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Why don't monocotyledonous plants increase in girth like that of dicotyledonous?

The entire shoot system, no matter how large or small, owes its beginnings to a small region of the plant called the shoot apical meristem.

An apical meristem is a region of high cell division (lots and lots of mitosis) that contributes to the extension of the plant.
The width of a plant, or its girth, is called secondary growth and it arises from the lateral meristems in stems and roots. As with apical meristems, lateral meristems are regions of high cell division activity.The lateral meristems that produce secondary growth are called cambiums, which just means a tissue layer that adds to plant growth. The two important ones for secondary growth are the vascular cambium and the cork cambium. The vascular cambium produces more vascular tissue (xylem and phloem), which provide support for the shoot system in addition to transporting water and nutrients. Because the xylem and phloem that come from the vascular cambium replace the original (primary) xylem and phloem, and add to the width of the plant, they are called secondary xylem and secondary phloem.
Monocots don’t have secondary growth.


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