After fertilization, several changes take place in the floral parts up to the formation of the seed. 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, to form the 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 .
Primary Endosperm Nucleus undergoes several mitotic divisions without cell wall formation thus a free nuclear condition exists in the endosperm. Examples: Coccinia, Capsella and Arachis
Primary endosperm nucleus divides into 2 nuclei and it is immediately followed by wall formation. Subsequent divisions also follow cell wall formation. Examples: Adoxa, Helianthus and Scoparia
Primary Endosperm Nucleus moves towards base of embryo sac and divides into two nuclei. Cell wall formation takes place leading to the formation of a large micropylar and small chalazal chamber. The nucleus of the micropylar chamber undergoes several free nuclear division whereas that of 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.
The endosperm with irregularity and unevenness in its surface forms ruminate endosperm. Examples :Areca catechu, Passiflora and Myristica
Functions of endosperm:
• 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.
Development of Dicot embryo
The Stages involved in the development of Dicot embryo (Capsella bursa-pastoris – Onagrad or crucifer type) is given in Figure 1.22. The embryo develops at micropylar end of embryo sac. The zygote undergoes transverse division to form upper or terminal cell and lower or basal cell. Further divisions in the zygote during the development lead to the formation of embryo. Embryo undergoes globular, heart shaped stages before reaching a mature stage. The mature embryo has a radicle, two cotyledons and a plumule.