INFLUENCE OF THE SHADE ON PLANTS. 27 



regarding the influence of plants in purifying the 

 air. His observation., that green plants emit car- 

 bonic acid in the dark, led De Saussure and 

 Grischow to new investigations, by which they 

 ascertained that under such conditions plants do 

 really absorb oxygen, and emit carbonic acid ; but 

 that the whole volume of air undergoes diminution 

 at the same time. From the latter fact it follows, 

 that the quantity of oxygen gas absorbed is greater, 

 than the volume of carbonic acid separated ; for if 

 this were not the case, no diminution could occur. 

 These facts cannot be doubted, but the views based 

 on them have been so false, that nothing, except the 

 total want of observation, and the utmost ignorance 

 of the chemical relations of plants to the atmo- 

 sphere, can account for their adoption. 



It is known, that nitrogen, hydrogen, and a num- 

 ber of other gases, exercise a peculiar, and, in ge- 

 neral, an injurious influence upon living plants. Is 

 it, then, probable, that oxygen, one of the most 

 energetic agents in nature, should remain without 

 influence on plants when one of their peculiar pro- 

 cesses of assimilation has ceased ? 



It is true, that the decomposition of carbonic 

 acid is arrested by absence of light. But then, 

 namely, at night, a true chemical process com- 

 mences, in consequence of the action of the oxygen 

 in the air, upon the organic substances composing 

 the leaves, blossoms, and fruit. This process is not 

 at all connected with the life of the vegetable or- 



