The innermost wall layer of microsporangium is the __________

Answer

**Step-by-Step Solution:**

1. **Understanding Microsporangium**: The microsporangium is a structure found in the anthers of flowering plants where pollen grains are produced. It is crucial to know the layers that make up the microsporangium to answer the question accurately.

2. **Identifying the Layers**: The microsporangium consists of several layers, including the outer epidermis, middle layer, and the innermost layer. The innermost wall layer is specifically what we need to identify.

3. **Innermost Layer**: The innermost layer of the microsporangium is known as the tapetum. This layer is essential for the development of pollen grains.

4. **Function of Tapetum**: The tapetum plays a vital role in nourishing the developing microspores (pollen grains). It is characterized by being multinucleated, which allows it to provide adequate nutrition to the microspores during their development.

5. **Conclusion**: Therefore, the correct answer to the question is that the innermost wall layer of the microsporangium is the **tapetum**.