|Contents; Size, Distribution, Habit, Importance, Leaves, Corolla, Androecium, Gynoecium, Fruit, Classification,||
Genera in northern Utah (Genera with native species in green): Amorpha, Astragalus, Caragana, Cladastris, Colutea, Coronilla, Cytisus, Dalea, Galega, Glycyrrhiza, Hedysarum, Laburnum, Lathyrus, Lotus, Lupinus, Medicago, Melilotus, Onobrychis, Oxytropis, Phaseolus, Pisum, Psoralidium, Robinia, Sophora, Thermopsis, Trifolium, Vicia, Wisteria,
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This family, interpreted in its narrow sense, includes approximately 10,000 species in 400 genera, and is global in distribution. Most of our native species are herbaceous, but the family includes many tropical and sub-tropical trees and shrubs. There are 90 species in northern Utah, of which 25 are introduced. The introduced species include some that are grown for food (e.g., Pisum sativum, pea; Phaseolus vulgaris, kidney bean), some for forage (e.g., Trifolium pratense, red clover; Medicago saliva, alfalfa), some for flowers (e.g., Lathyrus odoratus, sweetpea; Laburnum anagyroides, laburnum), and a few weeds that grow despite efforts to remove them (e.g., Medicago lupulina, black medic). On the west coast, Cytisus scoparius is considered a major problem.
Economically and ecologically, the Fabaceae is one of the most important plant families. Its economic importance stems from its importance to the human diet: peas, beans, and peanuts are all seeds of members of the Fabaceae. They are important not only for their caloric value, but also for their value as a source of protein. The food species are found in several different genera, e.g., Pisum, Phaseolus, Lens, and Glycine. Other species are indirect contributors to the human diet, being used as forage. Probably equally significant, if not more so in the global scheme of things, is the ecological importance of members of the Fabaceae in converting atmospheric nitrogen to a form that can be used by any plant. This process is carried out through a symbiotic association with a bacterium, Rhizobium, which forms nodules on the roots of fabaceous plants. Rhizobium bacteria are present in most soils, but one can purchase an inoculum to improve the growth of one's garden beans. More nitrogen will be fixed than is used by the bean plants, so the soil's nitrogen level is enhanced by growing a fabaceous crop. If practicing rotation farming, a fabaceous crop should always follow a crop that requires a lot of nitrogen such as corn.
Most members of the Fabaceae have compound, stipulate leaves. In some, the terminal leaflet has been converted into a tendril that is used in climbing (e.g., Vicia, vetch; Lathyrus, sweetpea).
The flowers of the Fabaceae are said to be papilionaceous, from a fancied resemblance to a butterfly (from the French papillon, 'butterfly'). What makes a flower papilionaceous? The corolla is bilaterally symmetric with 5 petals, 1-3 of which are free and two united. The uppermost petal is called the banner, and is generally erect, or at least turned upwards. The two side petals, called the wing petals, may spread sideways or point forwards. They are not attached to the banner petal, but may be attached to the two lower petals. These two lower petals, known as the keel petals, are fused together along both their edges, enclosing the reproductive portions of the flower. The lower edges are fused together more firmly than the upper edges, but both "seams" can be split open without too much effort, consequently the flowers are generally keyed out as having separate petals. Many families have bilaterally symmetric flowers; only in the Fabaceae does one find this combination of (1)3 separate petals and two fused petals.
The androecium consists of 9 or 10 stamens that are (usually) fused by their filaments. If there are 10 stamens, 9 may be fused together, the tenth stamen remaining free. If all ten stamens are fused together, they are said to be monodelphous (mono, 'one' and delph, 'brotherhood'); if nine stamens are fused together and one free, the androecium is said to be diadelphous.
The gynoecium consists of a single, simple pistil. At maturity, it usually forms a dry fruit that splits down one side only, differing in this respect from a follicle. This kind of fruit (i.e., a dry fruit derived from a simple pistil and splitting down one side) is called a legume. A pea pod is a typical legume. There are variations within the family, with some species having a fruit that splits into individual segments on either side of each seed. These transversely dividing fruits are called loments.
Legumes are found in what are now treated as three distinct families, the Mimosaceae, Caesalpinaceae, and Fabaceae, but not in any other families. These three families used to be treated as a single family with three large subfamilies and a few minor ones. The large sub-families were: Mimosoideae, Caesalpinoideae, and Faboideae. This single family was known as the Leguminosae, and is still known as such by sticks-in-the-mud such as myself, but those who want all family names to end in -aceae have to call it Fabaceae. This creates some confusion, because the Fabaceae can either mean all three units (Mimosa-group, Caesalpinua-group, and Faba-group) or just the Faba-group.
One of the arguments for treating the three subfamilies as three separate families should be that they represent different lines of evolution from a common ancestor. I am not sure what the data are that support this interpretation. Unfortunately recent work suggests that the traditional interpretation was a more accurate reflection of the phylogenetic history of the three families and their small relatives. In other words, all members of the old Leguminosae belong to the same general line, but that the traditional subgroups are not well delineated - at least, not if one wants a taxonomic treatment that reflects evolutionary history.