bubbles, the following experiments can be at once conducted:

(1) Substitute for the fresh water some which has been boiled a few
minutes before, and then allowed to completely cool: by the boiling,
all the carbonic acid has been expelled. If the plant is immersed in
this water and exposed to the sun's rays, no bubbles will be evolved;
there is no carbonic acid within reach of the plant 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