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Nodules in the family Leguminosae come in many different shapes and forms, depending partly on the host that bears them. This review focuses on the peripheral tissues that surround the infected tissues of four anatomically distinct classes of nodules: nodules with indeterminate meristems (from the Trifolieae and Fabeae tribes); desmodioid nodules with determinate meristems (from the Phaseoleae and Loteae tribes); aeschynomenoid nodules ( Arachis hypogaea L., peanut); and lupinoid nodules ( Lupinus sp. L., lupine). I have especially stressed the importance of the three-dimensionality of the organ, because not all nodules display a radial symmetry as it is often assumed. The goal of this review is to provide a strong base of nodule structure so that forthcoming molecular studies can integrate this information into their approach.

Delimitation of Medicago from its close relatives in tribe Trifolieae subtribe Trigonellinae remains controversial. Scanning electron microscope studies of the seed surface of Medicago, Trigonella, and Melilotus showed consistent differences between Medicago and the other two genera. Smooth and rough seeds occur in all three genera, the rough seeds proving taxonomically useful. The roughness of Medicago seeds is due to rugose surface bumps (usually wrinkled, always with rounded sides), except in the monotypic section Orbiculares. The roughness of seeds of Trigonella, Melilotus, and Medicago section Orbiculares is due to tuberculate surface bumps. The structure of the Malpighian layer shows completely parallel taxonomic differences. The Malpighian cells are traversed by the light line, and the length of the portion of the cell above the light line (a) and that below the light line (b) distinguishes smooth, rugose, and tuberculate seeds. In smooth seeds (of Medicago, Melilotus, and Trigonella), a and b are constant. In rugose seeds (in Medicago only), a is constant while b is greater under the testa bumps, varying with the height of the portion of the bump occupied. In tuberculate seeds (in Medicago section Orbiculares, and in Trigonella and Melilotus), b is constant while a is greater under the testa bumps. These studies confirm the appropriateness of circumscribing Medicago in a wide sense, except for the inclusion of Medicago section Orbiculares.

Numerical taxonomic studies were conducted on 164 species, including all of Medicago and Melilotus and most of Trigonella, the three genera of subtribe Trigonellinae, in order to clarify generic circumscription. The suite of 54 traditionally employed characters studied proved to be polythetic for all groupings of interest, i.e., no subset of individually fully discriminating characters was present, although the suite could be used collectively for discrimination. Medicago proved to be quite distinctive from Melilotus, but not from Trigonella. It was recently found that 51 species traditionally placed in Medicago and 32 species commonly excluded from this genus (and often placed in Trigonella) shared without exception a number of chemical, floral and seed characters, not studied here. That is, all of these 83 species can be assigned to Medicago, which is accordingly defined monothetically. In the present study it was found that characters used traditionally to circumscribe Medicago (with 51 species) by general morphological trends (i.e., polythetically) can also be used, although somewhat less effectively, to circumscribe Medicago in the broad sense (with 83 species). Therefore, prior to the recent discovery of characters that define Medicago monothetically, this genus was defineable only by general morphological trends, and these trends favored the narrower circumscription rather than the broader one now known to be justified. Variation between Trigonella and Melilotus also proved to be gradualistic for the characters studied. Unlike the Medicago–Trigonella problem, however, there are presently no known sets of individually diagnostic characters by which the continuum of variation should be split. These two genera are (arbitrarily) distinguishable as presently circumscribed, only tradition preventing the adoption of alternative, equally defensible ways of segregating them, or indeed of combining them. The study of Trigonella produced groupings essentially identical to the 12 sections of the classical monographer, Sirjaev, and clarified the relationships of several species. The study of Melilotus separated annuals and perennials, corresponding to the subgenera of the classical treatment of Schulz, but failed to reflect his sectional taxonomy. Melilotus bicolor is much closer to what is presently conceived of as Trigonella than to Melilotus.

Examination by scanning electron and light microscopy was carried out on all genera of the legume tribe Trifolieae (Medicago, Trigonella, Factorovskya, Melilotus, Trifolium, Ononis including Passaea, and Parochetus) and on some genera of tribe Loteae (Lotus, Tetragonolobus, and Dorycnium) which have been alleged to be closely related to the former assemblage. It was found that the stigmas of Medicago, including those of problematical species often placed in other genera, were fungiliform, with a basal circumferential ring of papillae and an apex apparently free of papillar outgrowths. The monotypic genus Factorovskya, a segregate from Trigonella, possesses a stigma substantially like that of Medicago. Moreover", several of the sections of Trigonella have stigmas like that of Medicago and quite unlike those of the bulk of Trigonella species. Medicago, Factorovskya, and these sections of Trigonella appear to form a distinct phylad, separable on the basis of stigma features from all of the other genera examined.

Gland-tipped trichomes were found to occur on the cotyledonary petioles of a number of species of Medicago, Trigonella and Melilotus. Their location, seemingly protecting the plumule, and the known insect resistance provided by glandular hairs on stems and fruits of Medicago suggest that they are defensive, and may be useful in plant breeding.Key words: Medicago, alfalfa, Trigonella, Melilotus, glandular trichomes, pest resistance (immunity)