Question

Trace the development of megaspore mother cell upto the formation of a mature embryo sac in a flowering plant.

Answer 1

• A typical pistil consists of an ovary, style, and stigma. Inside the ovary, there are one or more ovules Or megasporangia. the main body of the ovule consists of the parenchymatous tissue, the nucellus with one or 2 covering or integuments. the integuments surround the nucellus all around except at the apex leaving a narrow passage called micropyle. the ovule at first rises as a primordium on the placenta in the cavity of the ovary. owing to the meristematic activity of the cells of ovular primordial, the protuberances become prominent and constitute the nucellus. the initials of the 2 integuments arise at the base of the nucellus and surround it all around the apex.

• The process of megasporogenesis begins with the differentiation of primary archesporial cell in the nucellar hypodermis .the primary archesporial cell divides to form outer primary cell wall and inner primary sporogenous cell. this sporogenous cell directly behaves as the megaspore mother cell and gives rise to 4 potential megaspores by meiosis .the 4 megaspores are arranged in a linear tetrad. usually, one megaspore of the tetrad becomes functional and develops further . the functional megaspore is the first cell of the female gametophyte. it divides by 3 successive divisions to form an eight nucleate female gametophyte or embryo sac. 3 of the 4 nuclei nearest the micropyle differentiate into the egg apparatus. the egg apparatus consists of 3 cells: the egg cell and 2 flanking cells, called synergids .three of the 4 at the opposite end of the developing embryo sac, which is called the chalazal pole are called the antipodal cells. the 2 remaining nuclei one from each pole migrate to the middle of the embryo sac and are called the polar nuclei or secondary nucleus ( 2n). this is a monosporic type of development generally referred to as the Polygonum type because it was first found in Polygonum species. about 70% of the angiosperms have this type of development.