LIFE OF A PLANT 
called chlorophyll. It is the material which 
gives leaves their green colour. It is one of 
the most important substances in plantdom. 
Under the influence of sunlight, this chlorophyll 
takes the carbon dioxide of the air, and, with 
water and certain minerals, makes starch, the 
raw material of plant construction. This pro- 
cess, called photosynthesis, goes on while the 
sun shines, and stops with the approach of 
darkness. The necessity of plenty of light can- 
not be overestimated. 
In the manufacture of starch, oxygen occurs 
as a by-product. As the plant has no use for 
this element, it is breathed out from the surface 
of the leaves. From the standpoint of man, 
this makes plants atmospheric purifiers. At 
night, when the making of starch is suspended, 
there is often a superabundance of carbon diox- 
ide within plant structures. It is this gas which 
is now exhaled, though in very small amounts. 
Some authorities maintain that the excess of 
carbon dioxide is contained in water absorbed 
by the roots. In the daytime this is welcomed 
as additional starch material, but at night there 
is no use for it. 
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