Final answer:
Lignin form the insoluble layer in woody tissues, providing structural support to plants and making the tissue difficult to digest due to toxic breakdown products.
Step-by-step explanation:
The insoluble layer of aromatic alcohols found primarily in woody tissues is formed by lignins. Lignin is a complex, phenolic polymer that is synthesized in the secondary cell walls of plant cells, only after the cells have ceased growing. Due to its rigidity and resistance to degradation, lignin provides structural support, enabling plants to grow tall and withstand the compressive forces of gravity. This polymer not only makes tissues woody but also contributes to the difficulty in digesting plant material, as its breakdown products are phenolic compounds that are toxic to many organisms.