Fertilization

the nucellus, a corresponding number of tubes may be formed. However, typically only one tube reaches the female gametophyte. The tip of a pollen tube forces itself between the neck-cells and then ruptures, discharging the two male gametes, the tube nucleus, and stalk cell into the cytoplasm of the egg cell. Usually the smaller male gamete, tube nucleus, and stalk cell degenerate. Initially, the nuclei of the male and female gametes are separated by their nuclear

Figure 6.10. Reproductive cycle in the cli- envelopes. Following syngamy, the loid complement of chromosomes. These two sets of chromosomes soon become arranged along the equatorial region of a separate or common spindle, but with several poles. In the late metaphase a single spindle is observed. The interval between pollination and fertilization is about 30 days. The complete reproductive cycle of Picea extends over a two-year period (Fig. 6.10).

6.5.4. Embryogeny

The division of the zygote nucleus yields two nuclei, each of which promptly divides forming four free nuclei in middle of the egg cytoplasm. Soon afterward, the nuclei move to the lower end of the archegonium, where a third division occurs accompanied by cell wall formation (Fig. 6.11). The first wall formed is transverse to the proembryo axis. The eight nuclei lie in two tiers. The proembryo consists of a lower tier of four cells with walls, and an upper tier, devoid of walls adjacent to the egg cytoplasm. The lower tier is the primary embryonic tier. it gives rise to the three tiers, namely, the primary and secondary suspensor tiers, and embryonic tier. Divisions of the embryonic tier are simultaneous with the elongation of the secondary (proper) suspensor, developing into the embryonal tubes (Fig. 6.12). Synchronization of these processes is a characteristic feature for Picea and not for Abies. Proembryo development is complete about 40 days after pollination.

During the early stage of primary suspensor elongation, the apical tier of the embryonic cells gives rise to several additional cell tiers (E1, E2, etc., Fig. 6.12.), owing to a series of transverse divisions. These cells elongate and serve to push the growing embryo farther into the corrosion cavity of the gametophyte. The embryo system is supplied food materials stored in the female gametophyte cells, which are digested. The female gametophyte being a nutritive haploid tissue is sometimes incorrectly called endosperm.

mate conditions of Poland nucleus of the fertilized egg contains one paternal and one maternal hap-

Figure 6.11. A schematic representation of proembryo development (Dogra 1970)

PU - upper tier; PE - lower tier; St - primary suspensor tier; E - primary embryo tier;

Figure 6.12. Development of the embryo; schematic (Dogra 1970)

s - secondary suspensor; e - embryo; Rt - root tip; C - cotyledons; E i 2 ... t - derivatives of the primary suspensor tier (St on Fig. 6.11)

Six or seven weeks after pollination, the embryo is spherical in shape, and two weeks later the cotyledon primordia are already visible. In mid August the embryo is fully developed. During embryo growth, the integument differentiates into a seed coat. Simple polyembryony can occur in spruce; however, two embryos are rarely observed in ripe seeds. Simple polyembryony refers to the phenomenon in which each of the fertilized eggs of a single gametophyte produces separate heterozygotic embryos. Cleavage polyembryony, typical for pines, does not occur in spruce.

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