application of a stimulus, independent of 

 the will ; while, in the latter, the action takes place in the 

 muscle, in consequence of volition exercised through the 

 medium of the nerves. When the nerve of a muscle is 

 cut across, and direct communication with the brain thus 

 interrupted, the will ceases to exercise its controul over the 

 motions of the fibres ; but involuntary motion, or the pe- 

 culiar irritability of the muscle, continues in force. 



By zoologists in general, the muscular fibre is regarded 

 as deriving this power from nervous energy ; since, as they 



* SMITH'S Introduction to Botany, p. 325* 

 t Knglifsh Botany, vol. xix. No, 1331, 



