Development of heterocolpate pollen in Myosotis scorpioides L.(Cynoglosseae, Boraginaceae)

Abstract

Structure of mature pollen of Myosotis scorpioides and sporoderm development were examined using light, scanning and transmission electron microscopy. Pollen grains of M. scorpioides are heterocolpate with three colpori alternating with three pseudocolpi (pseudoapertures). Functional pseudoapertures take part in harmomegathy but not in pollen germination. Formation of colpori and pseudocolpi starts simultaneously. Structural difference between the aperture and pseudoaperture becomes clearly noticeable at the early free microspore stage when the endexine is initiated: at the colpori (at the area of ora) the endexine is composed of only a thin loose layer, at the pseudocolpi, the endexine is homogeneous and thick. Later, on the late free microspore stage, the difference appears in the structure of the intine: at the area of ora, the intine is three times thicker comparing with the pseudoapertures and mesocolpia. Besides zonal pseudoapertures, the triangular poroid areas are present at both poles of M. scorpioides pollen. Their structure and developmental pathway are similar to the zonal pseudoapertures. Pseudocolpi and polar pseudoapertures should be considered as the structures originated de novo specialised for harmomegathy.