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 white will show no starch. The
green coloring matter seems to have something to do with the starch
making, in fact starch is manufactured only where it is present. This
coloring matter is called chlorophyl or leaf green.

We are told by the chemists that this starch is made from carbon and