To solve this problem, we need to understand why vascular bundles in monocotyledons are considered closed.$\newline$Let's analyze the options:$\newline \bullet$Option 1: A bundle sheath surrounds each bundle.$\newline$ - While monocot vascular bundles often have a bundle sheath, this is not the reason they are considered closed.$\newline \bullet$Option 2: Cambium is absent.$\newline$ - In monocotyledons, vascular bundles are closed because they lack cambium.$\newline$ - Cambium is a lateral meristem responsible for secondary growth, which is absent in monocots.$\newline$ - Therefore, without cambium, the vascular bundles cannot increase in girth, making them closed.$\newline \bullet$Option 3: There are no vessels with perforations.$\newline$ - The presence or absence of perforations in vessels does not determine whether a vascular bundle is closed.$\newline \bullet$Option 4: Xylem is surrounded all around by phloem.$\newline$ - This is not a characteristic feature of closed vascular bundles in monocots.$\newline$Therefore, the correct answer is Option 2: Cambium is absent.