Objective   Occurrence, distribution, anatomical structure, and morphological characteristics of floral organs of red pitaya (Hylocereus monacanthus) were microscopically observed to study the differentiation and regulation of the organogenesis.

  Method   Flower buds on red pitaya plants at different developmental stages were examined under a VHX-5000 digital microscope.

  Result   The flower buds were formed mainly below the spines on the branches. The sepal and petal primordia were initiated spirally. The multi-round stamen primordia originated at the base of perianth on proximal before distal axis. Anthers, including 4 pollen sacs lined symmetrically and split longitudinally, grew at the bottom. There were as many as 30 carpels primordia appeared simultaneously. Multiple carpel primordia might form ovaries that contained a single locule. Two rows of circinotropous ovules were inserted on the abdominal suture. The mature stigma showed 24-30 shallow clefts with numerous papilla cells on the surface. The style had a hollow canal with an inner surface densely covered with passage cells.

  Conclusion   The sequence of the floral organogenesis of red pitaya started from sepal primordia, developed into petal primordia, then stamen primordia, and finally, carpel primordia. However, there were overlaps in time of appearance on some of the organs.