VEGETABLE PHYSIOLOGY. 33 



up oxygen from the atmosphere, by decomposing it and setting 

 ]3art of the nitrogen at liberty : a portion of the oxygen thus 

 absorbed, is fixed in the sap, and the other part, uniting with the 

 superfluous carbon in the plant, forms carbonic acid gas, which 

 escapes from the leaf and mixes with the atmosphere. This 

 will serve to explain, why the night air is less salubrious than 

 that of the day ; and the necessity of a large proportion of oxygen 

 being set at liberty during the day, to obviate the injury which 

 the atmosphere sustains by the operations of the night. 



Thus we perceive the leaves of plants perform very different 

 operations at different times ; since during the day, they are 

 giving out moisture, absorbing carbonic acid gas, and emitting 

 oxygen gas ; during the night, they are absorbing moisture, 

 giving out carbonic acid and nitrogen gases, and taking up 

 oxygen gas. By these operations assisted by the agency of 

 light, (which, independently of its imparting color to the leaf, 

 contributes essentially to its chemical changes,) the sap receives 

 all the primary principles which constitute the plant namely, 

 oxygen, hydrogen, carbon and nitrogen ; by the various com- 

 binations of which, nourishment to the plant is not only produced, 

 but also through the agency of secretion, those Other substances 

 are elaborated which we know can be extracted from vegetables ; 

 and which, taken from one description of vegetable or another, 

 amount to no less than thirty-one, exclusively of those which, 

 belonging to the mineral kingdom, have been denominated 

 extraneous ; while, by a very beautiful process, the purity of the 

 atmosphere is so balanced within the twenty-four hours, as to be 

 fitted for all the purposes of animal and vegetable economy. 

 Thus by a wonderful piece of mechanism that cannot be too 

 much admired and investigated, and in the construction of which 

 there is still a wide field for discovery, the sap in its simple state 

 is absorbed from the earth by the roots, and conveyed through 

 the cells and the tubes of the wood into the leaves ; where by the 

 processes of evaporation and absorption (as just explained) it 

 acquires new principles and becomes the true sap. It is then 

 taken up by the extreme vessels of the bark, and by them 

 conveyed back to the branches, stem and root ; depositing in its 



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