Fig. 250.—I Pollen-grains of Cupressus; at the top is seen one prothallium-cell. II Germinating; c pollen-tube; a the extine; b the intine.
2. The Macrospores. The prothallium in Salvinia and Marsilia is still rather large, green, and capable of the independent assimilation of carbon. It projects more or less from the macrospore and bears (in Marsilia only one, in Salvinia several) archegonia, which however are embedded to a greater degree in the prothallium, and are more reduced than the archegonia of the true Ferns and Horsetails (Figs. [215], [216]). The prothallium is still more reduced in Isoëtes and Selaginella; partly because it is smaller and is in a higher degree enclosed in the spore, it also contains less chlorophyll, or is entirely without chlorophyll, and in consequence incapable of independent existence, whilst the number of archegonia is less; and partly because the archegonia are themselves reduced, the cells of the neck are fewer and embedded to the level of the surface of the prothallium without any, or with only a very slight projection (Figs. [235], [236]).—Finally, the prothallium with its archegonia begins to develope in Selaginella while the macrospore is still within its sporangium, and before it is set free from the mother-plant. After the spores are set free and germination has commenced, the spore-wall ruptures and the prothallium is exposed.
Fig. 251.—Longitudinal section of ovule of Abies canadensis. Inside the integument (i) is seen the nucellus, n; m the micropyle. In the interior of the nucellus is seen an oval mass of cells, the endosperm, and at its top two archegonia, c. The ovule is turned in such a way that the micropyle points upwards, but usually it turns downward in the Abietineæ.
Fig. 252.—The apex of the nucellus (n) of an ovule of Abies: l long-shaped cells which guide the pollen-tube; s the wall of the macrospore (embryo-sac); h the neck-cells of the archegonium; k the ventral canal-cell; and c the central cell (oosphere). The archegonia of the Cryptogams should be compared with this (see pages [181], [208], [216]).
The GYMNOSPERMS go still further. The macrospore (embryo-sac) germinates and forms internally a cellular tissue, designated in former times by the name of albumen (endosperm), which is homologous with the prothallium. It always remains enclosed in the embryo-sac, and is a parenchymatous mass containing a large supply of nourishment. In the upper part of the endosperm a number of archegonia are developed which are in the main constructed in the same manner as those in the Cryptogams, but are still more reduced, the neck consisting only of 4, 2, or 1 cell (Figs. [251], [252]). The ventral canal-cell is also formed, in the majority, as a small portion cut off from the large central cell just beneath the neck; the larger remaining portion becomes the oosphere. When the pollen-tube has passed down to the oosphere (Fig. [253]) and fertilisation has been effected, the oospore commences a cell-formation, the final result of which is the formation of an embryo (the asexual generation) which is provided with a thinner, lower end, termed the suspensor. The embryo is forced more or less into the endosperm in which it may rest for a longer or shorter time, and generally is developed to such an extent that it has a distinct primary-root (radicle) and stem (plumule) with one or more embryo-leaves (cotyledons).
When the oosphere has been fertilised its nucleus sinks down to its lower end, and by repeated division into two, forms four cells lying in one plane (Fig. [253], see base of the left archegonium). Three tiers of cells are now formed by transverse division of these four. It is the intermediate one of these which elongates and forms the suspensor, or four suspensors, if they separate from each other, which push the lowermost four cells deep down into the endosperm. It is from these four lower cells that the embryo (or four embryos when the suspensors separate) is developed, but never more than one embryo attains full development. As several archegonia are contained in one and the same ovule, all of which are capable of forming embryos, there is the possibility that several embryos may be developed in a seed (polyembryony), but usually only one embryo attains perfect development.
Fig. 253.—The apex of the nucellus (n) of Abies in longitudinal section: c, c the oospheres of the two archegonia; the embryo-formation has commenced at the bottom of the left archegonium; s wall of the macrospore; p pollen-grains; r pollen-tubes.