Details of Developmental Anatomy and Embryogeny
The description will be based on the stages examined in N. menziesii, since the most complete series was obtained for that species.
Anthesis of the flowers on the one tree occurred over a short period. A few weeks before anthesis the ovary consisted of a mass of undifferentiated parenchyma, but at anthesis itself the beginning of loculus and ovule formation was to be seen. The first stages of ovule formation were as described in the section relating to the general sequence of seed development.
It is noted that Langdon (1939), describing the early stages in the development of Fagus americana pistillate flowers writes, “… the portion of the floral axis inclosed by the carpels produces a short, thick columnar structure, at the tip of which ovule development is initiated. Centripetal growth of the incurved carpel margins takes place concurrently with development of the placental axis, and as cushion-like masses of parenchyma they envelop the placentae and ovules.” No such sequence occurred in the species of Nothofagus examined, the loculi and ovules developing from an undifferentiated mass of parenchyma. It is possible that the undifferentiated mass might have been formed at an earlier stage by invagination; a sufficient series of material was not examined to determine this. But not long before anthesis this tissue was completely uniform.
Benson (1894) gives no account of the formation of ovules in Fagus sylvatica, but she did find that they were formed about a month before pollination and that the rudiments of the inner integuments were to be seen at that time. The nucellus and archesporial cells had also begun to form. Langdon found that in Fagus americana, “the formation of ovules takes place before the opening of the buds and the emergence of the pistillate heads.”
In the flowers of the Nothofagus species examined the development of the ovules was all of the same pattern, and at anthesis ovules were usually just beginning to appear, though in occasional flowers the ovary was still undifferentiated. Flowers examined from local Fagus sylvatica had ovules well developed at anthesis.
About four to five weeks after anthesis ovules were well developed and anatropous, and showed a single integument surrounding the nucellus (figs. 8 and 9). Single integuments only were seen in all Nothofagus material. According to Benson (loc. cit., t. LXVIII) there are two integuments in F. sylvatica, but I am unable to confirm this.
The fully developed nucellus is six to seven cells wide with an outer layer of regular cells and the inner cells in distinct parallel rows. The central core, about two cells wide, consists of more elongated cells, while at the base of this core, at the junction of the nucellus and integument, the cells are greatly elongated and more densely staining (fig. 11). The integument forms a narrow micropyle opening towards the base of the style.
The megaspore mother cell arises from one of the top cells of the central row of elongated cells (fig. 10). In one nucellus there was a suggestion of a linear tetrad of four megaspores arising from the megaspore mother cell (fig. 11). These were large cells, while the nucellar cells around them had degenerated.