Question

Explain the structure of megasporangium.

Answer 1

Hint: Plants consist of male reproductive organs named microsporangium and female reproductive organs named as megasporangium. Microsporangium produce male gametes or microspores and the megasporangium produce female gametes or megaspores.

Complete answer:
In flowering plants gynoecium represent the female reproductive parts and it consists of a pistil that can be single (monocarpellary) or multiple (multicarpellary) these pistils can be fused (syncarpous) in the case of multicarpellary condition or free (apocarpous). The pistil is further divided into stigma, style and ovary. Stigma is the site where pollen grains land and it elongates into stigma and its basal part is bulged into ovary. Ovary consists of the ovarian cavity inside which the placenta is present. Ovules arise from the placenta which are known as megasporangium.
Megasporangium is a small structure which is attached to the placenta with the help of a stalk-like structure known as funicle. The funicle fuses with the body of megasporangium at the hilum representing the junction between the funicle and megasporangium. Microsporangium is covered by one or more integuments which cover the nucellus (a mass of cells) leaving the tip which forms the micropylar end that aids in the entry of the pollen tube. The basal part of the microsporangium is the chalazal end which is present opposite to the micropylar end. Nucellus is a mass of cells present in the megasporangium and has abundant reserve food material. Embryo sac also known as the female gametophyte and is present inside the nucellus. Mostly a single embryo sac is present within a megasporangium that produces megaspores by the process of megasporogenesis.

Note: Megasporangium are borne mostly on specialised structures like cones in pteridophytes, sori in ferns, flowers in gymnosperms. Sometimes leaf-like structures bear megasporangium and are known as megasporophylls and those bearing microsporangium are known as microsporophylls.