Fill a five-inch test tube, provided with a foot, with fresh drinking water.  In this place a sprig of one of the following water plants,—­Elodea Canadensis, Myriophyllum spicatum, M. verticillatum, or any leafy Myriophyllum (in fact, any small- leaved water plant with rather crowded foliage).  This sprig should be prepared as follows:  Cut the stem squarely off, four inches or so from the tip, dry the cut surface quickly with blotting paper, then cover the end of the stein with a quickly drying varnish, for instance, asphalt-varnish, and let it dry perfectly, keeping the rest of the stem, if possible, moist by means of a wet cloth.  When the varnish is dry, puncture it with a needle, and immerse the stem in the water in the test tube, keeping the varnished larger end uppermost.  If the submerged plant be now exposed to the strong rays of the sun, bubbles of oxygen gas will begin to pass off at a rapid and even rate, but not too fast to be easily counted.  If the simple apparatus has begun to give off a regular succession of small 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 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.