ON THE SUPPOSED PERSPIRATION OF PLANTS. 353 



the moisture under the glass had evaporated. Itmight perhaps 



have given a little more in a shorter time, and the hygrometer 



would have marked a trifle more moisture; but it is forced 



from the plant, and, so far from giving it naturally, I have 



every reason to believe, that it acts as heat does, and tears 



its way through the cuticle, as animals in an air pump will 



sometimes have the blood forced through the pores of the 



skin. 



It is certain, that a plant cannot exist without air, and We cannot 

 , , . . ... n . . t i • i . J "dge of the 



that it languishes in a confined air. in this state how im- secretions of a 



possible 10 judge of its secretions. I cannot help being P lant in C0Ii " 

 persuaded, that excellent botanist Mirbel had many doubts Mirbel. 

 of its existence. The clear and simple account he gives of 

 the production of the gasses and juices of plants is such, 

 that but for one line, it would be the most perfect thing I 

 ever saw : I hope I may be excused translating the few 

 lines. " It is certain, that the carbonic acid gas, pro- 

 duced and renewed without ceasing by combustion, is dis- 

 solved in water, which the atmosphere holds suspended in 

 vapour; and which passes through the thin cuticle of the 

 leaves, and penetrates the albumen, and gains the nourish- 

 ing vessels. This absorption takes place when the sap and 

 other fluids (at first dilated by the heat of the day,) become 

 condensed by the cold of the night, and fall towards the 

 lower extremities of the tree ; for then the liquids take less 

 room, a sort of vacuum takes place in the higher parts ; 

 and the vapours flowing around enter the leaves by the 

 pores, as we see water force itself Into the pipe of a 

 pump by the help of the piston, that produces a vacuum. 

 But as soon as the sun appears above the horizon, these 

 same fluids, joined to those the roots have pumped up from 

 the earth, drawn by heat, are carried into the leaves, and 

 escape by the pores, and it is then that the water and tw- 

 ionic acid gas enforced by light are decomposed, and the 

 torrent of oxigen flows from the leaves." 



Now if the water escapes through the pores, how can it Water cannot 

 be there to be decomposed by the light, and to give out its j^.^omposed 

 oxigen ? Setting aside therefore this line, it is the clearest through th« 

 account of vegetation, and the most just, I had ever the pores * 

 pleasure of reading. But certain it is, that, if plants per- 



Vol. XXIII. — Supplement. 2 A spired. 



