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.

remain alive ready for growth the next year. These facts suggest another method of classifying plants, namely, as:

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

the stems and leaves above ground die; but if this root remains in the ground or is planted the next season, stems, leaves, and flowers develop rapidly, and finally seeds are

FIG. 81. Carrot. A, young seedling; B, enlarging root early in season; C, section of enlarged root late in season.

formed, the food stored up the preceding season being drawn upon for the development of these parts. Plants which have a life history like this and which live for two years only are