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 of lateral 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.

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Copyright © 1988, Botanical Society of America. This article first appeared in American Journal of Botany: 75:11 (1988), 1701-1713.

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