Some monocots (such as palms) are impressively thick and massive, yet botanists maintain that they don't have secondary growth. Why do botanists say this? How can it get so big without secondary growth?
The vascular system is different in monocots and dicots. In dicots the vascular tissues are arranged in concentric circles; one of these rings is meristematic cells (undifferentiated cells that can differentiate into any cell type). This ring of meristem tissue is called the vascular cambium and is where secondary growth occurs - xylem grows inwards and phloem grows outwards.
For palms specifically:
Parenchyma cells are type of cells found in plant ground tissue, which makes up the bulk of plant mass.