Which of the following layer of anther wall shows fibrous...

The anther wall consists of four layers: epidermis, endothecium, middle layers, and tapetum.

The endothecium is the sub-epidermal layer of the anther wall. The cells of this layer develop fibrous thickenings of $\alpha$-cellulose on their inner and radial walls.

These fibrous thickenings are hygroscopic in nature and play a crucial role in the dehiscence of the anther to release pollen grains.

Answer: Endothecium