As indicated above, many mushrooms are poisonous, but a few kinds are known to be edible.1 Mushrooms are not especially nutritious; that is, they cannot take the place of the cereals and other staple foods, but they serve to add to the variety of materials which are more valuable for their flavoring qualities than for the quantity of nutriment they contain. Commercially the cultivated mushroom is of considerable importance, especially in Europe. Paris is said to be the center for the sale of this product. In the year

FIG. 77.-Corn smut on an ear of corn.

1901 it was estimated that 10,000,000 pounds of cultivated mushrooms passed through the markets of Paris. In this country the mushroom is of commercial importance only in the regions of the larger cities.

164. Rusts and smuts. The fungi known as rusts receive their name from the rusty appearance in an early stage of their growth which they cause on the stems and leaves of plants which they attack. The cereals, wheat, oats, barley, and rye, are the crops which this fungus injures most. In the case of wheat, half of the crop or even more may be destroyed.

The very suggestive name of smut is given to another fungus which affects all the cereals named above, and corn as well. In the case of corn, this plant often affects the ears as well. The name is probably given on account of the appearance of the mass of black spores. If one touches these spores, especially those of corn smut, with the finger, and then rubs the finger on some white paper or

1 So many deaths are caused by using poisonous instead of edible mushrooms that it is never safe to eat wild forms until they have been identified by an expert.

cloth, a sooty mark is left. The damage done by smuts is very considerable. In case of the corn crop alone it has been estimated that a yearly loss of 20 per cent of the crop, or $20,000,000, is caused thereby, and in the other cereal crops the loss is even greater. It should be mentioned in closing this discussion that the rusts and smuts are only two of a large number of fungous diseases that affect plants.

CHAPTER X

PLANT CLASSIFICATION

I. COMMON METHODS OF CLASSIFICATION

165. Herbs, shrubs, and trees. One way of classifying the common plants with which we are most familiar is that of calling them either herbs, shrubs, or trees. This classifica

FIG. 78. Base of one of the giant trees of California. -(Tarr and McMurry.)

tion is based upon the general similarity in size, form, and texture of the plants which are assigned to each group. Thus when we think of a tree we have in mind a plant which, when

mature, is of large size, with a single woody trunk and branches. This trunk may extend up nearly to the top of the tree, as in the case of the pines and spruces, or some distance above the ground the trunk may divide into branches, as is true in the elms and maples.

A shrub, on the other hand, is usually of smaller size even when fully grown than is a tree; it commonly does not have a single trunk, but several woody stems which often start from

the ground level, as in the lilac, rose, and witch hazel. Both shrubs and trees are alike in that their stems and branches do not die down to the ground at the end of the season.

An herb, as the term is used in plant biology, is a plant of relatively small size, with comparatively little woody material in its stem, which dies down to the ground level at the close of the season. Such are beans, corn, and morning glories. The roots or underground stems of some herbs- for example, dahlias, carrots, and parsnipsremain alive ready for growth the next year. These facts suggest another method of classifying plants, namely, as:

FIG. 80.- An herb.

166. Annuals, biennials, and perennials. -When a plant attains its maturity in one season's growth and then dies, as do beans, corn, and morning glories, such a plant is called an annual. Many plants which have fleshy roots, like the beet, carrot, and parsnip, do not produce flowers and seeds until the second year. During the first season after the seed is planted the food manufactured in the leaves passes down the plant and is stored beneath the ground. At the end of the season