(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 themselves. [Footnote 1: Reader in Botany. XIV. Parasitic Plants.] Some plants are so made that they can use animal matter for food. This subject of insectivorous plants is always of great interest to pupils. If some Sundew (_Drosera_) can be obtained and kept in the schoolroom, it will supply material for many interesting experiments.[1] That plants should possess the power of catching insects by specialized movements and afterwards should digest them by means of a gastric juice like that of animals, is one of the most interesting of the discoveries that have been worked out during the last thirty years.[2] [Footnote 1: See Insectivorous Plants, by Charles Darwin. New York: D. Appleton and Co., 1875. How Plants Behave, Chap. III.