Reinventing the trunk

Aloes and yuccas have evolved even further than their Monocotyledon (Flowering plants whose seeds have one cotyledon.) cousins in seeking a tree-like shape. They have literally reinvented the trunk's secondary growth and don't need the cambium (Cells responsible for the increase in thickness of the trunk, stems and roots of many plants.) that dicotyledons rely on.

Over time, they add girth by building new xylem and phloem ducts inside their stems to transport water and nutrients. Just as new spectators at a crowded rock concert have to squeeze in and make room for themselves, these new vascular tissues (Specialized conducting tissue in plants.) make room for themselves by pushing aside the existing tissues. As a result, the trunk adds girth so that it can support the plant's tree-like growth.