28 STRUCTURE AND PROPERTIES 



fluid having penetrated the membrane with 

 greater velocity than the denser fluid." (Hens- 

 low's Principles of Botany, p. 159-60.) 



21. Vegetable existence has been supposed to 

 possess three vital properties, so termed from 

 their analogy with the powers similarly named in 

 the animal economy; viz. 1. Excitability. 2. 

 Irritability, and 3. Sensibility : by the first is 

 understood that peculiar state of the vegetable 

 tissue, which enables it to resist decomposition 

 by water much more energetically while living 

 than after death, and which also renders it 

 capable of supporting the action of air and heat 

 during life, in a manner totally different from 

 that in which their agency affects it afterwards. 

 Many phenomena common to all plants concur 

 to prove that this difference is inexplicable with- 

 out the admission of vital excitability ; such are 

 the rapid mounting of the sap in the living plant, 

 compared with the slow absorption of water in 

 the lifeless tissue ; the influence of light on the 

 ascent of the sap, &c. 



22. The quality to which the term Irritability 

 has been applied by some physiologists, is that 

 by which certain portions of some plants respond 

 to the agency of external objects, in a manner 

 somewhat similar to the sudden contraction of 



