The structure and development of incompletely closed carpels in an apocarpous species, Sagittaria trifolia (Alismataceae)

• Premise of the study: Carpel closure is commonly considered as a key innovation in angiosperms; however, some families continue to exhibit a variety of forms of incomplete carpel closure. The carpel of Sagittaria species contains an unusual structure. In this study, we confirm the closure type of the carpel of Sagittaria trifolia and discuss its development and evolution. • Methods: Scanning electron microscopy and light microscopy of semithin sections were used to observe the development and the mature structure of the carpel. Pollen tube growth in the carpel and seed germination in the achene was also studied. • Key results: During late carpel development, the middle parts of the carpel margins underwent postgenital fusion. However, at maturity the lowest and uppermost parts of the carpel margins remained open. The mature carpel was incompletely closed and contained a secretion‐filled canal, which extended from the stigma to the opening at the carpel base. At that site, pollen tubes could either grow to the ovule or exit the carpel and grow to other carpels. The basal opening also served as an exit point for the seedling to emerge. • Conclusions: Incomplete carpel closure by S. trifolia differs from the closure types recorded in previous studies because two entrances link the ovary in the carpel to the outside environment. This type of carpel closure occurs as a result of the lack of fusion of the carpel margins at the base of the carpel and could improve the seed set and seedling germination of S. trifolia .