If it is desired to continue this experiment some time, a small hole should be cut in the pasteboard before it is fastened to the jar.  This hole is for adding water to the jar from time to time.  The hole should be kept closed with a cork.  The amount of water added should always be weighed and account taken of it in the following weighings.  While this plant is growing it will be well to wrap the jar with paper to protect the roots from the light.

It has been found that the amount of water necessary to grow a plant to maturity is equal to from 300 to 500 times the weight of the plant when dry.

This gives us an idea of the very great importance of water to plants.

=Experiment.=—­Take a few leaves from a plant of cotton, bean, clover 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.