MEGASPOROGENESIS

                            MEGASPOROGENESIS


The process of formation of megaspore from megaspore mother cell by meiotic division is known as megasporogenesis. This process takes place in ovule.



(1) Structure of ovule:


Ovule is considered to be an integumented megasporangium. The ovule consists of the stalk and the body. The stalk is called funicle. One end of the funicle is attached to placenta and the other end to the body of the ovule. The point of attachment of funicle with the body is called hilum. Sometimes funicle gets fused with the body of the ovule one side and forms a ridge known as raphe. The body of the ovule shows two ends: the basal end, often called the chalazal end and the upper end is called micropylar end. The main body of the ovule is covered with one or two envelopes called integuments. These leave an opening at the top of the ovule called micropyle. The integuments enclose a large parenchymatous tissue known as nucellus.


(2) Development of female gametophyte (Megagametogenesis):


The process of development of female gametophyte or embryo sac from megaspore is called megagametogenesis.


(i) Monosporic type (Polygonum): In this type, only one megaspore situated towards chalazal end takes part in the development of embryo sac.


(ii) Bisporic type: In this type two megaspore nuclei take part in embryo sac formation.


(iii) Tetrasporic type: This type of embryo sac develops from four megaspore nuclei.