THE ACTION OF LIGHT UPON PLANTS. O 



SO novel as I had at first supposed. Priestley first 

 advanced the opinion that plants in certain circum- 

 stances emitted oxygen gas ; and Ingenhousz soon after 

 discovered that the leaves of plants, when immersed 

 in water, and exposed to the light of day, produced 

 an air, which he announced as oxygen gas. This 

 result, however, was doubted by Ellis, in his elaborate 

 treatise on Atmospheric Air, and, as he considered, 

 disproved.* The consumption of oxygen by animals 

 in respiration, and the emission of carbonic acid from 

 the lungs and skin, were well sho^vn by this writer, 

 who maintained, however, that this latter gas was also 

 emitted by the leaves of plants.f 



At the third meeting of the British Association, 

 held at Cambridge in 1833, Professor Daubeny com- 

 municated a notice of certain researches which he was 

 then pursuing, concerning the action of light upon 

 plants, and that of plants upon the atmosphere. " He 

 considered that he had established, by experiments on 

 plants immersed, sometimes in water impregnated with 

 carbonic acid gas, and at others in atmospheric air 

 containing a notable proportion of the same, that the 

 action of light in promoting the discharge of certain 

 of their functions, and especially that of the decom- 

 position of carbonic acid, is dependent neither upon 

 the heating, nor yet upon the chemical energy of the 

 several rays, but upon their illuminating power. 



" He regarded light as operating upon the green 

 parts of plants as a specific stimulus, calling into 

 action, and keeping alive those functions, from which 



* Inquiry, &c. p. 57 — 60. f lb. p. 203, et passim. 



