36 NEW ZEALAND NATURE-STUDY BOOK 
as shown—placing in it a small pot-plant and a glass 
vessel containing caustic soda. This substance will rapidly 
absorb any carbon-dioxide there may 
be inside the jar. Insert through the 
cork, which must be air-tight, a bent 
glass tube having the bend filled with 
pieces of caustic soda so as to absorb 
any carbon-dioxide which would 
otherwise find its way inside the jar. 
The pot-plant must previous to the 
experiment be kept in the dark for 
twenty-four hours to free it from 
starch. Jf now the whole be exposed 
to sunlight for a few hours, it will be 
found that no starch has been formed. 
By this and other experiments it can 
be proved that green plants obtain 
the whole of their carbon from the 
carbon-dioxide of the air, and—as we shall subsequently 
show—that sunlight is necessary for the process. 
Decomposition of Carbon-dioxide by Leaves, in 
the presence of Sunlight. When the leaves of a plant 
are spread out to the air, carbon-dioxide is absorbed by 
them, and. in the presence of sun 
light is decomposed or split up into 
its two parts, carbon and oxygen ; 
the carbon unites with other plant 
food to form starch &ec, and the 
oxygen is returned to the air. 
Take a bunch of any green water 
weed—watercress does admirably — 
and put it in a vessel of water. 
Cover the leaves with an inverted 
funnel in such a way that the Fig 32. 
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