Q.Explain the role of tapetum in the formation of pollen-grain wall.

The tapetum is the innermost layer of the microsporangium wall, directly surrounding the sporogenous tissue. It is a nutritive tissue, providing essential nourishment to the developing microspores and subsequently, the pollen grains.

Its role in the formation of the pollen grain wall is critical:

• The tapetum synthesizes and secretes sporopollenin, which is a highly resistant organic material.
• This sporopollenin is deposited to form the exine, the tough, outer layer of the pollen grain wall. The exine provides protection against harsh environmental conditions and enzymatic degradation. …

To understand the tapetum's role, we first need to set the scene within the anther, the part of the stamen that produces pollen. Inside the anther, there are microsporangia, which are essentially pollen sacs. Each microsporangium is surrounded by several wall layers that protect and aid in the development of the pollen grains. These layers, from outside to inside, are the epidermis, endothecium, middle layers, and finally, the tapetum.

The tapetum is the innermost layer of the microsporangial wall, directly surrounding the sporogenous tissue (which will eventually form microspores and then pollen grains). Its cells are distinctive: they typically have dense cytoplasm and often contain more than one nucleus, indicating high metabolic activity. This characteristic morphology hints at its vital function as a nutritive layer.

The pollen grain itself has a robust two-layered wall, essential for its survival and function. These layers are the outer exine and the inner intine. The tapetum contributes significantly to the formation of both:

• Formation of the Exine:

  The exine is the tough, outer layer of the pollen grain wall, known for its intricate patterns and extreme resistance. This remarkable durability is due to a unique biopolymer called sporopollenin. The tapetum is directly involved in the synthesis and secretion of sporopollenin. It produces and releases the precursors and enzymes necessary for the formation of this highly resistant material. These sporopollenin components are then deposited onto the surface of the developing microspores, forming the exine.

  Important

  Sporopollenin is one of the most resistant organic materials known. It can withstand high temperatures, strong acids, and alkalis, and no enzyme known so far can degrade it. This property, facilitated by the tapetum, ensures the pollen grain's survival in harsh environments and its preservation as microfossils.

• Formation of the Intine: …