620. Female prothallium of the pine.—To study this we must make careful longitudinal sections through the ovule (better made with the aid of a microtome). Such a section is shown in [fig. 358]. The outer layer of tissue, which at the upper end (point where the scale is attached to the axis of the cone) stands free, is the ovular coat, or integument. Within this integument, near the upper end, there is a cone-shaped mass of tissue. This mass of tissue is the nucellus, or the macrosporangium proper. In the lower part of the nucellus in [fig. 356] can be seen a rounded mass of “spongy tissue” (spt), which is a special nourishing tissue of the nucellus, or sporangium, around the macrospore. Within this can be seen an axile row of three cells (an: m). The lowest one, which is larger than the other two, is the macrospore. Sometimes there are four of these cells in the axile row. This axile row of three or four cells is formed by the two successive divisions of a mother cell in the nucellus. So it would appear that these three or four cells are all spores.
Only one of them, however, the lower one, develops; the others are disorganized and disappear. The nucleus of the macrospore now divides several times to form several free nuclei in the now enlarging cavity, much as the nucleus of the macrospore in Selaginella and Isoetes divides within the spore. The development thus far takes place during the first summer, and now with the approach of winter the very young female prothallium goes into rest about the stage shown in [fig. 358]. The conical portion of the nucellus which lies above is the nucellar cap.
Fig. 357.
Pollen grains of pine. One of them germinating. p¹ and p², the two disintegrated prothallial cells, = sterile part of male gametophyte; a.c., central cell of antheridium; v.n., vegetative nucleus or tube nucleus of the single-wall cell of antheridium; s.g., starch grains. (After Ferguson.)
Fig. 358.
Section of ovule of white pine. int, integument; pc, pollen chamber; pt, pollen tube; n, nucleus; m, macrospore cavity.
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], p¹ and p², 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.
