We often teach undergraduates that monocots do not have secondary growth. However, when you look at many Palms it becomes obvious that they must have some way to reinforce their stems and conduct water from their roots to their shoot apices.

Furthermore, Palm leaves can be extremely complex & gigantic. Consequently, they need a good water transport system to prevent desiccation. Coconut Palm and other arborescent monocots have a unique form of secondary growth in regard to Vascular Tissues in stems.

They also have a Periderm on Stems & Roots.

Cocos nucifera (Coconut Palm) has Secondary Growth

Hawaiian ki (Cordyline) & hala (Pandanus) have secondary growth.