20 PRODUCTION OF CHLOROPHYL IN LEAVES. 



presently begin to turn green, they unfold their leaves, and evolve their different parts 

 in a natural way. From day to day their weight increases, and chemical analysis shows 

 that they are fixing carbon, hydrogen, oxygen, and azote. If they be made to grow in 

 confined glass vessels, under such circumstances that an examination can be instituted 

 on the changes they are impressing on the atmosphere, it is discovered that they are con- 

 stantly abstracting carbonic acid from it, and as long as the sun shines on them, or as 

 long as they are exposed to bright daylight, they continue appropriating carbon and ex- 

 haling a mixture of oxygen and nitrogen. The continuance of their growth depends 

 on a continued supply of the acid gas in due quantities. The leading facts which are 

 here mentioned were discovered during the last century by PRIESTLEY, who found that 

 when leaves of any kind are placed in water, which holds carbonic acid gas in solution, 

 they evolve oxygen when in the sunshine. It is not pure oxygen, but a mixture of that 

 gas with azote (Ap., 794). It has been objected that no conclusion can be drawn from 

 experiments conducted in this way in regard to vegetable functions, for a plant which 

 is fitted to carry on its living processes in the atmosphere is placed in an unnatural con- 

 dition when immersed in a vessel of water. But there is much less force in this objec- 

 tion than might at first sight appear. A leaf, exposed to the air, does not absorb and act 

 upon carbonic acid gas as a gas; its tissues and parts are saturated with water, which has 

 been thrown up by the capillary force of the spongioles, or, on some occasions, obtained 

 directly from the atmosphere by the beautiful process of nightly radiation to the sky, and 

 corresponding precipitation of drops of dew ; this water, thus penetrating every part of 

 the succulent structure, is the medium through which carbonic acid is absorbed and de- 

 composed. In point of fact, therefore, though plants may live in air, their mode of dis- 

 charging this function is the same as though they were living in water, for the absorp- 

 tive force of that liquid is called into play, and carbonic acid is presented in a dissolved 

 state, and the case, in reality, becomes nothing more than a repetition of what goes on 

 when water-plants are digesting. In very many instances Nature takes extraordinary 

 pains to secure a rapid supply of the acid. In water-plants, which are often fixed, re- 

 liance is had on currents which are established by variations of specific gravity, so as 

 fast as one portion of water has approached to the place of decomposition and surren- 

 dered its gas, it is pressed away by those around, which are about to discharge the same 

 duty. But in aerial plants, the digestive organ unfolds a broad surface to the atmo- 

 sphere, and the calorific beams of the sun descending on it, by reason of its dark col- 

 our are readily absorbed ; a warm current, which is easily established in a mobile fluid 

 like air, rises rapidly; and, as if this were not enough in multitudes of instances, the 

 trembling leaves are set upon slender footstalks, which give way to every passing wind, 

 and are thus continually brought into an extensive and ever-changing gaseous contact. 

 It is scarcely possible to conceive a more simple and effective contrivance, or one which 

 reaches more perfectly its destined end. To animals powers of locomotion are given, 

 with a view of securing a possession of food ; at proper seasons the wild pigeon comes 

 up from the southern countries, and instinctively flies thousands of miles ; and in the 

 case of many beasts, to procure their prey seems to be a principal cause of movement. 

 An animal, an oxydating machine, is driven by Nature to expend his powers of loco- 



