Describe the structure of a typical angiospermic microsporangium with a diagram. Also, explain the function of each part.

Microsporangium is a structure located in the anther of the stamen that contains microspores. A typical anther consists of four microsporangia. Each microsporangium consists of four wall layers, i.e. the outer epidermis, endothecium, middle layers, and tapetum. Inside the wall layer, a mass of diploid sporogenous tissue is there. This sporogenous tissue gives rise to pollen mother cells and undergoes meiosis to form haploid microspores. These microspores give rise to pollen grains. Epidermis: It is the outermost covering of the anther. It provides protection and helps in the dehiscence of anther. Endothecium: It is the second layer of the wall of the anther inner to the epidermis that consists of large cells. In a typical anther, the endothelial cells develop a fibrous thickening. Thus, it is also called a fibrous layer. It performs the function of protection of the developing pollen grains and helps in the dehiscence of the anther that releases the pollen grains. Middle layers: Inner to the endothecium 2-3 layers of the middle layers are present. These layers develop fibrous thickening. It also provides protection to the developing microspores and helps in their dehiscence. Tapetum: It is the innermost layer of the wall layers that consists of more than one nucleus. This layer provides nourishment to the developing microspores.

What is triple fusion? What are the nuclei involved in triple fusion?

Triple fusion is the fusion of two polar nuclei with one male gamete to form a triploid primary endosperm nucleus (PEN). PEN develops to form triploid endosperm. The endosperm provides all the nutrition to the developing embryo. Triple fusion takes place in the embryo sac that is the female gametophyte of the flowering plants. Triple fusion involves one haploid male gamete and two haploid polar nuclei or a diploid secondary nucleus'(formed by the fusion of polar nuclei).