Ontogeny of the Cotyledonary Region of Chamaesyce maculata (Euphorbiaceae)

Development of the cotyledonary region in Chamaesyce maculata is described from germination of the seed through formation of the dense mat of branches which characterize this common weed. The cotyledonary node is trilacunar with split-lateral traces. Epicotyl development is limited to a pair ofleaves ("V-leaves") inserted directly above and decussate to the cotyledons. The two V-leaves are also vascularized by three traces and insertion of these traces relative to the vasculature at the immediately subjacent cotyledonary node is asymmetrical; four of the six V -leaf traces arise on one side of the intercotyledonary plane and two arise on the opposite side. Further shoot development is limited to lateral branches that develop sequentially from cotyledonary axillary buds, and then from de novo buds which arise at bases of previously developed lateral branches. The first axillary bud to develop is located on that half of the seedling which supplies the V-leaves with four traces. Development or insertion of the third and fourth branches (first and second de novo branches) relative to the first and second (cotyledonary) branches occurs in two patterns, termed cis and trans. Subsequent de novo branches generally develop between the two most recently developed branches on that half of the seedling, gradually forming a large branch plexus at the cotyledonary region. In mature robust specimens, however, the sequence oflateral branch development may become irregular. Structure of the cotyledonary region of C. maculata does not readily support widely held concepts of homology between the pleiochasium of Euphorbia subgenus Agaloma and the lateral branch system of Chamaesyce. STEM ONTOGENY in Chamaesyce S. F. Gray (or Euphorbia subgenus Chamaesyce Raf.) is unusual in that development of the epicotyl is extremely limited. Following seed germination and emergence of the cotyledons, one pair of leaves develops decussate to the cotyledons. The primary axis of the epicotyl exhibits no further extension growth; indeed, its apical meristem is commonly described as undergoing abortion (Degener and Croizat, 1938; Hurusawa, 1954; Webster, 1967; Koutnik, 1984, 1987). Growth resumes through development of lateral or secondary axes. According to Wheeler (1941), "Lateral branches arise from the apex without any particular relation to the leaves." On the other hand, Goebel (1931) and Webster (1967) describe the first pair oflateral branches to arise from axillary buds of the cotyledons. Croizat ( 1960: 982), however, has disputed the axillary origin of lateral branches. Disputes about their origin notwithstanding, the number and orientation oflateral branches 1 Received for publication 20 July 1987; revision accepted 25 February 1988. I am indebted to Thomas Felts, Ercle Herbert, and Sheila Hayden who assisted in the preparation of microscope slides. Sheila Hayden also drew Fig. 5-7 and l 7. Research was supported by Grant J-5 from the Jeffress.Memorial Trust. Microscopy and photomicrography were provided through NSF Grant BSR 84-07594. varies from species to species in Chamaesyce; some produce a small number of upright branches, whereas others, such as C. maculata (L.) Small (Fig. 1-4), produce a dense cluster of radiating prostrate branches. At the anatomical level, details of the alleged abortion of the epicotyl apical meristem and intricacies of branch stem ontogeny are not well known. For example, Gaucher's (1898) anatomical monograph of Euphorbia, sensu lato, does not address these issues. Aside from a briefreport on Chamaesyce hirta (L.) Millspaugh (Rosengarten and Hayden, 1983), the only previous publication on the subject is that of Veh (1928). Veh's study dealt largely with phyllotaxy of lateral branches and anatomical and embryological features of cyathia; he did, however, describe and illustrate some early ontogenetic events of the epicotyl and lateral branches. Nevertheless, Veh's descriptions do not include details of the vascular system of the seedling, nor do they document in detail the origin of successive lateral branches. Moreover, observations presented below dispute some ofVeh's interpretations of these unusual developmental phenomena. Asdiscussedmostrecentlyby Koutnik(l 984, 1987), elucidation of stem ontogeny of Chamaesyce bears critically on understanding its relationships with Euphorbia L. In concert with