HORTICULTURE

Detailed explanation-1: -Contrary to dicots, monocotyledonous plants’ vascular bundles do not have a layer of meristematic tissue (cambium). Thus, the vascular bundles of monocots are said to be closed since no new cells can grow inside of them, whereas the vascular bundles of dicot plants can.

Detailed explanation-2: -However, secondary growth from vascular cambium is completely absent in the monocot lineage, likely due to a loss of one or multiple crucial genes required for vascular cambium identity, patterning, differentiation, or development early in their evolutionary history.

Detailed explanation-3: -Vascular bundles in monocotyledons are considered as closed because cambium is absent in the vascular system and thus they lack secondary growth in the thickness. Therefore, the correct answer is option D.

Detailed explanation-4: -Due to the absence of cambium, the vascular bundles in monocots are said to be closed. This absence of cambium in monocots results in no secondary growth and no new cells are formed within the vascular bundles.