Fig. 4

The ovule and seed development of Gastrodia nantoensis. A The archesporial cell is differentiating into a megasporocyte. Cell division (arrow) near the ovule's chalazal end signifies the integument tissue's initiation. Scale bar = 20 μm. B The second meiotic division results in the formation of two megaspores of unequal size. At the same time, the initiation of integument tissue is becoming visible (arrow). Scale bar = 20 μm. C A longitudinal section through a mature embryo sac showing the egg apparatus (*). The integument tissue (arrow) has completely enclosed the embryo sac at this stage. Starch grains (arrowhead) start to accumulate in the integument tissue. Scale bar = 20 μm. D At the time of fertilization, the pollen tube (arrowhead) penetrates the embryo sac, and the integument tissue elongates further and becomes the seed coat (arrow). Scale bar = 20 μm. E Light micrograph showing a proembryo with a suspensor cell (S). Scale bar = 20 μm. F A longitudinal section through a developing globular embryo. At this stage, the nucellus (arrowhead) gradually compresses, and large starch grains (double arrowheads) are abundant in the cells of the embryo proper and the seed coat. The suspensor cell (S). Scale bar = 20 μm. G As the seed approaches maturity, starch grains (double arrowhead) are prominent within the embryo cells, and the suspensor cell (S) has reduced its size and begins to degenerate. At this stage, the nucellus (arrowhead) has compressed and degenerated. Scale bar = 20 μm. H At maturity, the embryo has smaller cells near the chalazal end and larger cells in the micropylar end. The suspensor has degenerated at this stage, and the embryo proper is enveloped by a shriveled seed coat (arrow). Scale bar = 20 μm. I Nile red staining fluorescence micrograph of a mature seed at the same stage as that seen in Fig. 4I. The seed coat (arrow) and the surface wall (arrowhead) of the embryo proper react positively to the stain. Scale bar = 20 μm