414 BOTANY OF CROP PLANTS 



and, together with other chemical elements, made a part of 

 its protein. It is probable that the legume bacteria, while 

 active in the nodule, are throwing off continuously nitroge- 

 nous substances which are absorbed directly by the plant 

 upon which they are growing. Moreover, when the nodules 

 decompose, their protein contents are ammonified, and 

 nitrified, and finally there is left in the soil, nitrates which are 

 available as a source of nitrogen for green plants. Legumes 

 are regularly employed as rotation crops with cereals, and 

 root crops. Since they are heavy soil feeders, they make 

 excellent crops to plow under. 



Habit. — ^Leguminosae are either annual, biennial or peren- 

 nial; and are either herbs (peas, beans, alfalfa, etc.), shrubs 

 {Genista, dye-weed or green-weed), or trees {Rohinia and 

 Gleditsia, locusts), and a very few are vines {Vicia spp., 

 vetches). 



Leaves. — These are alternate on the stems, stipulate, and 

 mostly compound. They are generally odd-pinnate, that 

 is, a leaflet terminates the rachis of the leaf, as in Rohinia 

 (locust). Astragalus (vetches) and Aragallus (loco) ; sometimes 

 they are even-pinnate, that is, terminated by a tendril or 

 bristle, as in Vicia (vetch) and Lathyrus (wild and sweet 

 peas) ; or they may be trifoliate, as in clovers, or digitate, as 

 in Thermopsis (buckbean). 



Liflorescence. — The flowers are nearly always arranged in 

 racemes (pea), sometimes in a head (clovers), or spike-like 

 raceme (alfalfa), or spike (Glycyrrhiza, licorice). 



Flowers. — These are irregular (Fig. 172); they have a 

 butterfly-hke shape, and for this reason, flowers of the pea 

 type are often spoken of as "papiHonaceous." The calyx 

 is normally four- to five-toothed or cleft, the teeth or lobes 

 being equal or unequal. The petals are usually five in 

 number, a broad upper one (standard, banner or vexillum), 



