Which wall slows the spread of decay toward the pith?

The correct answer highlights the significance of the wall in limiting the progression of decay toward the pith of a tree. In tree biology, as decay organisms invade the wood, trees react by forming specialized structures called barriers or walls to contain the spread of decay.

In the context of this question, Wall 2 is particularly vital because it represents the layer that is most effective at halting the advance of decay towards the tree's vital central core—the pith. Often, this wall can be composed of denser or more chemically resilient wood fibers that can inhibit pathogen growth and decay processes. Trees have evolved various defense mechanisms, including the ability to compartmentalize decay, which allows them to isolate infected areas and protect healthy tissues.

Additionally, the characteristics of the other walls, while important in understanding the overall structure and health of the tree, do not possess the same capacity to effectively slow decay's movement towards the pith as Wall 2. This specific focus on Wall 2's role in providing a robust barrier against decay is what establishes it as the optimal choice in this scenario.