216 RECKNT RESEARCHES ON THE RESPIRATION OF PLANTS. 



3. Exhalation of Carbonic Acid by the Leaves under the 

 influence of the Solar Rays. 



Of all the facts relating to the respiration of the green parts 

 of plants, the exhalation of carbonic acid by the leaves, when 

 acted upon by the direct rays of the sun, is by far the most 

 curious, and worthy of the attention of physiologists. Being 

 most peculiarly organs of reduction, they set free the oxygen of 

 the gas and retain its carbon, under the influence of the sun's 

 rays ; and mider the very same influence as was ascertained in 

 1849 i^Atm. des Set. Nat., 1850), they give out a portion of their 

 carbon in the form of acid gas. Since then I have paid more 

 especial attention to the influence of temperature, of shade, and 

 of dull weather, on the respiration of these organs during the day, 

 neglecting for a time that of the direct rays of the sun ; and I 

 now lay before the public an account of the observations made 

 by me in this matter during the present year (1851). The ex- 

 periments consisted in making green branches and leaves of 

 plants, in the open ground, breathe in the limited atmosphere of 

 a tolerably capacious flask containing some baryta-water. 



The apparatus consisted of a flask of the capacity of 6000 

 cubic centimetres. The mouth of the flask, sufiiciently large to 

 receive a branch covered with leaves without bruising them, was 

 turned down, and closed with a cork having a groove cut in it 

 to fit the stem. Through the middle of the cork passed a safety- 

 tube, the object of which was first to facilitate the introduction of 

 baryta-water for the purpose of fixing the carbonic acid, and, 

 secondly, to prevent the continual rarefaction of the air caused 

 by the formation and fixing of the acid gas. 



Baryta-water was preferred to lime-water, as carbonate of 

 baryta is less soluble than carbonate of lime. The carbonic 

 acid obtained by this means does not however accurately repre- 

 sent all the acid that might have been fixed ; for I ascertained 

 that carbonate of baryta is soluble to an appreciable extent in an 

 excess of base. 



In all the experiments made in the way mentioned, the herba- 

 ceous parts tried were fresh and vigorous, and seldom exceeded 

 in size ^Jtrth part of the volume of the confined air in which 

 they breathed. After an exposure to the sun for six hours, 

 the branch was detached from the plant, and the baryta-water 

 agitated so as to fix the last portions of acid gas mixed with 

 the air. 



This water, charged with carbonate in suspension, was then 

 collected in a closed funnel, and the deposit was put into a gra- 

 duated tube, and decomposed over mercury by means of some 

 pieces of citric acid. This acid was preferred to tartaric acid 



