Serial Section-Based Three-Dimensional Reconstruction of Anaxagorea (Annonaceae) Carpel Vasculature and Implications for the Morphological Relationship between the Carpel and the Ovule

Elucidating the origin of flowers has been a challenge in botany for a long time. One of the central questions surrounding the origin of flowers is how to interpret the carpel, especially the relationship between the phyllome part (carpel wall) and the ovule. Recently, consensus favors the carpel originating from the fusion of an ovule-bearing part and the phyllome part that subtends it. Considering the carpel is a complex organ, the accurate presentation of the anatomical structure of the carpel is necessary for resolving this question. Anaxagorea is the most basal genus in a primitive angiosperm family, Annonaceae. The conspicuous stipe at the base of each carpel makes it an ideal material for exploring the histological relationships among the receptacle, the carpel, and the ovule. In the present study, floral organogenesis and vasculature were delineated in Anaxagorea luzonensis and Anaxagorea javanica, and a three-dimensional model of the carpel vasculature was reconstructed based on serial sections. The results show that in Anaxagorea, the vasculature in the carpel branches in the form of shoots. The radiosymmetrical vasculature pattern is repeatedly presented in the receptacle, the carpel, and the funiculus of the ovule. This provides anatomical evidence of the composite origin of the carpel.