Variation in the tracheary elements in species of Coryphantha (Cacteae-Cactoideae) with contrasting morphology: the bottleneck model
2016
Terrazas, Teresa | Escamilla-Molina, Rocío | Vázquez-Sánchez, Monserrat
A comparative analysis was conducted of the tracheary elements of the stem and tubercle of six species of Coryphantha. The aims of the study were to identify the micromorphological characters and to determine whether the bottleneck model applied based on the similarity of the diameters of the tracheary elements in the stem and in the tubercle. We collected individuals of C. bumamma, C. clavata, C. erecta, C. glanduligera, C. ottonis, and C. radians, which were species with contrasting morphologies of the stems and tubercles. Sections and macerations were used to prepare the vascular cylinder and the cortical vascular bundles of the tubercle for observation. Our results showed that wide-band tracheids and vessel elements with annular or helical secondary walls predominated in wood. The cortical vascular bundles had primary or both primary and secondary growths, and the tracheary elements had diameters of 10–27 µm, with the pattern and size of the wide-band tracheids more heterogeneous than those of wood. These wider and shorter wide-band tracheids are interpreted as analogous to the dilated tracheids in the veinlets of eudicotyledons. The length and diameter of both tracheary elements (vessel elements and wide-band tracheids) in the tubercles were shorter and narrower than those of the tracheary elements in the vascular cylinder of the stem (P < 0.05). The diameters of the vessel elements in the tubercles were two- or threefold wider than those of the vessel elements in the cortical vascular bundles. Therefore, the results for the species of Coryphantha are consistent with the bottleneck model observed for the diameters of vessel elements for the non-succulent stems of other eudicots.
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