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Volume 80, 1952
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Embryo Sac

From the slides examined it seems that the chalazal megaspore* enlarges to form the functional megaspore, while the remaining megaspores and surrounding nucellar cells degenerate and form a densely staining tissue around the sac. At the two-nucleate stage the sac becomes vacuolate, with one nucleus at the micropyle and one at the chalazal end (fig. 12). At the four-nucleate stage a pair of nuclei surrounded by cytoplasm is situated at either end of the sac with a marked vacuole in between (fig. 13). With further division of the nuclei the sac becomes much enlarged, the nucellar tissue disappears, and the sac comes in direct contact with the inner layer of the integument.

The final organization of the sac is normal, with an egg apparatus of two densely staining synergids, and the egg, all situated at the micropylar end, three antipodal cells, and a large secondary nucleus in the centre of the sac (fig. 14). Later this nucleus takes up a position on the sac wall. In N. fusca and N. truncata the egg apparatus, before fertilization took place, moved down the wall of the sac a short distance from the micropylar end (fig. 18).

A distinctive feature is that at the antipodal end of the sac the basal elongated nucellar cells remain jutting into the sac as a “column” of tissue (fig. 17). The antipodal cells are situated on top of this (fig. 19). In N. menziesii ovules this column gradually disappears and at the time of, or just after, fertilization is not to be seen, but in N. fusca ovules it remains for some time as a cone-shaped body of cells (fig. 18). This type of structure was first described and named “Postament”—an old German word for base, pedestal, or stand—by von Westermaier (1890). Dahlgren (1940) gives an account of it in a number of families. Of the two possible methods of origin, a resistant remnant of the nucellar tissue or a special growth of tissue from the chalazal end of the embryo sac, Dahlgren favours the former. As far as could be ascertained in Nothofagus, the Postament consists of a resistant part of the nucellus. Dahlgren mentions the elongation sometimes of the cells of the Postament and advances this and other evidence in support of a contention that it serves the function of conducting tissue. In Nothofagus this is possibly so, because the basal cells of the Postament were connected with the vascular tissue leading from placenta to ovule and cytoplasm surrounded the Postament.

A flat-topped Postament is also seen present in the ovules of N. obliqua and N. procera (fig. 17). In both these species some of the basal cells of the column have very thick, dark-staining walls.

* The terminology of Maheshwari (1948) is used.

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Fig. 1—N. menziesii Longitudinal section of a pistillate flower at the time of pollination Ovules just beginning to form × 50. Fig. 2—N. obliqua Longitudinal section of pistillate flower at the time of pollination Ovules just beginning to form × 50 Fig. 3—N. truncata Part of transverse section of an ovary at the time of pollen shed. First stage in the appearance of ovules is the organization of two rows of cells in the centre of the ovary × 500 Figs 4–7—N. procera Transverse sections of a flower shortly after pollen shed. × 100 (4) Near the base of the ovary, two young ovules in one loculus (5) Near the middle of the ovary where the perianth begins to separate from the ovary. (6) Above the ovules and at the base of the style with the perianth completely separated from the ovary (7) Halfway along the style. showing the first signs of division into two lobes. Fig. 8—N. fusca Longitudinal section of developing nut showing the ovules becoming anatropous, four weeks after pollination × 20. f. fibre layer in the wall of the nut Fig. 9—N. menziesii. Longitudinal section of nut five weeks after pollination × 20.

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Figs 10–16—N. menziesii (10) Nucellus with megaspore mother cell × 150 (11) Nucellus with linear tetrad of 4 megaspores with degenerated nucellar cells around them × 150 (12) Two-nucleate embryo sac showing large vacuole between the nuclei, six weeks after pollination × 200. (13) Embryo sac at the 4-nucleate stage, five weeks after pollination × 300 (15) Five weeks after pollination Embryo sac showing the fusion of the polar nuclei. The apical part of the nucellar tissue is still present × 400. (16) Five weeks after pollination Embryo sac fully developed but not fully enlarged Egg apparatus, secondary nucleus and antipodal cells showing. × 400

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Fig. 17—N. obliqua eight weeks after pollination Embryo sac showing the Postament of tissue at the chalazal end of the sac with thick-walled cells at the base Fig. 18—N. fusca Ovule with a fully developed sac Ten weeks after pollination. The secondary nucleus has moved to the wall of the sac The Postament of tissue juts into the sac at the chalazal end × 100 Figs 19–23—N. obliqua (19) Fully developed embryo sac showing the egg apparatus, secondary nucleus and 2 antipodal cells adjacent to the chalazal column of tissue × 70 (20) Second division of zygote. The first division must have been transverse, basal cell has divided longitudinally. × 1,200 (21) Proembryo at a several-cell stage with a large basal cell and dividing terminal cells × 600 (20) Proembryo at an obovate stage × 600 (23) Spherical stage of the proembryo with light-staining basal cells. × 400.

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Fig. 24—N. menziesii Embryo sac with spherical proembryo at the micropylar end, secondary nucleus on the sac wall and one antipodal cell at the chalazal end × 50 Fig. 25—N. truncata Developing embryo showing two cotyledons and radicle Thin-walled endosperm does not fill the sac. × 50 Figs 26 and 27—N. solandri (2G) Transverse section of a mature nut (27) Details of the mature nut wall, inner fibre layer and outer parenchymatous cells Fig. 28—N. solandri Almost fully developed cotyledons folded inside the ovule wall From a triquetious nut about six weeks before the nut-fall

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Fig. 29—N. obliqua embryo sac with flat-topped Postament and thick-walled cells at the base. The secondary nucleus can be clearly seen. × 250.
Fig. 30—N. fusca embryo sac with Postament and secondary nucleus on the wall of the sac. × 250.

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Describing the embryo-sac formation in F. sylvatica ovules, Benson (1894) stated that central strands of the nucellus arising from a layer of sub-epidermal cells maintained regular periclinal division. They stood out from the surrounding tissue and formed the “sporogenous tissue.” Some of the cells of the central core elongated without dividing, and it was suggested that one of these long cells formed the embryo sac. The embryo sac had in any case an inconspicuous beginning and “there is no other mark by which to distinguish it from its sister cells above and below it in an axile row than the formation of the two nuclei which take up their position at either end of the cell.”

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