Pollen Wall Development of Xiphidium coeruleum (Haemodoraceae) and its Systematic Implications

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Abstract
The development of the pollen grain wall in Xiphidium coeruleum (Haemodoraceae) was studied using TEM and cytochemical staining techniques. Microsporocyte ontogeny initiates with the degradation of the cellulosic cell wall and subsequent deposition of a thick callosic cell wall. Following callose deposition, successive meiosis occurs, resulting in a tetragonal tetrad of microspores. during meiosis, the cell walls of the tapetum break down, releasing the syncytial periplasmodium. Irregular non-sporopollenous globular bodies are deposited in this peripheral periplasmodium, which is rich in ER, golgi bodies, vesicles, and characteristic starch plastids. Within the microspore cytoplasm, vesicles, golgi bodies, and plastids are plentiful during the early tetrad stage. At this time the plasma membrane of the microspore develops characteristic evaginations. An extracellular membrane, the ‘white line’, is secreted outside the microspore plasma membrane, followed by callose wall degradation. Bead-like deposits of exine or primexine are deposited at points along the ‘white line’ simultaneously on inner and outer surfaces and opposite the original plasma membrane evaginations. The bead-like exine deposits continue to grow during the release of the microspores and develop into laterally appressed, rod-shaped ektexinous elements having a tangentially oriented commissure, the vestige of the original ‘white line’. The mature intine is two-layered, the outer exintine containing radially oriented vesicular structures, which are apparently derived from plasma membrane extensions. Exine development in Xiphidium is similar to ‘nexine 1’ development in Lilium and may have evolved from an ancestral tectate-columellate condition by the loss of the sexine. Wall development in members of the Zingiberales is strikingly similar to that reported here for the Haemodoraceae—evidence of a possible relationship between the two taxa.