To manufacture starch by combining some of the water sent up by the
roots with carbon taken from the air.

To digest the starch and food sent up by the roots.

To do these things well leaves must be connected with a strong,
healthy root system and must have plenty of light and air.

We are now ready to give reasons for the facts about leaves mentioned
in the first part of the chapter (see page 109).

Leaves are green because the green coloring matter is necessary for
the leaf to do its work.

Leaves are flat and thin and broad in order that they may present a
large surface to the air and sunlight.

[Illustration: FIG. 61.
To show that growing leaves contain starch. 1. Represents a green
cotton leaf as picked from the plant. 2. Is the same leaf after taking
out the green coloring matter; the leaf is white. 3. The same leaf
after treatment with weak iodine turned to a dark purple, showing the
presence of starch. (Drawings by M.E. Feltham.)]

[Illustration: FIG. 62.
To show that starch disappears from the leaf when the plant is placed
in the dark. The plant from which was taken the leaf represented in
Fig 61, was immediately placed in a dark closet for 24 hours. Then
leaf 4 was taken from it; 5 represents this leaf after the chlorophyl
was taken from it: it is white; 6 is the same after treatment with