Serial Section-Based 3D Reconstruction of Anaxagorea (Annonaceae) Carpel Vasculature and Implications on Integrated Axial-Foliar Origin of Angiosperm Carpels

The carpel is the basic unit of the gynoecium in angiosperms and one of the most important morphological features distinguishing angiosperms from gymnosperms; therefore, carpel origin is of great significance in angiosperm phylogenetic origin. Recent consensus favors the interpretation that the carpel originates from the fusion of an ovule-bearing axis and the phyllome that subtends it. It has been confirmed by morphological and molecular evidence that foliar homologs are involved in carpel evolution. Consequently, if axial homologs can be traced in the carpel, it would more likely be derived from an integrated axial-foliar structure. This study aimed to reveal the axial structures in carpels by analyzing the continuous changes in vasculature from the receptacle to the carpels and ovules. Anaxagorea is the most basal genus in a primitive angiosperm family, Annonaceae. The conspicuous carpel stipe at the base of each carpel makes it an ideal material for exploring the possible axial homologous structure in the carpel. In this study, floral organogenesis and the topological vasculature structure were delineated in Anaxagorea luzonensis and Anaxagorea javanica, and a 3D-model of the carpel vasculature was reconstructed based on the serial sections. The results show that (1) at the flowering stage, the number of vascular bundles entering each Anaxagorea carpel from the receptacle was significantly higher than three, arranged in a radiosymmetric pattern, and forming a basal ring at the base of each carpel. (2) All carpel bundles were only connected with the central stele. (3) At the slightly upper part of the carpel, all lateral bundles from the basal ring were reorganized into two groups, each forming a lateral bundle complex below the respective placenta. Bundles in each lateral bundle complex were also ringed. (4) The ovule bundles were composed of non-adjacent bundles in the lateral bundle complex. The results of the present study suggest that the circular arrangement of bundles in the receptacle, carpel stipe, and placenta of Anaxagorea are in line with the composite axial-foliar nature of the carpel, and provide a morphological basis for further research on the origin of the carpel.

The anatomy of the leaf of red pine, Pinus resinosa. II. Vascular tissues

The anatomy and ultrastructure of the endodermis and enclosed vascular tissues of the midregion of the mature secondary needle-leaf of Pinus resinosa are described. Within the uniseriate endodermis are two vascular traces surrounded by transfusion tissue. The endodermal cells have differentially thickened walls which lack Casparian strips but are lignified. Plasmodesmata traversing pit regions form a symplastic interconnection between mesophyll, endodermal, and transfusion parenchyma cells. In the lateral bundle region plasmodesmata extend this symplastic pathway across the cell walls of subjacent transfusion parenchyma and richly protoplasmic albuminous cells to the metaphloem. Four distinct types of transfusion tracheids have been defined on the basis of cell shape and location. Transfusion tracheids in the lateral bundle regions form direct radial connections between metaxylem and endodermis.