Essential Parts of Flower: Androecium - Transverse Section of Mature Anther (Microsporangium)

T.S. of Mature anther (Microsporangium):

• Microsporangium is the sporangial structure which contains microspores which are the pollen sacs giving rise to male gametes in an angiosperm.
• In a transverse section, a typical microsporangium appears near circular in outline. 
• In the young stage, the microsporangium is surrounded by a multi-layered wall consisting of the epidermis, endothecium and the tapetum. The centre is occupied by sporogenous tissue.

• The transverse section of mature anther reveals the presence of anther cavity surrounded by an anther wall.
• The anther is a bilobed structure with each lobe containing four pollen sacs or microsporangia.
• Each lobe has 2 thecae structure that is joined together by a non-sporogenous tissue i.e., connective tissues are called the dithecous anther.
• Often a longitudinal groove runs lengthwise separating the theca.
• Each lobe of the anther has two vertical pollen chambers which are called pollen sacs or pollen chambers. Each pollen sac contains a number of pollen grains.
• A typical anther has four pollen sacs called tetrasporangiate.
• The four pollen sacs (microsporangia) are aligned at the four corners of a dithecous anther is the microsporangium, two in each lobe.  
• The T.S. of Mature anther is given in below figure.

1. Anther wall:

• Anther wall is of four layers of cells - Epidermis, Endothecium, Middle layers, and Tapetum.

1. Epidermis:  It is the outermost layer of the anther wall. It is only one cell in thickness and protective in function. The cells undergo repeated anticlinal divisions to cope up with the rapidly enlarging internal tissues.
2. Endothecium: Endothecium is generally a single layer of radially elongated cells found below the epidermis. The cells of the endothecium are having fibrous thickening arising from their inner walls. These fibrous thickenings are of α- cellulose (alpha). The cells along the junction of the two sporangia of an anther lobe lack these thickenings. This region is called stomium. This region along with the hygroscopic nature of endothecium helps in the dehiscence of anther at maturity.
3. Middle layers: Two to three layers of cells next to endothecium constitute middle layers which are variable. Ranges from 1-6 layers. They are generally ephemeral. The middle layer degenerates at the time of maturity of the anther.  
4. Tapetum: It is the innermost layer of anther wall and attains its maximum development at the tetrad stage of microsporogenesis. It is derived partly from the peripheral wall layer and partly from the connective tissue of the anther lining the anther locule. Thus, the tapetum is dual in origin. It nourishes the developing sporogenous tissue, microspore mother cells and microspores. The cells of the tapetum may remain uninucleate or may contain more than one nucleus or the nucleus may become polyploid. It also contributes to the wall materials, sporopollenin, pollenkitt, tryphine, and number of proteins that control incompatibility reactions. Tapetum also controls the fertility or sterility of the microspores or pollen grains.
   
   Two types of tapetum:

1. Secretory tapetum (parietal/glandular/cellular): The cells of this type of tapetum remain in contact with the anther wall throughout and cellular integrity and nourishes the developing microspores.
   
2. Invasive tapetum (amoeboid/ plasmodial/periplasmodial): The cells of this type of tapetum separate from the wall and move freely in the pollen chamber. The protoplast of all tapetal cells coalesces to form a periplasmodium.
   
   Functions of Tapetum:

• It supplies nutrition to the developing microspores.
  
• It contributes sporopollenin through ubisch bodies and thus plays an important role in pollen wall formation.
• The pollenkitt material is contributed by tapetal cells and is later transferred to the pollen surface.
• Exine proteins responsible for ‘rejection reaction’ of the stigma are present in the cavities of the exine. These proteins are derived from tapetal cells.

2. Anther Cavity: The anther cavity is filled with microspores in young stages or with pollen grains at maturity. The meiotic division of microspore mother cells give rise to microspores which are haploid in nature.

3. Connective: It is the column of sterile tissue surrounded by the anther lobe. It possesses vascular tissues. It also contributes to the inner tapetum.