=621. Male prothallia.=—By the time the pollen is mature the male prothallium is already partly formed. In fig. 343 we can see two well-formed cells. Two other cells are formed earlier, but they become so flattened that it is difficult to make them out when the pollen grain is mature. These are shown in fig. 357, _p_¹ and _p_², and they are the only sterile cells of the male prothallium in the pines. The large cell is the antheridium wall, its nucleus _v.n._ in fig. 357. The smaller cell, _a.c._, is the central cell of the antheridium. During the summer and autumn the male prothallium makes some farther growth, but this is slow. The larger cell, called the vegetative cell or tube cell, which is in reality the wall of the antheridium, elongates by the formation of a tube, forming a sac, known as the pollen tube. It is either simple or branched. It grows down into the tissue of the nucellus, and at a stage represented in fig. 358, winter overtakes it and it rests. At this time the central cell has divided into two cells, and the vegetative nucleus is in the pollen tube. [Illustration: Fig. 359. Section of nucellus and endosperm of white pine. The inner layer of cells of the integument shown just outside of nucellus; _arch_, archegonium; _en_, egg nucleus. In the nucellar cap are shown three pollen tubes. _vn_, vegetative nucleus or tube nucleus; _stc_, stalk cell; _spn_, sperm nuclei, the larger one in advance is the one which unites with the egg nucleus. The archegonia are in the endosperm or female gametophyte. (After Ferguson.)] =622. The endosperm.=—In the following spring growth of all these parts continues. The nuclei in the macrospore divide to form more, and eventually cell walls are formed between them making a distinct tissue, known as the _endosperm_. This endosperm continues to grow until a large part of the nucellus is consumed for food. [Illustration: Fig. 360. Last division of the egg in the white pine cutting off the ventral canal cell at the apex of the archegonium. _End_, endosperm; _Arch_, archegonium.] =623. Female prothallium and archegonia.=—The endosperm is the female prothallium. This is very evident from the fact that several archegonia are developed in it usually on the side toward the pollen chamber. The archegonia are sexual organs, and since the sexual organs are developed on the gametophyte, therefore, the endosperm is the female gametophyte, or prothallium. In fig. 359 are represented two archegonia in the endosperm and the pollen tubes are growing down through the nucellus. The archegonia are quite large, the wall is a sheath or jacket of cells which encloses the very large egg which has a large nucleus in the center.