Fig. 254.—Embryo-sac of Carex præcox: syn synergidæ; kb the oosphere; c the central nucleus; ant the antipodal cells.
At the same time that the embryo is being developed, other changes are taking place in the ovule, especially in the integument which becomes the shell of the seed (testa). The endosperm grows, and the embryo-sac supplants the cells of the nucellus. The seed is now formed, and it consists in its most complete development, as in this instance, of three parts:
(1) The testa of the seed, formed by the enveloping integuments, with the remainder of the tissue of the nucellus lying outside the embryo-sac (the macrosporangium).
(2) The endosperm or prothallium.
(3) The embryo.
Fig. 255.—Diagrammatic longitudinal section through an anatropous ovule shortly after fertilisation; a and i are the two integuments; f the funicle; k the nucellus; S the embryo-sac, with the incipient formation of nutritive-tissue; E the embryo; P the pollen-tube passing through the micropyle (n) to the oosphere.
The reduction in the Angiosperms is carried to the extreme limit. In the embryo-sac (the macrospore) the nucleus by continued division produces a prothallium consisting of primordial cells (Fig. [254]). In the upper end of the embryo-sac (which is nearest the micropyle) are three cells, two of which are termed the “co-operating cells” (synergidæ) and the third is the oosphere. Three others are placed at the opposite end of the embryo-sac and are therefore termed the “antipodal cells.” Finally, a large cell is also formed, which occupies the space between the two groups and whose cell-nucleus, the central definitive nucleus, lies in the centre of the embryo-sac. These primordial cells are the slight remnant of the prothallium. The entire structure of the archegonium, with its neck and canal-cells, has disappeared, and nothing is left but the indispensable oosphere. When the oosphere has been fertilised, and has commenced the cellular divisions which lead to the formation of the embryo (Fig. [255]), the synergidæ and antipodal cells are absorbed, and a cell-formation begins by a new process which emanates from the definitive nucleus and by which a parenchymatous cell-tissue, the nutritive-tissue, arises which may perhaps be considered as homologous with the endosperm of the Gymnosperms. The difference is that the nutritive-tissue of the Angiosperms is formed in two parts with an intervening interruption; the primary nutritive-tissue is first formed, and after fertilisation is absorbed, with the exception of one cell, which continues the development and gives rise to the nutritive-tissue proper, which is formed in the first instance of primordial cells, and later on of a cellular tissue; this nutritive-tissue formed in the embryo-sac is termed “endosperm”; in a few instances[23] a tissue which is derived from the nucellus functions as nutritive-tissue, and is termed “perisperm.” In many plants the seeds, when ripe, contain a very rich nutritive-tissue, in addition to the embryo, for the purpose of its nourishment during germination. These are termed albuminous (endospermous) seeds, in distinction to the ex-albuminous, or those in which the nutritive-tissue is stored in the embryo itself, before it is completely developed, and used for its sustenance.
In addition to the changes which fertilisation produces in the ovule itself, it also gives the impetus to a series of changes in the entire shoot which bears the ovule. The perianth, stamens, and style, generally wither, because the part they play is at an end; the wall of the ovary grows and becomes the wall of the fruit (pericarp). The entire gynœcium of a flower, transformed as a consequence of fertilisation, is termed a fruit. It consists of two parts, the pericarp and the seeds, and according to the nature of the pericarp, the fruit is termed a capsule, nut, berry, or drupe.