Development Leading to Seed Formation

The trimerous nuts of Nothofagus develop three and the dimerous nuts two loculi, each loculus having two pendulous ovules which finally become anatropous. The loculi of an individual nut are not clearly separated and might strictly be considered a single loculus. The young ovary consists of an undifferentiated mass of parenchyma. The first appearance of change is the enlargement about the centre of the ovary, and in both transverse and longitudinal sections, of two adjacent rows of cells—in a bimerous flower (fig. 3). A space develops between these two rows. Then, in transverse section, young ovules appear at each end of the rows of cells and on either side of the space (figs. 4 and 5). The ovules then grow by quick division of the cells, forming bulges. In longitudinal section each end of the space, or in other words the developing locules, appears half-moon shaped with the bulge to the outside of the ovary and the developing ovules on the axile side (figs. 1 and 2).

About the time of pollination the slits only, or the first stages in the formation of the ovules are showing.

Ovules develop slowly after pollination. They become pendulous from the roof of the loculus or the top of the placenta, which consists of tissue remaining between the developed loculi. Before fertilization, which occurs some nine to ten weeks after pollination, the ovules become anatropous and develop a single integument (figs. 8 and 9). After fertilization all except one ovule in a nut abort; it is not known whether the ovule which does develop is the first one to be fertilized or not. The divisions between the three loculi finally break down as the single seed develops. This seed grows to occupy the whole internal space of the nut, thus forming an exalbuminous one-seeded nut, with the vascular strands of the placenta lying along one side. The aborted ovules shrivel and almost disappear.