The Ultrastructure and Ontogeny of Pollen in Helleborus Foetidus L

Abstract

The ontogeny of the tapetum and Ubisch bodies in Helleborus foetidus L. has been examined at the ultrastructural level, and their development has been closely linked with that of the sporogenous cell and pollen grains. During development the tapetum passes through successive phases of synthesis, maturity and senescence, ending in complete dissolution. During the anabolic phase of growth, precursors of the Ubisch bodies are formed as spheroidal vesicles of medium electron density within the tapetal cytoplasm; they are associated with a zone of radiating ribosomes, which, as development proceeds, can clearly be seen to be situated on strands of endoplasmic reticulum. The callose special wall round the microspores and the tapetal cell wall now disintegrate and the pro-Ubisch bodies are extruded through the cell membrance of the tapetal cells, where they remain on the surface of the anther cavity and soon become irregularly coated with sporopollenin. Deposition of sporopollenin continues on the Ubisch bodies at the same time as upon the exines of the developing pollen grains. In both cases, the later stages of sporopollenin deposition are associated with electron-transparent layers of unit-membrane dimensions appearing in section as white lines of uniform thickness. Continuing deposition of sporopollenin leads to the formation of compound or aggregate Ubisch bodies. It is conjectured that the sporopollenin is synthesized from the compounds of low molecular weight released into the anther loculus by the breakdown of the callose special wall and the tapetal cell wall. The final stages of tapetal autolysis involve the disappearance of all the cell organelles. An attempt is made to relate the findings to those described in other recent studies on Ubisch body formation and to combine them in a common ontogenetic pattern.