280 On the Structure and Functions of the Organs of Respiration. 



Daubeny, however, which were performed with the greatest care, show that 

 in fine weather, a plant consisting chiefly of leaves and stems, if confined 

 in the same portion of air, day and night, and duly supplied with carbonic 

 acid gas during sunshine, will go on adding to the proportion of oxygen 

 present as long as it continues healtliy ; the slight diminution of oxygen 

 and increase of carbonic acid, which take place during the night, bearing 

 no considerable ratio to the degree in which the opposite effect occurs by 

 day. 



Professor Burnett has ascertained, by an extensive series of experiments, 

 that the production of carbonic acid is constant during the life of a plant ; 

 that it takes place both by day and night, in sunshine and in shade j and 

 that this process is essential to its life ; for if deprived of oxygen, and con- 

 fined in' carbonic acid gas, plants quickly die ; at least, in ordinary circum- 

 stances. This process, which he considers as strictly analogous to the 

 respiration of animals, is performed by the leaves and petals ; and in a less 

 degree, by the stems and roots. On the other hand, the green parts of 

 vegetables, at certain times, and under certain circumstances, decompose 

 carbonic acid, and renovate the air by the restoration of its oxygen ; but 

 this occasional renovation may be referred, not to the respiratory, but to 

 the digestive system ; and so completely is the process dependent upon 

 the stimulus of sun-light, that plants made to grow in the dark, although 

 they increase in size, do not augment the absolute volume or weight of 

 their solid contents, and even lose some of their carbon by respiration. 

 Here again, the analogy holds good between the functions of respiration 

 and digestion, in animals and plants ; for to both is carbonic acid delete- 

 rious when breathed ; and in both is it invigorating to the digestive system 

 when absorbed as food. Plants, when exposed to strong sun-light, will 

 thrive in an atmosphere containing seven or eiglit per cent, of carbonic 

 acid ; but when placed in the shade, they quickly die. 



This fact has an interesting connection with the luxuriance of the fossil 

 flora of the coal formations, which abounds in gigantic ferns, lycopodiacea?, 

 equiseta, &c. ; — plants whose development is now much inferior, even in 

 tropical climates. It is the opinion of Adolphe Bronguiart, that the atmos- 

 phere of that epoch was highly charged with carbonic acid, as well as with 

 humidity j and that these vegetables, by the assistance of powerful sunlight, 

 drew from it that nutriment which there was not sufficient soil to afford 

 them. He considers that by thus purifying the atmosphere, they prepared 

 the earth for the residence of the higher classes of animals. 



Tliere are two periods in the life of plants, in which the function of re- 

 spiration appears to go on with remarkable activity. The first of these is 

 germination, or the process by which a new plant originates from seedj 

 and during its continuance, a large quantity of the surrounding oxygen is 

 converted into carbonic acid, by the liberation of carbon from tlie seed j 

 and at the same time, a considerable degree of heat is generated. This 



