130 ELEMENTARY BOTANY 
to the influence of sunlight for some hours, and then apply 
the iodine test as above. 
Result. No starch detected. 
Conclusion. Starch is only formed when the stomata are open. 
Place a shoot of watercress in a beaker of water ; 
cover it with an inverted funnel over the end 
of which is a test-tube filled with water. Place in sunlight. 
Bubbles of gas are 
_ given off from the 
-_ watercress and pass up the test- 
tube, where they may be collected. 
If tested, the gas is found to be 
oxygen. 7 
Experiment E. 
Result. 
Oxygen is given off 
by green leaves in sun- 
light-—that is to say, under the same 
conditions necessary for the taking 
in of carbon dioxide by the plant. 
We may now assert that starch 
is formed by the green parts of leaves, 
Fic. 146.—Apparatus To which take in carbon dioxide from 
oe the air, but only in the presence of 
OXYGEN WHEN MAKING sunlight, and if the stomata of the 
oes leaf are open. 
Starch, chemists tell us, is built up from carbon and water ; 
the green part of the leaf has the power of separating the 
carbon from the carbon dioxide and keeping it for the pur- 
pose of making starch, the oxygen of the carbon dioxide being 
at the same time given back to the air. ‘The water necessary 
for the making of starch is absorbed by the roots, passes 
through the plant (Chapter XVII.) to the leaves; thus with 
carbon and water the leaf makes starch. 
Plants in using up carbon dioxide help to keep the air 
pure; this is the reason that it is good to have them in the 
room during the day, but it is unhealthy to have them in a 
bedroom at night; for thon, sunlight being absent, they are 
not using carbon dioxide at all. 
Conclusion. 

