=624. Pollen tube and sperm cells.=—While the endosperm (female prothallium) and archegonia are developing the pollen tube continues its growth down through the nucellar cap, as shown in fig. 359. At the same time the two cells which were formed in the pollen grain (antheridium) from the central cell move down into the tube. One of these is the “generative” cell, or “body” cell, and the other is called the stalk cell, though it is more properly a sterile half of the central cell. The nucleus of the generative cell, about the time the archegonium is mature, divides to form two nuclei, which are the sperm nuclei, and the one in advance is the larger, though it is much smaller than the egg nucleus. =625. Fertilization.=—Very soon after the archegonia are mature (early in June in the northern United States) the pollen tube grows through into the archegonium and empties the two sperm nuclei, the vegetative nucleus and the stalk cell, into the protoplasm of the large egg. The larger of the two sperm nuclei at once comes in contact with the very large egg nucleus and sinks down into a depression of the same, as shown in fig. 361. These two nuclei, in the pines, do not fuse into a resting nucleus, but at once organize the nuclear figure for the first division of the embryo. Two nuclei are thus formed, and these divide to form four nuclei which sink to the bottom of the archegonium and there organize the embryo which pushes its way into the endosperm from which it derives its food (fig. 362). [Illustration: Fig. 361. Archegonium of white pine at stage of fertilization, _en_, egg nucleus; _spn_, sperm nucleus in conjugation with it; _nb_, nutritive bodies in cytoplasm of large egg; _cpt_, cavity of pollen tube; _vn_, vegetative nucleus or tube nucleus; _stc_, stalk cell; _spn_, second sperm nucleus: _pr_, portion of prothallium or endosperm; _sg_, starch grains in pollen tube. The sheath of jacket cells of the archegonium is not shown. (After Ferguson.)] =626. Homology of the parts of the female cone.=—Opinions are divided as to the homology of the parts of the female cone of the pine. Some consider the entire cone to be homologous with a flower of the angiosperms. The entire scale according to this view is a carpel, or sporophyll, which is divided into the cover scale and the ovuliferous scale. This division of the sporophyll is considered similar to that which we have in isoetes, where the sporophyll has a ligule above the sporangium, or as in ophioglossum, where the leaf is divided into a fertile and a sterile portion.