Sensation in Plants. 141 



in other beings, to attribute them, in this instance, to mere 

 mechanism, actuated solely by external impulse, is to 

 deviate from the soundest rule of philosophising, which 

 directs us not to multiply causes, when the effects appear to 

 be the same. Neither will the laws of electricity better 

 solve the phenomena of this animated vegetable : for its 

 leaves are equally affected by the contact of electric and 

 non-electric bodies ; show no change in their sensibility, 

 whether the atmosphere be dry or moist ; and instantly 

 close when the vapour of volatile alkali or the fumes 

 of burning sulphur are applied to them. The powers 

 of chemical stimuli to produce contractions in the fibres of 

 this plant, may perhaps lead some philosophers to refer 

 them to the vis insita, or irritability, which they assign to 

 certain parts of organised matter, totally distinct from, and 

 independent of, any sentient energy. But the hypothesis 

 is evidently a solecism, and refutes itself. For the presence 

 of irritability can only be proved by the experience of irri- 

 tations, and the idea of irritation involves in it that of 

 feeling.' 



Speculations on the Perceptive Power of Vegetables. By 

 Thomas Percival, M.D.> F.R.S. Read February 18, 

 1784. Vol. ii. p. 114. 



' Vegetables bear so near a similitude to animals in 

 their structure, that botanists have derived from anatomy 

 and physiology almost all the terms employed in the 

 description of them. 



' A tree or shrub, they inform us, consists of a cuticle, 

 cutis, and cellular membrane, of vessels variously disposed, 

 and adapted to the transmission of different fluids ; and of 

 a ligneous, or bony substance, covering and defending a pith 



