THE NUTRITION OF PLANTS 153 
plastid uses this energy in the work of photosynthesis. It 
is evident, therefore, that photosynthesis goes on only in 
the sunlight, and is suspended entirely at night. It is 
found that any intense light can be used as a substitute 
for sunlight, and plants have been observed to carry on 
the work of photosynthesis in the presence of electric 
light. 
(3) Result of photosynthesis. The result of this work 
can be stated only in a very general way. Carbon dioxide 
is composed of two elements, carbon and oxygen, in the 
proportion one part of carbon to two parts of oxygen. 
Water is also composed of two elements, hydrogen and oxy- 
gen. In photosynthesis the elements composing these sub- 
stances are separated from one another, and recombined in 
a new way. In the process a certain amount of oxygen is 
liberated, just as much as was in the carbon dioxide, and a 
new substance is formed, known as a carbohydrate. The 
oxygen set free escapes from the plant, and may be re- 
garded as waste product in the process of photosynthesis. 
It will be remembered that the external changes in this 
process are the absorption of carbon dioxide and the giving 
off of oxygen (see 25). 
(4) Carbohydrates and proteids. The carbohydrate 
formed is an organic substance ; that is, a substance made 
in nature only by life processes. It is the same kind of 
substance as sugar or starch, and all are known as carbohy- 
drates ; that is, substances composed of carbon, and of hy- 
drogen and oxygen in the same proportion as in water. 
The work of photosynthesis, therefore, is to form carbohy- 
drates. The carbohydrates, such as sugar and starch, rep- 
resent but one type of food material. Proteids represent 
another prominent type, substances which contain carbon, 
hydrogen, and oxygen, as do carbohydrates, but which also 
contain other elements, notably nitrogen, sulphur, and 
phosphorus. The white of an egg may be taken as an ex- 
ample of proteids. They seem to be made from the carbo- 
