50 OF THE SAP, 



have never been found to throw any light upon the 

 vegetable circulation. This great facility in the sap to 

 run is the first step towards the revival of vegetation 

 from the torpor of winter; and its exciting cause is 

 heat, most unquestionably by the action of the latter 

 on the vital principle, and scarcely by any mechanical 

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

 author*, who has applied this principle very happily to 

 the elucidation of the animal oeconomy, excitability, is 

 exhausted, as Mr. Knight well remarks, and they re- 

 quire a stronger stimulus to grow with vigour. See 

 p. 69. Hence vegetation goes on better in the increas- 

 ing heat of spring than in the decreasing heat of au- 

 tumn. And here I cannot but offer, by way of illustra- 

 tion, a remark on the theory advanced by La Cepede, 

 the able continuator of Buffon, relative to serpents. 

 That ingenious 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 atmo- 

 sphere than is perceptible in the autumn, when, seem- 

 ingly from the increasing cold, they become benumbed ; 

 and he explains it by supposing a greater degree of 

 electricity in the air at the former season. Dr. Brown's 

 hypothesis, of their irritability being as it were accu- 



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

 of Dr. Darwin's P/iytulogia on this subject. 



