THE DEVELOPMENT OF THE GYNOPHORE OF THE PEANUT PLANT, ARACHIS HYPOGAEA L. I. THE DISTRIBUTION OF MITOSES, THE REGION OF GREATEST ELONGATION, AND THE MAINTENANCE OF VASCULAR CONTINUITY IN THE INTERCALARY MERISTEM

THE PEANUT plant, Arachis hypogaea L. (fig. 1) is very similar to many others of the Leguminosae in its manner of growth and its general morphological characteristics. It may be either "bunch" or "runner" in habit. The bunch type grows erect and bears its fruit around the base of a single main stem. The runner type has its branches trailing over the soil and bears its fruit from numerous nodes along these trailing branches. The most distinctive and noteworthy characteristics of the peanut plant are only evident with the appearance of the typically papilionaceous, yellow-orange flowers and with the phenomena following, their fertilization, for the peanut is one of the few plants (Theune, 1916)which are geocarpic (i.e., the ovary must be underground before maturation of the fruit will occur). The transfer of the ovary from its originally aerial to its finally hypogeal position is accomplished through the growth and positive geotropism of an organ called a gynophore. The gynophore is formed proximal to the ovary from the short ovarian stalk by an intercalary meristem which becomes active after pollination. Various stages in the growth of gynophores are shown in fig. 2. In unopened buds a group of epidermal cells at one side of the base of the single, terminal style becomes radially elongated. Shortly after fertilization, these cells push aside the base of the style and form a pointed, hard, protecting cap. The author has found that this overgrowth is always from the side of the ovary nearer to the main stem. The remainder of the epidermal cells near the tip of the ovary become lignified in their outer walls and increase in radial diameter. The structure resulting from these changes later serves to protect the ovules during the penetration of the soil by the gynophore. A few days after pollination, the first sign of elongation of the gynophore is ordinarily visible, but rapid elongation and the positive geotropic curvature do not start until about the sixth day, when the gynophore is a centimeter or so long. The rapidly (longating gynophore continues to push the ovary toward the ground. Meanwhile, the floral envelopes have withered and dropped away, with the result that the hard, pointed tip of the ovary more readily penetrates, the soil. Hypogeal elongation to a depth of 2 to 5 cm. occurs. The gynophore ceases its elongation and only then does enlargement and maturation of the ovary begin. In the variety used in our 1Received for publication January 27, 1947. The author wishes to express his gratitude to Professor Ralph H. Wetmore for his unflagging patience, encouragement, and assistance and understanding. studies, a one-sided growth of the base of the developing fruit causes the latter to lie more or less parallel to the surface of the ground. The epigeal portion of the gynophore has a typical herbaceous stem structure with the important exception that there are no appendages. Eleven to thirteen separate collateral bundles, each with heavy, peripheral development of phloem fibers, run lengthwise through the gynophore (fig. 3). The hypogeal portion of the gynophore differs from the epigeal portion mainly in the early occurrence in the former of numerous root-hair-like outgrowths from the epidermis and of a later formed periderm in the outer cortex. Thus, the gynophore is noteworthy for the disparity between its general stem-like anatomy and its root-like behaviour. Chevalier (1933, 1934, 1936) in his monograph on the peanut has summarized the earlier literature and incl-udes an extensive bibliography. The anatomy of the epigeal portion of the gynophore has been described in detail by Pettit (1895). For the most part, her account agrees with those of later workers, although Richter (1899), Waldron (1919), Reed (1924) and Sprecher von Bernegg (1929) found that interfascicular cambium occurs in the older portions of the gynophore. It is with regard to the exact location and extent of the intercalary meristem that most disagreement occurred. Pettit (1895) reported that there was a "cluster of meristematic cells lying just below the ovarv." Those cells giving rise to the pith consisted of several layers lying immediately below the loculus of the ovary. She could not determine whether the meristem of the vascular strands coincided longitudinally with that giving rise to the pith. The cortical portion of the meristem was said to be so irregular in its divisions that no definite longitudinal limits could be set to it. However, despite, this uncertainty as to the vertical extent of the various regions of the meristem, she later states that the meristem in toto is less than 1 mm. from the apex of the gynophore. Theune (1916) reported that the zone of elongation lay between the tenth and fifteenth cellrows below the last ovule, while Waldron (1919) stated that the meristem was mostly just below and around the base of the ovary, though a "few dividing cells forming the cortex were found well up around the ovarian cavity." Shibuya (1935) had found all elongation to occur within the apical 7 mm. of the gynophore. The present author felt that a clearer picture of the extent of the intercalary