OF THE SAP, AND INSENSIBLE PERSPIRATION. 65 



and its exciting cause is heat, most unques- 

 tionably by the action of the latter on the 

 vital principle, and scarcely by any mechani- 

 cal operation, or expansive power upon the 

 fluids. The effect of heat is in proportion to 

 the degree of cold to which the plant has been 

 accustomed. In forced plants the irritabi- 

 lity, or, to use the words of a late ingenious 

 author*, who has applied this principle very 

 happily to the elucidation of the animal eco- 

 nomy, excitability ', is exhausted, as Mr. Knight 

 well remarks, and they require a stronger sti- 

 mulus to grow with vigour. See. p. 91. Hence 

 vegetation goes on better in the increasing 

 heat of spring than in the decreasing heat of 

 autumn. And here I cannot but offer, by 

 way of illustration, a remark on the theory 

 advanced by La Cepede, the able continua- 

 tor of Buffon, relative to serpents. That in- 

 genious writer mentions, very truly, that 

 these reptiles awake from their torpid state in 

 the spring, while a much less degree of heat 

 exists in the atmosphere than is perceptible 



* Dr. John Brown, formerly of Edinburgh. See the 

 14th Section of Dr. Darwin's Phytologia on this sub- 

 ject. 



F 



