THE RELATION OF THE PLANT TO THE AIR 129 
two. Take a leaf off each. Boil the leaf in water for a 
minute, then put into alcohol in order to get out the green 
colouring matter of the leat. Lastly, put the leaves into iodine. 
The leaf from A does not turn dark-blue, that 
from B does. Starch is therefore present in B, 
but not in A. 
Result. 
Leaves exposed to air containing carbon 
dioxide form starch, whilst leaves exposed to 
air freed of carbon dioxide do not form starch. Evidently, 
carbon dioxide is necessary for the production of starch. 
Cover some of the leaflets of Clover with tin- 
Fxseriment B. foi] on both sides, and expose the plant to the sun. 
Cut off leaves, boil, decolorise, and test with iodine as above. 
The leaflets which have been covered over, so 
that the sunlight could not reach them, do not 
form starch. The others do. | 
Plants take in carbon dioxide only in the 
presence of sunlight. 
Young Tropolum plants also give good results, and this 
experiment may he varied by cutting the tinfoil in patterns 
and covering only part of the leaf. The part of the leaf that 
is exposed makes starch, and therefore turns blue when the 
iodine test is applied; the covered part of the leaf does not 
make starch, and therefore does not turn blue. 
Take a variegated leaf, keep in the dark for 
some time in order that there may be no starch 
in the leaf to begin with ;* then expose to the light, and after 
some time test for starch as above. 
Only the green portions of the leaf turn blue 
with iodine—that is, starch is formed only in 
the green parts of the leat. 
Carbon dioxide can be taken in only by the 
green parts of a plant. 
Take a leaf with stomata on one side only ; 
- smear this side with vaseline, expose the leaf 
Conclusion. 
Result. 
Conclusion. 
Experiment C. 
Result. 
Conclusion. 
Experiment D. 
* It isalways better in experiments on assimilation to keep the plant in 
the dark for some little time to secure there being no starch. 
