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]