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.

A bibliography of the most important works on the subject will be found in
Physiological Botany, page 351, note.]

[Footnote 2:  Reader in Botany.  XV.  Insectivorous Plants.]

5. Respiration.—­Try the following experiment in germination.

Place some seeds on a sponge under an air-tight glass.  Will they grow? 
What causes them to mould?

Seeds will not germinate without free access of air.  They must have free oxygen to breathe, as must every living thing.  We know that an animal breathes in oxygen, that the oxygen unites with particles of carbon within the body and that the resulting carbonic acid gas is exhaled.[1] The same process goes on in plants, but it was until recently entirely unknown, because it was completely masked during the daytime by the process of assimilation, which causes carbonic acid to be inhaled and decomposed, and oxygen to be exhaled.[2] In the night time the plants are not assimilating and the process of breathing is not covered up.  It has, therefore, long been known that carbonic acid gas is given off at night.  The amount, however, is so small that it could not injure the air of the room, as is popularly supposed.  Respiration takes place principally through the stomata of the leaves.[3] We often see plants killed by the wayside dust, and we all know that on this account it is very difficult to make a hedge grow well by a dusty road.  The dust chokes up the breathing pores of the leaves, interfering with the action of the plant.  It is suffocated.