Development of the Tapetum in Pinus banksiana Preceding Sporogenesis

Abstract

Early in sporangial ontogeny, the cells destined to become the sporogenous and tapetal tissue differentiate in a strikingly similar manner. The first conspicuous step in development is a contraction of the protoplasts, beginning at the centre of the microsporangium and moving radially to its periphery. Similar development of the two groups of cells ceases as the callose wall is formed around the meiocytes. At this point the original walls investing the tapetal cells become gelatinous, and lipid synthesis commences within the contracted protoplasts. The bulk of this lipid is secreted from the cells, and becomes lodged in the loculus, either as globules in the expanded radial and inner cell walls, or as a continuous layer on the inside of the middle lamella separating the loculus from the wall of the microsporangium. This lipoidal layer forms the basement of a peritapetal membrane, believed to serve as a container for the fluid in which the young sporogenous cells are immersed. Examination of protein levels and ribosome numbers in the tapetal cells reveals that protein synthesis proceeds at an increasing rate throughout the development preceding meiosis, but apparently ceases as the pollen mother cells become enveloped in callose.