THE DEVELOPMENTAL ANATOMY OF STEGNOSPERMA (PHYTOLACCACEAE)

• Main Author: Horak, Karl Emanuel
• Other Authors: Mason, Charles T. Jr.
• Language: en_US
• Published: The University of Arizona. 1981
• Subjects: Stegnosperma.; Phytolaccaceae.
• Online Access: http://hdl.handle.net/10150/281969

Most dicotyledonous species with secondary growth possess a single cylindrical vascular cambium. However, anomalous types are not uncommon. The various classification systems for anomalous patterns of secondary thickening are compared and the distribution of successive cambria in about 20 families of 13 orders is summarized. The distribution of this feature within the Phytolaccaceae is detailed including new observations showing successive cambia in Stegnosperma. Serial sections of different developmental stages of plant axes permitted precise three-dimensional reconstruction of the vascular system. Macerations of increments adjacent to the plant of transverse sectioning allowed comparison of xylem-cell dimensions between successive increments, different parts of individual plants, and all three species. Comparisons between species reveal that S. cubense has secondary xylem with the largest diameter pores, longest vessel elements, longest fiber-tracheids, and largest diameter fiber-tracheid pits. S. watsonii has the greatest number of pores per square mm, smallest fiber-tracheid diameter, and the largest ratio of fiber-tracheid to vessel-element length. Pore diameter, fiber-tracheid length, and the ratio of fiber tracheid to vessel-element length were less in small stems than in large stems and less in large stems than in roots. Pores per square mm, fiber-tracheid diameter, and vessel-element length were more or less constant between small stems and roots. Most xylary cell dimensions remain constant between successive increments of wood at a given level. The anomalous cambia arise in the conjunctive tissue and function bidirectionally. They are first evident as dedifferentiated parenchyma and then as small fascicular areas. Tangential expansion of existing regions produces larger arcs and rings of vascular tissue. Radial and tangential interconnections between vascular bundles are common in Stegnosperma. Dye movement traced through the axis confirms the presence of a complex network. At the upper and lower limits of a given ring or arc of anomalous vascular tissue, a radial connection occurs with the adjacent inner increment. The resulting pattern is a series of concentric cylinders of cambial zones, which anastomose with inner layers at their top and bottom. The normal increment of secondary thickening is the innermost cylinder and each successive layer forms a shorter but wider sheath around the previous layers. The apical organization of Stegnosperma consists of four zones: tunica, central mother cells, peripheral, and pith-rib meristem. A cambium-like or transition zone was not observed. The primary shoot vasculature is described as open, lacking direct vascular connections between sympodia, and followed a 2/5 phyllotaxy. Patterns of differentiation in shoot apices are detailed. The organization of the root apex as determined from seedling studies is analyzed by the constituent histogens. The vasculature of seedlings was found to follow the common dicotyledonous pattern. The initiation of secondary growth in the hypocotyl and its acropetal development is also typical of other dicotyledons. Secondary growth ascends the shoot helically according to phyllotaxy. The vascular cambium descends in the root, first appearing opposite the phloem and then in the pericycle. These observations are discussed in light of their differences with earlier studies, compared with other Phytolaccaceae, and examined with regard to the physiological function of the anomaly. Systematic implications are summarized and a tentative phyllogeny of the Phytolaccaceae is given.