OVULE FORMATION Mr. J.L. Terlabie

OVULE FORMATION The ovule consists of a central parenchymatous core (nucellus) surrounded by a varying number of rather massive parenchymatous covering or integument. Each integument begins as collar around the base of the nucellus and grows upwards to form a close fitting slit. The micropyle is the pore where the two (2) leaves of the integuments sometimes meet or fail to meet.

Ovule Formation contd. Majority of ovules have two (2) integuments; Inner integument and Outer integument Most of them have a stalk called funiculus. Nucellus is the macrosporangium just as the pollen sac is the microsporangium. The ovule when matured transforms into the seed and the integuments at maturity transforms into the seed coat or testa.

Ovule Formation contd. The life or the emergence of the ovule appears as a hemispherical projection of cells from the placenta. Very early in the developmental stages of the ovule, a cell with a prominent nucleus and a dense cytoplasm appear just below the apex of the nucellus, the nucellar apex. This cell divides mitotically into two (2); Outer tapetal and Inner megaspore mother cell

Ovule Formation contd. The megaspore mother cell divides mitotically and form a file of haploid megaspore which are arranged in a linear tetrad of four megaspore. Three (3) of the cells usually disintegrates and the one (1) further form the micropyler end survives. The functional megaspore begins to enlarge at the expense of the nucellus. The rapid increase in size is due to the protoplasm and partly due to vacuolation (formation of vacuoles)

Ovule Formation contd. The nucleus of the megaspore divides mitotically, each of the resulting nuclei also divide mitotically. The four nuclei that are formed at this stage divides mitotically forming eight (8). At the end of the 3rd division, the eight (8) are arranged into two (2)groups of four; One group near the micropyler end and Other group close to the chalazal end.

Ovule Formation contd. One nucleus from each end migrates to the center of the eight (8) nucleated cell. These two nuclei are known as polar nuclei. The three (3) remaining at the micropyler end become organized as the egg apparatus consisting of the egg cell and the two (2) cellular synergids. The three (3) left at the chalazal end develops into antipodals.

Ovule Formation contd. Cell walls are formed around all the nuclei. The cell containing the polar nuclei remain binucleate. The eight (8) nuclei, the seven (7) cell structure is the female matured gametophyte (the embryo sac). The life of flowering plant begins with a double fertilization; which is peculiar or unique to the angiosperms (flowering plants)

Ovule Formation contd. The pollen tube grows around the ovule and gets into the embryo sac through the micropyle. The pollen tube contains two nuclei, the generative and the vegetative. The generative nucleus divides into two (2) forming the two male gamete nuclei. One of them effects fertilization by fusing with the haploid egg nucleus. The fertilized ovum develops into the zygote which is diploid (2n) and this later develops into the embryo.

Ovule Formation contd. The 2nd male gamete fuses with polar nuclei of the embryo sac to give rise to a triploid - Primary endosperm nucleus. This later transforms into the endosperm of the developing seed. After fertilization , the zygote remains inactive whiles the primary endosperm nucleus undergoes rapid mitotic division until an extensive material, the endosperm is developed.

Ovule Formation contd. The time at which the endosperm may be formed before the zygote commences its activity differs greatly among plants. The early stages of the embryo development are essentially the same in the dicot and monocot. Formation of the embryo begins with the division of the zygote. In most flowering plants, the first division of the zygote is transverse with regards to the long axis of the zygote.

Ovule Formation contd. The first division establishes the polarity of the embryo; the upper chalazal pole and the lower micropyler pole. The chalazal end is the main seat for the development of the embryo. The lower micropyler produces a stalk-like suspensor which anchors the embryo to the micropyler end. From an orderly progression of division, the embryo eventually develops into a spherical structure called the embryo proper and the suspensor.

Ovule Formation contd. Before this stage, the embryo is referred to as pro-embryo. Until recently, the suspensor is believed to be the structure that only pushes the developing embryo into the nutritive tissue. However, it is now known that, it absorbs nutrients and also protein is synthesized in the suspensor. The proteins synthesized by the suspensor are utilized by the embryo proper during periods of rapid growth.

Ovule Formation contd. The embryo proper in its initial formation consist of a mass of relatively undifferentiated cells. However, changes in the internal structure of the embryo results in the initial development of the tissue system of the plant. The future epidermis is formed from the protoderm by the periclinal division of the outermost cells of the embryo proper.

Ovule Formation contd. Anticlinal division?? Periclinal division?? The differences in the time of vacuolation and density of the cells within the embryo results in the initiation of the procambium and ground meristem. The highly vacuolated , less dense ground meristem (cortex) gives rise to the ground tissues which surrounds the less vacuolated and denser procambium which is the precursor of the vascular tissue.

Ovule Formation contd. The development of the cotyledon may begin either during or after initiation of the primary meristem. Before the cotyledon development, the monocots and the dicots cannot be distinguished. The stage of the embryo development preceding the cotyledon development is known as the globular stage. With the initiation of the cotyledons the globular stage in the dicots gradually assumes a two(2) lobed form.

Ovule Formation contd. This stage of the embryo development in the dicot is referred to as the heart stage. Because embryo in the monocot forms only one(1) cotyledon, they do not have the heart shape stage instead, the embryo becomes cylindrical in shape. As the embryo develops, the cotyledon and the axis elongates and the primary meristem extends along with them.

Ovule Formation contd. This stage is referred to as the ‘’torpedo stage’’. The embryo during its elongation either remains straight or curved. In the monocots the single cotyledon becomes so large in comparison with the embryo. It becomes the most dominating structure. The suspensor is gradually crushed with the enlargement of the embryo.

Ovule Formation contd. Cell division takes place throughout the young sporophyte as the embryo develops. However, as the embryo develops, the addition of new cells gradually becomes restricted to the apical meristem of the root and the shoot. In dicots, the apical meristem of the shoot emerges between the two(2) cotyledons. In monocots ,it arises on either side of the cotyledon and is completely surrounded by a sheath-like extension from the base of the cotyledon.

Ovule Formation contd. The apical meristem of the shoot or the root are of great importance since these tissues are the source of virtually all the new cells responsible for the development of the seedling and the adult plant from the embryo. Throughout the period of embryo formation, there is a continual flow of nutrients between the parent plant and the tissues of the ovule resulting in a massive buildup of food reserves within the endosperm, the perisperm and the cotyledon of the developing seed.

Ovule Formation contd. Eventually, the stalk or the funiculus connecting the ovule and the ovary wall separates from the ovary and the ovule becomes a nutritionally closed system. Finally, the seed becomes desiccated as it looses water to the surrounding environment. The seed coat then becomes hardened encasing the embryo and the stored food in a protective armour.