Structure of Microsporangium (or Pollen sac)
In a transverse section a typical microsporangium appears circular in outline. It has mainly two parts- anther wall and pollen chamber.
1. Epidermis : It is the outermost layer. It is only one cell in thickness.
2. Endothecium : It is situated below the epidermis. Mostly there is a single layer of endothecium, but sometimes it is two layered. The cells of endothecium are tangentially elongated and attain their maximum development when pollen .grains mature. The radial and inner tangential walls of these cells have characteristics fibrous thickening bonds. Due to these bonds, the endothecium cells are hygroscopic in nature, hence, play an important role in the dehiscence of anthers.
3. Middle layer : It lies inside the endothecium and made-up of 1-3 layers of thin-walled cells. The cells of these layers survive for short-time. The middle layer may store food material in some species which is utilized by pollen mother cells.
4. Tapetum : This is the innermost layer of cells with dense cytoplasm. Tapetum nourishes the developing pollen grains and secrete both enzymes and hormones. It contains two types of cells :
- Amoeboid or Periplasmodial cells
- Secretory or Glandular cells
Sporogenous Tissue : The primary sporogenous cell undergo several mitotic divisions to form microspore mother cells. A massive mass of sporogenous cells inside anther walls, called sporogenous tissue. Many cells of this tissue undergo meiosis to form microspores.
