Abstract
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.
Highlights
Since Darwin’s time, the elucidation of the origin and evolution of flowering plants has been a primary goal of plant science [1]
Observations of the continuous changes in vasculature from the receptacle to the carpel revealed that (1) all the carpel bundles were only connected with the central stele of the receptacle, (2) vascular bundles at both the carpel stipe and the ovule/placenta are in a radiosymmetrical pattern, (3) the young concentric bundles develop into a ring-arranged bundle complex with carpel maturation, and (4) all the radiosymmetric vasculatures in the carpel were fed by a larger radiosymmetric bundle system
The radiosymmetric vasculature patterns in the carpel of Anaxagorea are repeatedly presented in the pedicel, the receptacle, the base of the carpel, and the placenta
Summary
Since Darwin’s time, the elucidation of the origin and evolution of flowering plants has been a primary goal of plant science [1]. In the origin of flowers, the emergence of carpels was a key innovation. In the evolutionary origin of angiosperms, the emergence of carpel was the first step, followed by double fertilization and the emergence of flowers [2]. The carpel protectively encloses the ovules and sets angiosperms apart from other seed plants, which develop exposed ovules [3,4,5]. How to explain the relationship between carpel and ovule is of significance to the origin of flowers. Many studies have attempted to discuss the relationship between angiosperm carpel and ovule from phylogeny, molecular biology, and ontogeny perspectives [13,14,15,16,17,18,19]. Considering the carpel is a complex organ, more accurate methods are required for the description of the actual anatomical structure of carpels, besides discontinuous sections and line drawings
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