for the
assimilative process. But,

(2) If breath from the lungs be passed by means of a slender glass
tube through the water, a part of the carbonic acid exhaled from the
lungs will be dissolved in it, and with this supply of the gas the
plant begins the work of assimilation immediately.

(3) If the light be shut off, the evolution of bubbles will presently
cease, being resumed soon after light again has access to the plant.

(5) Place round the base of the test tube a few fragments of ice, in
order to appreciably lower the temperature of the water. At a certain
point it will be observed that no bubbles are given off, and their
evolution does not begin again until the water becomes warm.

The evolution of bubbles shows that the process of making food is going
on. The materials for this process are carbonic acid gas and water. The
carbonic acid dissolved in the surrounding water is absorbed, the carbon
unites with the elements of water in the cells of the leaves, forming
starch, etc., and most of the oxygen is set free, making the stream of
bubbles. When the water is boiled, the dissolved gas is driven off and
assimilation cannot go on; but as soon as more carbonic acid gas is
supplied, the process again begins. We have seen by these experiments
that sunlight and sufficient heat are necessary to assimilation, and that
carbonic acid gas and water must be present. The presence of the green
coloring matter of the leaves (chlorophyll) is also essential, and some
salts, such as potassium, iron, etc., are needful, though they may not
enter into the compounds formed.

The food products are stored in various parts of the plant for future use,
or are expended immediately in the growth and movements of the plant. In
order that they shall be used for growth, free oxygen is required, and
this is supplied by the respiration of the plant.

Some plants steal their food ready-made. Such a one is the Dodder, which
sends its roots directly into the plant on which it feeds. This is a
_parasite_.[1] It has no need of leaves to carry on the process of making
food. Some parasites with green leaves, like the mistletoe, take the crude
sap from the host-plant and assimilate it in their own green leaves.
Plants that are nourished by decaying matter in the soil are called
_saprophytes_. Indian Pipe and Beech-Drops are examples of this. They need
no green leaves as do plants that are obliged to support themsel