Flowers: The flowers of all leguminous plants are alike in general construction and totally different from the flowers of other plant families. The lower part (Fig. 7, Sep.) is insignificant. It is composed of five green, toothlike organs, called sepals, connected at their base. The upper part, popularly called the flower, consists of five mostly showy petals. One of these is much larger than the others and encloses them in the bud. It is called the standard (Fig. 7, St.). The lateral ones are irregular in shape and are called wings (Fig. 7, W.). The two others grow together, forming a boatlike organ called the keel (Fig. 7, K.), which encloses the stamens (Fig. 7, St.) and the pistil. Nine of the ten stamens grow together in their lower parts, forming a tube that encloses the pistil. Each flower has only one pistil. It consists of a broad lower part, the ovary (Fig. 7, O.) and a narrow upper part, strongly knee-bent and developed at its top into a stigma (Fig. 7, P.).
Fig. 7 - The different parts of a flower of Pea. Natural size.
Sep. - Sepals W. - Wing. St. - Stamens.
S. - Standard. K. - Keel. O. - Ovary of pistil.
P. - Stigma of pistil.
Fertilization: Fertilization in leguminous plants is never performed by air currents. In a few genera, such as peas and vetches, the flowers are self-fertilized; that is, the pollen automatically fertilizes the pistil of its own flower. In most leguminous plants, however, the pollen is transported from one flower to another by insects, which visit the blossoms for the nectar stored at their base. When the flowers are large and showy, the standard acts as a sign, announcing to the insect the location of the honey. In other species the comparatively small flowers are very numerous, and are thus visible at a long distance. Still others have insignificant flowers borne close to the ground. Such plants, like Trefoil, grow under taller neighbours, and are therefore more or less hidden. But in spite of their humble appearance and secluded position, insects are attracted by the fragrance of the blossoms.
A brief description of the fertilization, which varies in different genera, is given in connection with Alfalfa on page 114, and with Red Clover on page 99.
Fruit: The fruit is a pod; that is, a narrow fruit with leathery or papery walls. When ripe and dry, the pod splits its entire length and lets the seeds out. Its two halves often twist like a corkscrew, sometimes with such violence that the seeds are thrown a considerable distance. In some species and genera there is only one seed, when the pod falls off without breaking up, but generally the seeds are numerous.
Agricultural value: On well prepared land, stored with a fair supply of plant food, especially potash and phosphoric acid, leguminous plants yield heavy crops of great nutritive value, relished by all kinds of stock. Putting aside their value for soiling, leguminous plants can be used to advantage for either hay or pasture. Their suitability for fodder depends largely on their mode of development. As a rule their nutritive value is highest when they are in bloom or shortly before. If intended for hay they should therefore not be cut too late. It is true that sometimes the crop is larger if cutting is delayed until shortly after the plants have completed flowering; but, on the other hand, the hay is coarse and more or less woody. It lacks palatability and fat and milk producing constituents, and in spite of its larger quantity it is of smaller total value than if cut at the proper time. Late cutting also spoils the second growth. When Red Clover and Alfalfa, for instance, begin to bloom, new shoots start from the crown of the root. If cutting is delayed until these shoots are high enough to be caught by the mower, it is evident that the second growth will be seriously affected.
Some species, like White Clover, are suitable for pasture, as the tramping of stock encourages the plants to new growth. Others, like Red Clover and Alfalfa, with a crown a little above the ground, must be pastured more carefully, tramping being apt to injure the plants if the soil is not in the proper condition. As the new growth starts from the crown, the plants should not be pastured too close, at any rate not late in the fall.
It is well known that leguminous plants enrich the soil. This faculty used to be attributed to their rather deep root system. It was claimed that the taproots gathered from the subsoil great quantities of food inaccessible to plants with shallower roots. The substances thus removed from the subsoil were said to be used in building up the superficial roots and the overground parts of the plants, which parts, when ploughed down, added this material to the surface soil. There is no doubt that plant food is removed from the subsoil and stored in the upper parts of the plants and that the above explanation should be considered. But the soil-enriching faculty of leguminous plants is connected with phenomena that render this explanation insufficient. Generally Alfalfa will not thrive on soil where it has never been grown before. The plants soon stop growth, turn yellow and finally die. If, however, some soil from an old Alfalfa field is sown on the land, a crop will be produced without any trouble. If the plants are examined, it will be found that the roots of those grown on old Alfalfa soil are provided with numerous nodules, whereas the roots of weak plants on virgin soil are destitute of them. Only quite recently have the origin and significance of these tubercles been understood. It has been proved that they are a kind of gall produced by certain bacteria. These bacteria live in the ground, attack the root hairs, break through their thin walls, and make their way to the interior of the root branches. There they propagate rapidly, forming masses within the nodules. Later on, most of the bacteria decompose and are used by the plants, which thus obtain additional food. As the bacteria are very rich in nitrogenous substances, the source of which is the air contained in the porous soil, leguminous plants are able to secure, indirectly through the bacteria, their nitrogen from the air. They are therefore able to accumulate nitrogen without robbing the soil and, when dying, to leave a supply of nitrogenous substances for succeeding crops.
When soil from land where Alfalfa, for instance, has been successfully grown is put on a field, that field is supplied with the bacteria necessary for the development of Alfalfa. The amount needed is not large, two hundred pounds being sufficient for an acre. Instead of soil from old fields, artificial cultures of bacteria are now available at many botanical laboratories. These cultures, with directions for their use, are on sale in bottles at a low price.
Nodule-forming bacteria are necessary for the proper development of all kinds of leguminous plants. But this does not mean that bacteria which will serve for a certain plant will satisfy another kind. On the contrary, there are different species and races of nodule-forming bacteria, and each species or race is able to produce nodules only on a certain kind of leguminous plant. Thus the bacteria which work on the roots of Red Clover are different from those which produce nodules on the roots of Alfalfa and are quite unable to benefit the latter plant. In using artificial cultures of nodule-bacteria therefore, care should be taken to procure the right kind.