562 Cultivation of Plants. 



perties which should distinguish it. It has yet to undergo 

 important modifications in its chemical composition. A certain 

 temperature is indispensable to accomplish this change ; but 

 it is sunlight that plays the principal role. 



Delicate and complicated experiments have demonstrated 

 the fact, that (in many instances, at least) during the day, 

 and especially when exposed to the direct rays of the sun, all 

 green parts of plants exhale oxygen ; and that, on the contrary, 

 during the night, or when in darkness, they give off carbonic 

 acid. Oxygen is obtained by the decomposition of the carbonic 

 acid contained in the sap, which the plants have drawn from 

 the soil or absorbed directly from the atmosphere through their 

 leaves. Under the influence of light, the oxygen of the carbonic 

 acid is liberated, and the carbon fixed in the tissues of the 

 plant, entering itself into fresh combinations, termed proxi- 

 mate principles. Although plants exhale a certain proportion 

 of carbonic acid under the conditions here indicated, and 

 perhaps also under other conditions, the quantity of oxygen 

 disengaged is greatly in excess, as the immense annual growth 

 of wood in all parts of the world goes to prove. In fact, it 

 is beyond dispute that plants are mainly instrumental in pre- 

 serving the equilibrium of the constituent parts of the atmo- 

 sphere by their decomposition of the carbonic acid respired by 

 animals, and restitution of the greater portion of the oxygen 

 to the air. 



All parts of plants naturally deprived of light, as roots, 

 tubers, rhizomes, etc., do not themselves fix the carbon con- 

 tained in their tissues, nor become green ; but it is assimilated 

 for them in the leaves, and returned through the circulating 

 channels. Nevertheless, these parts when partially exposed to 

 light become green, and are capable of elaborating their sap. 

 There is a considerable variation in the degree of light re- 

 quired by different plants to enable them to form the chloro- 

 phyll, or green colouring matter common to most vegetable 

 organisms, of which carbon is one of the chief ingredients. 

 For the greater part, direct exposure to the sun in a climate 

 suitable to their nature, other things being equal, is necessary 

 for the perfect development of plants. But Ferns, and some 

 other plants, will flourish and reproduce themselves in semi- 

 darkness. This well-known action of light and darkness on 

 plants is taken advantage of by gardeners in blanching Endive, 

 Seakale, etc., and thereby rendering them more fit for food, 



