82 AKATOMY AND PHYSIOLOGY OF 



tat that their green-coloured organs, consequently their leaves in 

 particular, possess in a high degree, so long as they are exposed to 

 lisfht, the facnltv of absorhins^ carbonic acid from the medium, be 

 it air or water, in which they are placed, and of secreting oxygen 

 gas in its place. 



We owe the more accurate knowledge of this process espe- 

 cially to the admirable experiments of Saussure, which have been 

 fully confirmed by later ones of Grischow, Boussingault, and 

 others. The phenomena may be summed up in the following 

 statements- 



When green-coloured plants are exposed to the influence of sun- 

 light, under water containing carbonic acid, they exhale oxygen 

 gas. This exhalation of oxygen does not take place in boiled 

 water, 



"When plants are exposed to the influence of sunlight in atmos- 

 pheric air to which carbonic acid (up to l-12th of its volume) has 

 been added, they remove the carbonic acid and exhale oxygen in 

 its place. This absorption of carbonic acid takes place very soon. 

 Boussingault Q^ JEconomie rurale" i 66) placed a shoot of a Vine 

 bearing twenty leaves in a glass globe, and while the sun shone 

 upon the apparatus, drew through it in an hour fifteen litres of 

 atmospheric air which contained ,0004 to ,0004i5 of carbonic acid, 

 and at the exit of the air firom the globe, the carbonic acid was 

 diminished to ,0001 or ,0002. According to Chevandier's calcu- 

 lations, the trees of a forest, during the five summer months in 

 which they bear leaves, withdraw from the column of air standing 

 above the forest l-9th of its contents of carbonic acid. 



When a leafy shoot with its lower end dipping in water con- 

 taining carbonic acid, is enclosed in a glass globe, its leaves exhale 

 more oxygen than when its lower end is dipped in common water. 

 A leafy shoot still connected with a tree, enclosed in a glass globe, 

 increases the oxygen gas in the globe. Therefore in both cases 

 the carbonic acid carried up with the ascending sap into the leaves 

 is retained by the latter, and oxygen gas given oif in proportion 

 to it. 



The exhaled carbonic acid is not contained in the plant in the 

 form of gas, before its separation, for plants which contain no air, 

 like Confervmi or leaves from wliich the air has been exhausted 

 by the air-pump, exhale oxygen in like manner. Pieces of torn 

 leaves possess this function as well as entire leaves; leaves, on the 

 contrary, which have had their organization destroyed by pres- 

 sure, give gS no carbonic acid, neither does the epidermis of the 

 leaf. The quantity of oxygen gas wMch leaves give off depends 

 upon their superficial extent and not on their mass. 



The secretion of oxygen varies much in abundance under illu- 

 mination by different rays of the solar spectrum. According to 

 the researches of Draper (" Treatise on the forces which produce 

 the organwationof plants/' Appendix, 177), the following amounts 



