LEGUMES. (Figs. 58G-592).
The leguminous plants have lately come into great agricultural prominence because of the power that some, perhaps all, of them have of fixing the free atmospheric nitrogen contained in the soil, and thereby enriching the land in this valuable element when they decay, to the great advantage of plants that do not possess this power. These are plants of the great natural family, Leguminosm, which contains several thousand species in all parts of the world, some of them being great trees, as mahog any, locust, Kentucky coffee-tree. Some of them bear very gaudy flowers, pla cing them among the most showy of all plants, as, for ex ample, the royal poinciana of the tropics. The essential botanical characteristic that distinguishes the Leguminosm from other plants lies in the struc ture of the fruit. It is the kind of fruit known to botanists as a "legume," being a simple pistil ripening into a dry pod that opens on both su tures and bears a row of seeds on the ventral side. The bean (Fig.
586) is a typical exam ple. The most typical of the Leguminosm have a papilio naceous or but tertiy-likeflower, as in the peas and beans, the corolla having an upper mostly broad and ascend ing part called a standard, two side-pieces called wings, and two other petals below, united into a keel (Fig. 587).
The stamens are usually ten, and in the greater part of the common species these form a tube about the pistil, one of them, however, being free. The Mimosa or acacia sub-family has regular (not papil ionaceous) flowers and few or many stamens, but it agrees with the other members of the family in the legume. The leaves of practically all legumes are compound; but in some of the acacias they are reduced, on mature plants, to phyllodia (expanded petioles).
The field crops belonging to the Leguminosm may be found in this Cyclopedia under the arti cles alfalfa, beans, beg garweed, berseem, clover, cowpea, forage, lespedeza, lupine, medic, melilotus, pea, peanut, sainfoin, ser radella, soybean, spurry, velvet bean, vetch. Other leguminous plants are mentioned in the arti cles on cover-crops, dyes and medicinal plants; also on meadows and pastures. Many species
are grown in greenhouses and open gardens for ornament. The most popular is the sweet pea. The everlasting flowering pea is an old favorite.
Legume Root-tubercles. (Figs. 5SS-592.) The legume root-tubercles, or "nodules," are small galls on the roots of leguminous plants, which are caused by the activities of minute bacteria present in the soil wherever leguminous plants grow. The galls vary in form on dif erent species or gen era, being oval on the red clover, rounded and slightly lobed on the soybean, cylindri cal or club -shaped, simple or branched once or twice, on the vetch (Vida saliva), or many times dichot omously branched into a rounded mass, as in ittedicago den liculata. They are whitish or of a pale flesh-color, sometimes sordid brown in age. They occur on the roots of nearly all leguminous plants, but are absent on some, as, for example, on the honey locust (Gleditschia triacanthos).
History of the study of root-tubercles.
While the history of the study of these root tubercles of leguminous plants is extremely inter esting, reference can be made here only to a few of the diverse views which have been entertained as to their nature, origin and significance. Some of the early observers thought that they were galls produced by insects, or by eel-worms. By others they were regarded as lateral roots with dwarf growth, or swollen lateral root organs for the purpose of absorbing food, while others held that they were ]enticels which played some physiological role in the life of the plant. They were also thought by others to be imperfect buds which could repro duce the plant. They were classed as fungi of the genus Sclero tium by some, or as pathological out growths. Since Wor onin, in 1866, discov ered in the nodules bacteria-like bodies, which he thought to be the cause of their formation, the theory has been generally accepted that they are galls produced by the presence of fungi or bac teria, which enter through root-hairs and stimulate the tissues of the root to the production of an abnormal rootlet, which is called the tubercle or nodule.