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 the roots to the shoot apex.

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)

Hawaiian ti (Cordyline sp.) & hala (Pandanus) have secondary growth as well.

Pandanus (hala)Windward Oahu

Hawaiian ti (Cordyline sp.)