PROOFS OF VITALITY. 37 



temperature by hot-houses, inclosures, shades, etc. The vi- 

 tal energies of the plant are nearly suspended during the win- 

 ter season. But on the return of spring, the heat which it 

 brings with it, excites the living functions, and enables the plant 

 to put forth its leaves ; and, as the heat increases, its flowers 

 and fruit ; all of which not only prove the existence of such a 

 principle, but also exhibit a most important property of it. 



3. Ehitricity has probably a much more powerful effect 

 upon the functions of vegetables than has been generally sup- 

 posed, and tends powerfully to quicken their vital energies. 

 Davy proved that seeds germinate sooner in water, charged 

 with positive than with negative electricity. 



4. Artificial stimulants, such as manures and saline com- 

 pounds, and even acids and alkalies, produce effects upon the 

 functions of vegetables which can only be accounted for on 

 the ground that they contain a vital power which is peculiar 

 to them, and different from the ordinary agents of dead matter. 



III. Irritability is another property of vegetables which 

 proves the existence of the vital principle. This property is 

 conspicuous in the leaves of certain plants, as the sensitive 

 plant and Venus' jly trap ; but is more generally found in the 

 stems, stamens and tendrils, as in the pea, bean and vine. 



IV. The productions of the vegetable kingdom are decisive 

 proofs of the existence of a vital power. Most of these pro- 

 ductions cannot be formed by any known chemical agents.* 

 The chemist can analyze them and show their composition to 

 be oxygen, hydrogen, carbon and nitrogen, with a small quan- 

 tity of alkalies and metallic oxides ; but he has no means of 



* There is hardly an exception to the rule, that in producing orga- 

 nic substances, as they are called, the chemist must employ other or- 

 ganic substances which are as yet beyond his art — which, so far as we 

 know, can only be formed under the direction of the living principle. 

 In no one case can he form the substances of which animals and plants 

 chiefly consist, out of those on which animals and plants chiefly live.^ — 

 Johnsons Lectures, p. 190. 



