252 PRINCIPLES OF GARDENING. [CH. VIII. 



leaves operates instantaneously upon the sap. The 

 changes which occur have been detailed in pre^dous 

 pages, and there it has been shown, that as oxygen is 

 the vital air of animals, so that gas and carbonic acid 

 gas are equally essential to plants. If animals be 

 placed in a situation where they inhale pure oxy- 

 gen, their functions are highly excited and mcreased 

 in rapidity ; but it is an exhilaration speedily termi- 

 nating in exhaustion and death, if the inhalation be 

 contmued for a protracted time. So plants will 

 flourish with increased vigour in an atmosphere con- 

 taining -/irth of carbonic acid, but even this brings 

 on prematui'e decay ; and if it exceeds that pro- 

 portion, destruction is still more rapidly induced. 

 During sleep, animals exhale less carbonic acid 

 than during their waking hours, so plants emit a 

 much diminished amount of oxvgen duiing the 

 night. 



We might now proceed to enumerate the obsen-a- 

 tions and facts demonstrative that plants are gifted 

 with sensation, if these had not already been re- 

 corded at p. 104. In addition to those I mil only ob- 

 serve, that plants are obviously stimulated by light. 

 Everybody must have observed, that they bend to- 

 wards the point whence its brightest influence pro- 

 ceeds. M. Bonnet, the French botanist, demon- 

 strated this by some very satisfactory experiments, in 

 which plants, growing in a dark cellar, all extended 



