Boots and tubers contain the reserve material stored up by the plant, and some of them are among the important food-stuffs. Their nutritive value is largely due to starch and sugar. On account of the small proportion of protein and fat, and the large proportion of water, they are inferior in nutritive value to both legumes and cereals. The mineral matter is an important constituent of these, as of other vegetable foods. Sodium, potassium and iron salts, and sulphur and phosphorus compounds, are the common ash constituents. In combination with organic acids, etc., they contribute much to the flavor of these foods.

As a class they may be divided into the following groups:

1. Starch-yielding vegetables, as potatoes and sweet potatoes.

2. Succulent roots, as beets, carrots, parsnips and onions.

3. Condimental or flavoring roots, as horse-radish and ginger.

The potato is the most important of the starch-yielding class of vegetables. It is a tuber or thickened underground stem.

The composition of the potato (Letheby) is as follows:

Water ................................. 75.00 per cent.

Starch ................................. 18.80 per cent.

Nitrogenous matter ..................... 2.00 per cent.

Sugar ................................. 3.00 per cent.

Fat .....................................20 per cent.

When pared before cooking, there may be a considerable loss of nutritive material, especially of mineral matter. By cooking in the skin, this loss is largely prevented. To be easily digestible, a potato must be mealy, so as to be readily acted on by digestive juices. This is best accomplished by baking in an oven at 380 to 400° F. When a patient begins to take solids, the vegetable usually first prescribed is a baked potato.

Beets, carrots, parsnips, salsify, turnips and onions are the most common of the class of succulent roots. They contain, as a rule, more water than the starch-yielding class of vegetables, and their carbohydrates are frequently in the form of sugars, pectins, and other polysaccharide carbohydrates than true starch, some of which have no nutritive value. The percentage of crude fiber is greater than in the starch-yielding class.

The characteristic flavors and odors are in many cases due to the presence of volatile organic compounds of sulphur. These are often advantageous in making the vegetables palatable, and adding variety to the diet.

The beet contains a large percentage of starch and sugar.

Carrots and parsnips also contain much sugar, and when young and tender form a very nutritious food.

Onions contain considerable nutriment, but are most valuable for their pungent oil, which is rich in sulphur. They have diuretic properties, and are useful in constipation.

Radishes contain a large amount of cellulose and should not be eaten by invalids. They are chiefly used as a relish.