If a commercial museum is available, a trip should be planned to work over the topics in this chapter. The school collection may well include most of the examples mentioned, both of useful and harmful plants. A study of weeds and poisonous plants should be taken up in actual laboratory work, either by collection and identification or by demonstration. Green Plants have a "Dollar and Cents" Value.--To the girl or boy living in the city green plants seem to have little direct value. Although we see vegetables for sale in stores and we know that fruits have a money value, we are apt to forget that the wealth of our nation depends more upon its crops than it does on its manufactories and business houses. The economic or "dollars and cents" value of plants is enormous and far too great for us to comprehend in terms of figures. We have already seen some of the uses to mankind of the products of the forest; let us now consider some other plant products. [Illustration: Cabbage Onions Lettuce Leaves used as food.] Leaves as Food.--Grazing animals feed almost entirely on tender shoots or leaves, blades of grass, and other herbage. Certain leaves and buds are used by man as food. Lettuce, beet tops, kale, spinach, broccoli, are examples. A cabbage head is nothing but a big bud which has been cultivated by man. An onion is a compact budlike mass of thickened leaves which contain stored food. [Illustration: Celery Kohl-rabi Potato Sugar cane Stems used as food.] Stems as Food.--A city child would, if asked to name some stem used as food, probably mention asparagus. We sometimes forget that one of our greatest necessities, cane sugar, comes from the stem of sugar cane. Over seventy pounds of sugar is used each year by every person in the United States. To supply the growing demand beets are now being raised for their sugar in many parts of the world, so that nearly half the total supply of sugar comes from this source. Maple sugar is a well-known commodity which is obtained by boiling the sap of sugar maple until it crystallizes. Over 16,000 tons of maple sugar is obtained every spring, Vermont producing about 40 per cent of the total output. The sago palm is another stem which supports the life of many natives in Africa. Another stem, living underground, forms one of man's staple articles of diet. This is the potato. Roots as Food.--Roots which store food for plants form important parts of man's vegetable diet. Beets, radishes, carrots, parsnips, sweet potatoes, and many others might be mentioned. The following table shows the proportion of foods in some of the commoner roots and stems:--