Fig. 7

Section through the stigma, style, and ovary showing pollen tube growth/path through the transmitting tract of the pistil and the degree of ultrastructure preservation by HPF, PF, and ChF. The PT path through the stigma surface (a–c), the stylar TT (d–f), and the ovary TT (g–i). (a–f) HPF/Epon sections and (g–i) PF/Epon sections. b and c are increasing magnifications of (a) showing PTs that have grown through the apoplastic matrix of stigmatic papillar cells. d shows very densely packed cells in the style. Here, PTs need to overcome the mechanical resistance from the tightly packed surrounding cells in a fashion that requires more invasive growth potentials than elsewhere in the TT. Higher magnifications show PTs displacing TT cells and growing through the intercellular matrix (e) or the apoplastic space (f). (g) shows more loosely distributed cells within the ovary TT, where spent PT and degenerating TT cells are also visible. Higher magnifications show that cells are lodged in an extensive extracellular matrix, in which PTs can be seen to have cell walls with layers of different electron densities (h and i). (j) shows two PTs within a spent TT cell, one of the tubes is apparently degenerating while the other seems to have an active cytoplasm and fills up the larger volume. The extracellular matrix is clearly more preserved in the cryo-fixed section (h and i) than in the ChF section (j). (k and l) show transverse sections of in vitro-grown PTs contained within another with a region where their two cell walls are merged (mcw). The ultrastructure details presented of mcw appear different in the ChF (extensive fibrous, k), PF (almost electron transparent, l) and HPF (fibrous/dark gray, m) sections. l also reveals the artefactual loss of the primary (outer) cell wall layer in PF sections. Higher magnifications show anti-callose gold particle labelling (indicated by arrowheads) revealing callose distribution/extent in the unmerged cell wall of the outer PT (n), the unmerged cell wall of the inner PT (o), and the merged cell walls (p) corresponding to the marked regions in (m), respectively. pg = pollen grain, pt. = pollen tube, cw = cell wall, aps = apoplastic space, icm = intercellular matrix, ecm = extracellular matrix, dc = spent/degenerating cells, mcw = merged cell walls. Scale bar (a, d, g) = 20 μm; (b, c, e, f, h–m) = 2 μm; (n–p) = 200 nm