Microsporogenesis in Taxus baccata L.: The Development of the archaesporium

An ultrastructural and cytochemical investigation of the development of the mierosporangium of Taxus baccata reveals many resemblances to comparable events in other land plants, but also several distinctive features. During early development the archaesporium contains groups of degenerate cells which ultimately disappear, presumably by resorption. Scattered elsewhere in the cells of the archaesporium are highly ordered bodies, possibly crystal-like formations of tubulin. The tapetum differentiates from the outer layer of the archaesporium. Mitoses coincidentally cease to be detectable within the sporangium and the sporogenous cells become recognizable about three weeks before their nuclei enter meiosis. Although, as in other gymnosperms investigated, the early development of the archaesporial cells is not synchronized, there is a tendency for synchrony to be acquired by the sporogenous cells and diad formation takes place more or less simultaneously throughout the sporangium. Measurements by microfluorometry of RNA levels during development of the archaesporium show a progressive rise in the RNA content of the cells up to the beginning of meiosis.