OF THE LEAVES. 751 



tions to the atmosphere by their leaves and other external 

 organs, "which are constantly absorbing from, or exhaling into, it 

 certain ga-ses. The atmosphere, it should be remembered, is 

 brought into communication with the interior of the leaves by 

 the stomata ; it indeed fills the whole intercellular structiu'e of 

 those organs much in the same way as the air fills the lungs of 

 an animal, to, which both in structure and function they bear some 

 sort of analogy. The gases which are thus absorbed and exhaled 

 by the leaves have been proved, by a vast number and variety of 

 experiments, to be essentially carbonic acid and oxygen. The 

 experiments of Boussingaidt would also indicate that in some 

 cases, at least, carbonic oxide is evolved with the free oxygen. 

 Draper, Mulder, Cloez and Gratiolet, and others, also believe 

 that leaves and other parts exhale nitrogen when exposed to 

 sunlight. The experiments also of M. Ville would lead to the 

 conclusion, that plants, under certain circumstances, may absorb 

 nitrogen from the air ; but the investigations of Lawes, Grilbert, 

 Daubeny, and Pugh, so far as they extend, do not confirm his 

 results, but tend, on the contrary, to negative them. The ex- 

 periments of Boussingault appear also to show that the gas 

 thus supposed to be nitrogen is in reality carbonic oxide, which 

 is likewise accompanied by a certain proportion of carburetted 

 hydrogen. The whole matter connected with the subject of the 

 absorption and exhalation of gases other than carbonic acid 

 and oxygen is in an undetermined state, and our future remarks 

 will almost entirely apply to the latter. 



The absorption and exhalation of carbonic acid and oxygen 

 gases by the leaves vary according to the circumstances in 

 which they are placed. Thus, when the green leaves of a 

 healthy plant are exposed to sunlight, all experiments show, that 

 carbonic acid gas is absorbed from the atmosphere and decom- 

 posed, leaving its carbon, which is the residt of the decomposi- 

 tion, behind, and evolving its ox}-gen. It is in this way that by 

 far the largest proportion of carbon, which, as will be presently 

 shown, forms so large a part of plants, is taken up by them. 



This evolution of oxygen by the green leaves and other green 

 organs may be readily observed taking place in the form of 

 bubbles, when a submersed aquatic plant or some freshly gathered 

 leaves placed in water are exposed to the direct rays of the sun. 

 No such evolution of oxygen takes place unless the water con- 

 tains carbonic acid gas, and not, therefore, in pure distilled 

 water, or in that which has been recently boiled. It has been 

 found, also, that there is a constant relation between the amount 

 of carbonic acid gas absorbed and the oxygen exhaled. These 

 experiments prove therefore, not only the exhalation of oxvgen 

 by the leaves, but also that it must be derived from the de- 

 composition of the absorbed carbonic acid. 'J!hese changes do 

 not take place in the deep-seated tissues of the plant, nor in 



