or other plant that has been growing in the sunlight; boil them for a
few minutes to soften the tissues, then place them in alcohol for a
day or until the green coloring matter is extracted by the alcohol.
Wash the leaves by taking them from the alcohol and putting them in a
tumbler of water. Then put them in saucers in a weak solution of
iodine. The leaf will be seen to gradually darken; this will continue
until it becomes dark purple or almost black (Fig. 61). We have
already learned that iodine turns starch this color, so we conclude
that leaves must contain starch. (Five or ten cents worth of tincture
of iodine from a drug store diluted to about the color of weak tea
will be sufficient for these leaf experiments.)

=Experiment.=--If a potted plant was used for the last experiment, set
it away in a dark closet after taking the leaves for the experiment. A
day or two after, take leaves from it before removing it from the
closet. Boil these leaves and treat them with alcohol as in the
previous experiment. Then wash them and test them with iodine as
before. No starch will be found in the leaves (Fig. 62). The starch
that was in them when placed in the closet has disappeared. Now paste
some thick paper labels on some of the leaves of a plant exposed to
the sunlight. After a few hours remove the leaves that have the labels
on them, boil, treat with alcohol and test with the iodine. In this
case starch will be found in all parts of the leaf except the part
over which the label was pasted (Fig. 63). If the sunlight is intense
and the label thin, some starch will appear under it.

According to these last experiments, leaves contain starch at certain
times, and this starch seems to appear when the leaf is in the
sunlight and to disappear when the light is cut off. The fact is that
the leaves manufacture starch for the plant and sunlight is necessary
for this work. The starch is then changed to sugar which is carried by
the sap to other parts of the plant where it is again changed to
starch to be built into the plant structure or stored for future use.

=Experiment.=--Take leaves from a plant of silver-leaf geranium
growing in the sunlight. If this plant cannot be had, the leaves from
some other variegated white and green leaved plant will do. Boil these
leaves, treat with alcohol, wash and test with iodine (Fig. 64).
Starch will be found in the leaf wherever there was green coloring
matter in it, while the parts that were