Ovular development and perisperm formation in Phytolacca americana （Phytolaccaceae） and their systematic significance in Caryophyllales

Phytolacca is the biggest and most original genus in Phytolaccaceae and an important genus in plant systematic studies. Light microscopy results show that the Phytolacca americana L. ovule arises from the caulis （floral receptacle）. The perisperm and hypostase are simultaneously initiated from the top several layers of cells of chalaza after fertilization, and the perisperm is located between the nucellus and hypostase. In the early stages of development, the hypostase cells are thin-walled with dense cytoplasm, clear nuclei, and some reserve granules. Later, at the heart-shaped embryo stage, the hypostase cells are dead and thick-walled. The main functions of the hypostase may be to maintain cellular division and perisperm growth without delivering nutrient materials to the perisperm. An evolutionary picture of placentation in Caryophyllales is also presented.