Post fertilization changes
The process of fertilization and triple fusion result in the formation of a diploid zygote and a triploid primary endosperm nucleus respectively. Diploid zygote is the first cell of the sporophytic generation and it develops into a mature embryo. The triploid primary endosperm nucleus forms the endosperm.
Development of embryo:
The process of development of mature embryo from zygote is known as embryogeny. After fertilization, the primary endosperm nucleus begins to divide to give rise to a multicellular endosperm. The fertilised egg grows into an embryo. The growth of the embryo slows down because the zygote enters a period of dormancy after fertilization. The period of dormancy may vary from few hours to several days. The endosperm grows by means of food supplied by the parent plant and in return it nourishes the embryo.
The first division of zygote is transverse. It produces a basal cell towards the micropyle and an apical cell towards the chalaza. The basal cell divides by a transverse wall and the apical cell divides by a longitudinal wall. This results in the formation of four-celled T-shaped proembryo.
The two cells formed by the basal cell divide by transverse division to produce 6 to 10 cells long suspensor. Of these, the upper most cell is swollen and forms a vesicular cell while the lower most cell of the suspensor is called hypophysis. Hypophysis forms part of the radicle and root cap.
The growth of the suspensor pushes the growing embryo into the endosperm where abdundant food is available. The suspensor ultimately disintegrates.
The two cells formed by the apical cell divide by another vertical wall at right angles to the first division forming a four-celled quadrant. All the four cells of the quadrant divide by a transverse wall producing an eight-celled embryo, called octant. These cells are arranged in two tiers of four each. Four cells of the lower tier give rise to the plumule and cotyledons while the upper four cells produce hypocotyl.
Each of the eight cells of the octant divides periclinally forming a 16-celled embryo known as globular embryo. It has eight peripheral and eight central cells. The peripheral cells divide anticlinally and form a single-layered dermatogen. It eventually gives rise to epidermis. The derivatives of the inner eight cells differentiates into periblem and plerome. The periblem forms the ground meristem whereas the plerome gives rise to the procambium of the hypocotyl and cotyledons.
The globular embryo becomes heart shaped due to the differentiation of cotyledons at its distal end. The mature embryo consists of an axis bearing two cotyledons. The tip of this axis above the cotyledonary node, has the plumule which represents the apex of the embryonic shoot. The plumule usually consists of an apical meristem together with several embryonic leaves. On germination, plumule forms the shoot. The basal tapering end of the axis is the radicle which develops into primary root when the seeds germinates.
Example of embryogeny in dicotyledons is that of Capsella bursa, a member of Brassicaceae.
The embryonal axis in the monocotyledons has a single cotyledon, therefore, development of monocotyledons embryo is different from that of diocotyledons. Example of monocotyledons embryo is Luzula.
The zygote in monocotyledons divides transversely into a basal cell and an apical cell. The basal cell divides by a transverse wall into two cells. One of the cells divides once again to form two cells, which develops into suspensor and a part of root cap. The other cell contributes to the remaining part of the root cap and a part of the radicle.
The apical cell divides by two vertical walls, at right angle to each other which results in the formation of a quadrant. Cells of the quadrant divides periclinally differentiating into the peripheral cells and the inner group of cells. Repeated division in both peripheral and central groups of cells results in the formation of scutellum and a part of coleoptile.
The remaining part of the coleoptile develops from the cells derived from the basal cell. Monocots have only a single cotyledon which is situated on one side of the embryonal axis. The part of the embryonal axis below the level of scutellum is the radicle. It is enclosed in an undifferentiated sheath, called coleorrhiza.
Endosperm developes from the primary endosperm nucleus which is formed as a result of the fusion of two polar nuclei with one of the male gametes. It is a nutritive tissue that supplies food material to the growing embryo. In angiosperms, the endosperm is generally triploid.
Types of endosperm:
Depending upon the mode of its formation, angiospermic endosperm is of three types – nuclear, cellular and helobial.
Nuclear endosperm: In this type of endosperm, primary endosperm nucleus divides freely into large number of nuclei without any immediate cell wall formation. The endosperm, thus, consists of large number of nuclei, arranged mostly towards the peripheral region of the embryo sac. At a later stage, wall formation begins from periphery and progresses towards the center. It is mostly found in the members of Polypetalae (e.g., Arachis hypogea, Primula, Citrus, Mangifera, Malva, Glycine max etc.).
Cellular endosperm: In this type of endosperm, division of primary endosperm nucleus is immediately followed by wall formation. The first division results in the formation of two equal-sized chambers – chalazal and micropylar chambers. The subsequent divisions are followed by regular cell wall formation. Cellular endosperm is found in the members of gamopetalae, e.g., Adoxa, Scutellaria, Verbascum etc.
Helobial endosperm: This type of endosperm is an intermediate between cellular and nuclear types. The division of primary endosperm nucleus is followed by wall formation which results in the formation of two chambers – micropylar and chalazal chambers. Free nucleus division occurs in both the chambers. These divisions are not followed by wall formation. Such type of endosperm is characteristics of Helobiae, an order of monocotyledons, e.g., Asphodelus.