Post Fertilization structure and events
After fertilization, several changes take place in the floral parts up to the formation of the seed (Figure 1.20).
The events after fertilization (endosperm, embryo development, formation of seed, fruits) are called post fertilization changes.
The primary endosperm nucleus (PEN) divides immediately after fertilization but before the zygote starts to divide, into an endosperm. The primary endosperm nucleus is the result of triple fusion (two polar nuclei and one sperm nucleus) and thus has 3n number of chromosomes. It is a nutritive tissue and regulatory structure that nourishes the developing embryo. Depending upon the mode of development three types of endosperm are recognized in angiosperms. They are nuclear endosperm, cellular endosperm and helobial endosperm (Figure 1.21).
Nuclear endosperm: In the nuclear type, the Primary endosperm nucleus (PEN) divides into two without any wall formation. The subsequent division of these two nuclei are free nuclear so that the endosperm consists of only free nuclei and cytoplasm around them. The nuclei may either remain free or may become separate by walls in later stages. Examples: Coccinia, Capsella and Arachis.
Cellular endosperm: In this type the primary endosperm nucleus(PEN) divides into 2 nuclei which are immediately followed by a wall formation. Subsequent divisions are also followed by walls. Examples: Adoxa, Helianthus and Scoparia.
Helobial endosperm: In helobial type, the primary endosperm nucleus (PEN) moves towards the base of the embryo sac where it divides into two nuclei. These 2 nuclei are separated by a wall to form a large micropylar chamber and a small chalazal chamber. The nucleus of the micropylar chamber undergoes several free nuclear divisions whereas that of the chalazal chamber may or may not divide. Examples : Hydrilla and Vallisneria
The endosperms may either be completely consumed by the developing embryo or it may persist in the mature seeds. Those seeds without endosperms are called non- endospermous or ex- albuminous seeds. Examples: Pea, Groundnut and Beans. Those seeds with endosperms are called endospermous or albuminous seeds. The endosperms in these seeds supply nutrition to the embryo during seed germination. Examples: Paddy, Coconut and Castor.
Ruminate endosperm: The endosperm with irregularity and unevenness in its surface forms ruminate endosperm (Example: Areca catechu). The activity of the seed coat or the endosperm itself results in this type of endosperm. The unequal radial elongation of the layer of seed coat results in the rumination of endosperm in Passiflora. In Annonaceae and Aristolochiaceae definite ingrowth or infolding of the seed coat produces ruminate endosperm. The irregular surface of the seed coat makes endosperm ruminate in Myristica.
· It is the nutritive tissue for the developing embryo.
· In majority of angiosperms, the zygote divides only after the development of endosperm.
· Endosperm regulates the precise mode of embryo development.
Another interesting feature of the endosperm is the presence of haustoria. In the case of helobial endosperm the chalazal chamber itself acts as a haustorial structure.
In cellular and nuclear endosperm special structures are produced towards the micropylar, chalazal, both micropylar and chalazal which may be in lateral direction depending on the species. These absorb nutrients from other outer tissue or from ovary tissue and supply them to the growing embryo.
The development of Dicot embryo (Capsella bursa-pastoris) is of Onagrad or crucifer type. The embryo develops at micropylar end of embryo sac.
The Zygote divides by a transverse division forming upper or terminal cell and lower or basal cell. The basal cell divides transversely and the terminal cell divides vertically to form a four celled proembryo. A second vertical division right angle to the first one takes place in terminal cell forming a four celled stage called quadrant. A transverse division in the quadrant results in eight cells arranged in two tiers of four each called octant stage.
The upper tier of four cells of the octant is called epibasal or anterior octant and the lower tier of four cells constitute hypobasal or posterior octants. A periclinal division in the octants results in the formation of 16 celled stage with eight cells in the outer and eight in the inner.
The outer eight cells represent the dermatogen and undergoes anticlinal division to produce epidermis. The inner eight cells divide by vertical and transverse division to form outer layer of periblem which give rise to cortex and a central region of pleurome which forms stele
During the development, the two cells of the basal cell undergoes several transverse division to form a six to ten celled suspensor. The embryo at this stage become globular and the suspensor helps to push the embryo deep into the endosperm. The uppermost cell of the suspensor enlarge to form a haustorium. The lowermost cell of the suspensor is called hypophysis. A transverse division and two vertical division right angle to each other of hypophysis results in the formation of eight cells. The eight cells are arranged in two tiers of four cells each The upper tier give rise to root cap and epidermis. At this stage embryo proper appears heart shaped, cell divisions in the hypocotyl and cotyledon regions of the embryo proper results in elongation. Further development results in curved horse shoe shaped embryo in the embryo sac. The mature embryo has a radicle, two cotyledons and a plumule(Figure 1.22).
The fertilized ovule is called seed and possesses an embryo, endosperm and a protective coat. Seeds may be endospermous (wheat, maize, barley and sunflower) or non endospermous. (Bean, Mango, Orchids and cucurbits).
The mature seeds are attached to the fruit wall by a stalk called funiculus. The funiculus disappears leaving a scar called hilum. Below the hilum a small pore called micropyle is present. It facilitates entry of oxygen and water into the seeds during germination. Each seed has a thick outer covering called seed coat. The seed coat is developed from integuments of the ovule. The outer coat is called testa and is hard whereas the inner coat is thin, membranous and is called tegmen. In Pea plant the tegmen and testa are fused. Two cotyledons laterally attached to the embryonic axis are present. It stores the food materials in pea whereas in other seeds like castor the endosperm contains reserve food and the cotyledons are thin. The portion of embryonal axis projecting beyond the cotyledons is called radicle or embryonic root.
The other end of the axis called embryonic shoot is the plumule. Embryonal axis above the level of cotyledon is called epicotyl whereas the cylindrical region between the level of cotyledon is called hypocotyl(Figure 1.23 a). The epicotyl terminates in plumule whereas the hypocotyl ends in radicle.
The seed of paddy is one seeded and is called Caryopsis. Each seed remains enclosed by a brownish husk which consists of glumes arranged in two rows. The seed coat is a brownish, membranous layer closely adhered to the grain. Endosperm forms the bulk of the grain and is the storage tissue. It is separated from embryo by a definite layer called epithelium. The embryo is small and consists of one shield-shaped cotyledon known as scutellum present towards lateral side of embryonal axis. A short axis with plumule and radicle protected by the root cap is present. The plumule is surrounded by a protective sheath called coleoptile. The radicle including root cap is also covered by a protective sheath called coleorhiza. The scutellum supplies the growing embryo with food material absorbed from the endosperm with the help of the epithelium (Figure 1.23 b).