Leafy spurge (Euphorbia esula L.) is an invasive, noxious weed that causes significant agricultural and economic losses in the Northern Great Plains. Despite its clear agricultural importance, virtually no studies have addressed the reproductive biology of this invasive plant. This report is one of a series aimed at understanding the reproductive biology of leafy spurge. Flowers were collected at various stages of development, chemically fixed, and prepared for brightfield and epifluorescence microscopy. Ovules are orthotropus with a distinct caruncle. An extensive obturator is also formed. The embryo sac is of the Polygonum type but antipodals appear to be short lived. A single embryo develops with a highly reduced basal cell. An hypostase is present shortly after embryo sac formation and is retained through seed maturation. Endosperm is initially free nuclear but eventually becomes entirely cellular. Histochemical staining reveals that protein is the major storage nutrient in the endosperm. Protein bodies are also found in the embryo except for the protoderm and procambium. Carbohydrates are primarily localized within hypostase cells. Methods for quantifying nuclear DNA and potential endoreduplication of the endosperm genome will be presented. Characteristics of leafy spurge will be compared with those of other members of the Euphorbiaceae in order to better understand reproductive diversity within this family.

Key words: endosperm, Euphorbia, leafy spurge