Synchronous pollen mitosis and the formation of the generative cell in massulate orchids

Abstract

In orchd species forming microspores in aggregates, the pollen mitotic division occurs synchronously in all cells of each massula, as do the earlier meiotic divisions. The synchroneity can be traced to the persistence of cytoplasmic connexions between the cells, from the meiotic prophiase until pollen maturation. The mitosis giving the generative nucleus is asymmetrical, and the spindle is truncated at one side, where the microtubules converge towards an amorphuos polar structure lying against the spore wall. The cell plate formed after pollen mitosis is hemispherical, and its curved growth is related to a radial spread of the microtubules of the phragmoplast after the telophase of the division. The plate itself, and the wall derived from it, is identifiable as callose by its fluorescence properties. In the later development of the gametophyte, growth of the callose wall continues until the originally hemispherical generative cell becomes separated from the spore wall. The cell then assumes a spherical shape and moves to the vicinity of the vegetative nucleus, where it remains freely suspended, bathed in the cytoplasm of the vegetative cell but insulated from it by the completely ensheathing callose wall.