It must be explained that all plants are made of a framework of numerous cells, something like a honey-comb. The cells in plants are of many different shapes, according to the plant, or the part of the plant, in which they are found. They are usually so small that they cannot be distinguished without a microscope; but occasionally they are large enough to be seen without one. Pass sections of orange or lemon, where the cells are visible. 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.
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.