48 A TEXTBOOK OF GENERAL BOTANY 
he breathes, and that it is also the source of his clothing and 
such of his other necessities as come from plants and animals. 
Energy. In photosynthesis, energy derived from light is 
used in the liberation of oxygen. It is a fundamental principle 
in physics that energy is not destroyed, but that one form of 
energy may be changed to another, as when electricity is trans- 
formed into heat. The energy 
from the light that is used 
in photosynthesis is therefore 
not destroyed but is stored in 
the resulting products as poten- 
tial energy. All that is neces- 
sary to release this energy is to 
combine the compounds result- 
ing from photosynthesis with 
the amount of oxygen that was 
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cara liberated in photosynthesis, and 
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thus change them back into car- 
bon dioxide and water. We 
may say, in general, that energy 
is used in separating oxygen 
from carbon or hydrogen, and 
liberated in combining oxygen 
with carbon or hydrogen. The 
combining of oxygen with an- 
other substance is known as 
oxidation. 
When we burn wood to produce heat or light, we make use 
of the fact that energy is liberated in oxidation. Wood is com- 
posed largely of compounds containing carbon and hydrogen. 
The burning of the wood is the oxidation of these compounds, » 
with the production of carbon dioxide and water and the liber- 
ation of energy in the form of heat and light. The energy that 
is released is the energy that was derived from sunlight and 
stored in the process of photosynthesis. Coal consists of the 
remains of plants, and the burning of coal releases energy 










Fia.37, Apparatus for collecting the 
bubbles of gas given off by a sub- 
merged plant during photosynthesis 
