Step 1: Understanding the Problem:
A typical microsporangium (which develops into a pollen sac) in an angiosperm is surrounded by four distinct concentric wall layers. These layers function in protection, dehiscence, and nourishment.
Step 2: Approach and Formula:
Recall the histological structure of the microsporangium wall from the outermost protective periphery to the innermost nutritive center.
Step 3: Detailed Explanation:
Moving from the outside to the inside, the layers are arranged as follows:
1. Epidermis: The outermost, single-layered protective boundary.
2. Endothecium: Situated right below the epidermis, featuring fibrous thickenings that assist in the dehiscence of the anther.
3. Middle layers: 1 to 3 layers of short-lived cells lying inside the endothecium.
4. Tapetum: The innermost layer consisting of large, specialized cells that nourish the developing microspores (pollen grains).
Therefore, the correct sequential order is Epidermis $\rightarrow$ Endothecium $\rightarrow$ Middle layer $\rightarrow$ Tapetum.
Step 4: Final Answer:
The correct option is (B).