LIFE OF PLANTS. 



239 



But others are reared by the internal powers of their own 

 constituent parts,— occasion, by means of motions, which me- 

 chanics and chemistry have not yet explained, the ascent of the 

 sap, its resolution, and an evident change of their own exter- 

 nal proportions^ — and even extend their agency beyond the 

 bounds of their own existence, by propagating themselves. 

 These three operations — the assimilation and maintenance of 

 the sap,^ — activity arising from internal impulse, — and the pro- 

 duction of new individuals, are the characteristics of the life 

 of plants, which we have partly already considered (343.), 

 and which we have partly yet to illustrate. At the same time, 

 we must pay some attention to the slight traces of sensitive 

 life, which is the highest degree of vital activity in plants. 



365. 



The vital activity of organized bodies is excited by stimuli, 

 and receives the name of Irritability, since we cannot explain 

 its effects either upon mechanical or chemical principles ; for 

 these last admit neither of extension nor contraction of parts, 

 — of no remarkable attraction nor removal of matters, — and we 

 consider the power of producing these changes, therefore, as 

 one of a higher kind, than the lower powers with which mere 

 matter is endowed. The explanations which we give of this 

 peculiar power we term Dynamical : yet it is impossible to 

 determine sometimes, whether a particular phenomenon is to 

 be explained purely on dynamical principles, or whether it 

 may also be illustrated by mechanical and chemical means. 

 It cannot be denied, that the recent discoveries of chemistry, 

 respecting many operations of Nature, have also given more 

 light to the science of plants, than was formerly communi- 

 cated by dynamical explanations. In particular, we are in- 

 debted to the introduction of the science of imponderable 

 bodies, for a more connected view of many vegetable pro- 

 cesses, than we formerly possessed. But still these imponder- 

 able substances act partly dynamically, at least contrary to 

 the laws of gravity, and not always according to the laws of 

 chemistry. 



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