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 réle. 
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 
vther 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, ete., and therehy rendering them more fit for food, 
