Page:The Oak.djvu/146

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128
THE OAK.

above diagram will serve to represent the lateral buds which develop male inflorescences only, or if we suppose the three bracts F, a, and β away, it would serve for a terminal bud.

Each single female flower stands in the axil of a minute scale on the floral axis, as said, and its general structure has been described. When the pollen grains have been dusted on to the trifid stigma, about the end of May or beginning of June, each grain germinates and sends a minute tube down the style, and this pollen-tube soon reaches the cavity of the ovary, and its end becomes applied to one of the ovules. While the pollen-tube is descending the style, the ovules have arisen as minute cellular outgrowths from the angles of the three chambers of the ovary (Fig. 34, d). There are two in each chamber. Each ovule is at first a mere solid lump of cells (nucellus), which curves and becomes enveloped in two thin investing layers, called integuments, as shown in the figures A-D (in Fig. 35). Inside the solid nucellus, n, of the ovule there soon arises a small cavity filled with nucleated protoplasm, and termed the embryo-sac, e, because the embryo is to be developed in it.

This embryo-sac contains, among other structures, a minute, nucleated, naked mass of protoplasm, called the oösphore, or egg-cell. The pollen-tube has carried down in its apex also a nucleated mass of protoplasm, and it passes this over into the egg-cell in the embryo-sac; the union of the nucleus from the pollen-tube with the nucleus of the egg-cell constitutes the act of fertiliza-