Page:An introduction to physiological and systematical botany (1st edition).djvu/95
OF THE SAP, AND INSENSIBLE PERSPIRATION.
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 economy, excitability, is exhausted, as Mr. Knight well remarks, and they require a stronger stimulus 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 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 atmosphere than is perceptible