Pollen development in Epilobium (Onagraceae): Late microspore stages (a review)

Development of the exine of Epilobium angustifolium L. [=Chamaenerion angustifolium (L.) Holub] prior to microspore mitosis and formation of the intine results in a mature-like pollen grain. Several aspects of this process are particularly analyzed here. They are (1) Development of viscin threads is especially pronounced during vacuolated stages. They are found to be structurally the same as columellae (tufts). (2) In early microspore stages endexine tufts extend outward into the ectexine. Later there is development of a foot layer and a white line between endexine and foot layer. After formation of the foot layer and the adjacent white line there is no further interchange between tufts in the endexine and ectexine. We find that endexine tufts have a radial orientation, at least throughout microspore stages. (3) There are extensive bulges in the endexine that aid in the transport of material through the exine of microspores as indicated by the accumulation of vesicles and globules in the cytoplasm under bulge regions. Light microscopical observations of living microspores showed that bulges extend like eskers for many microns and change positions both laterally and radially with closure of a bulge region. The bulges close up apparently due to a reduction in energy, e.g., experimental temperature reduction, and the endexine returns to its usual solid appearing morphology. The bulges and especially their recovery attest to the great plasticity of the exine. During and after each bulge event there are white lines at either side of each bulge. Our interpretation is that white lines are the result of broken and repaired tufts. Since white lines are resistant to acetolysis and are not contrasted by stains we suggest that they are composed of sporopollenin. (4) The tectum forms during mid- to late microspore stages. Supratectal knobs are formed at least in large part by the core subunits but their surfaces become smooth during this microspore period. After acetolysis and potassium permanganate exposure only 10–15 nm subunits remain in the knobs showing that the smooth surface has been due to secondarily deposited (receptor independent) sporopollenin that is not resistant to the permanganate exposure. Secondarily deposited sporopollenin that coats exines like a spackle or stucco, smoothing and enlarging the exine surface without greatly changing its form, is deposited during the late microspore stages. (5) Evidence is shown for the transfer of a tracer from the loculus through both the ectexine and endexine.