This section is from the book "Wild Flowers Of New York", by Homer D. House. Also available from Amazon: Wild Flowers Of New York.
Herbs, often with perennial roots and erect or ascending stems. In a few species the stems trailing. Leaves three-foliolate. Flowers small, purple or whitish, in axillary clusters, heads or panicles. Frequently the flowers are of two kinds intermixed, one petaliferous but sterile, the other minute, apetalous and fertile. Calyx lobes nearly equal, those of the petaliferous flowers usually longer than those of the apetalous flowers. Upper petal (standard) obovate or oblong, the lower petals forming an incurved keel. Stamens usually diadelphous (nine and one); anthers all alike. Ovary one-ovuled, ripening into a flat, indehiscent, reticulated, mostly one-jointed and one-seeded pod.
Corolla purple or purplish; plants bearing both petaliferous and apetalous flowers Stems prostrate or trailing Foliage glabrous or somewhat appressed-pubescent................L. repens
Foliage downy-pubescent or tomentose...................L. procumbens
Stems erect or ascending
Peduncles distinct and mostly longer than the leaves
Bushy-branched; petaliferous flowers paniculate...........L. violacea
Stems simple or little branched; flowers racemose or subspicate
Stems tomentose; leaves tomentose beneath...........L. brittonii
Stems and leaves glabrate or appressed-pubescent; leaflets oval to suborbicular....................................L. nuttallii
Peduncles shorter than the leaves or the flower clusters sessile
Leaflets densely tomentose beneath; calyx of the petaliferous flowers less than one-half as long as the pod..........................L. stuvei
Leaflets appressed pubescent beneath or glabrate
Calyx of petaliferous flowers less than one-half as long as the pod; leaflets oval to oblong........................L. frutescens
Calyx of the petaliferous flowers two-thirds as long as the pod or more; leaflets silvery-pubescent beneath..................L. simulata
Corolla whitish or yellowish; flowers all complete; pod included or scarcely exserted from the calyx Leaflets oblong, ovate-oblong, or nearly orbicular
Peduncles mostly exceeding the leaves............................L. hirta
Peduncles shorter than the leaves............................L. capitata
 
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