ible. Make a drawing on the black-board of the cellular
formation of a potato. Lead the class to understand that, in every case,
the cell walls must be broken to get out the cell contents. To
illustrate this, they may use potatoes, and break the cell walls by
grating the potatoes. After they have broken up the framework, the cell
contents should be strained through cheesecloth into a glass. They have
now two parts to examine--cell walls and cell contents.

[Illustration: Cellular structure of a potato]

Wash the framework to free it of any cell juice and study it first. Give
its name, and note its colour and texture. Compare the framework of
potatoes, strawberries, lettuce, trees, etc. Tell the class that in some
cases part of the cellulose is so fibrous that it is used to make
thread, cloth, or twine; for instance, _flax_ and _hemp_.

Cellulose is most difficult to dissolve, so that practically little of
it is digested. It serves a mechanical purpose in the digestive tract by
helping to fill the organs and dilute the real food. If fibrous, it acts
as an irritant and overcomes sluggishness of the intestines known as
constipation. The outer coats of cereals are an example of coarse
cellulose, as used in brown bread and some kinds of porridge.

Examine next the juice which was contained in the cells of the potato.
The liquid shows much water; the colour indicates mineral matter in
solution; the odour suggests a flavour; the white sediment is starch.

COMPOSITION OF POTATO JUICE

Water, mineral matter, flavouring matter, starch.

Draw attention to the fact that the potato is the part of the plant
which acts as a storehouse. In such parts, starch is always found as the
stored form of sugar; but, in parts which are not storehouses, sugar
will be found in its stead. In rare cases both are found, as in the
parsnip.

NOTE.--This is a good time to impress the fact
that plants are the source of starch for
manufacturing purposes. In England, potatoes
are largely used; in Canada, corn. It will be
interesting to state that the early settlers
obtained their starch for laundry purposes at
home from potatoes, by chopping or grinding
them.

The insolubility of starch in cold liquids may be effectively reviewed
at this part of the lesson. The starch has been lying in the water of
the potato cells for several months, yet has not dissolved. Let two or
three of th