Question

Why are the vascular bundles in monocots said to be closed?

Answer 1

Vascular bundles contain tissues which are important for the transport system of the plants. The plants having vascular tissue are known as vascular plants. The vascular tissues are of two types, that are, xylem and phloem. Both of which are present in the vascular bundle. Other than xylem and phloem it consists of some protective cells.

Complete answer:
The vascular tissue system as mentioned contains xylem and phloem. Xylem which accounts for water and mineral transportation while phloem is responsible for the transportation of organic materials. Transportation through xylem is generally unidirectional while the phloem is bi-directional.
In the dicotyledonous stem, there is a cambium present between phloem and xylem. This cambium interestingly provides the capability of secondary growth of xylem and phloem tissue. The secondary growth in plants causes the increase in girth of the stem. They are free for secondary growth, so they are also known as free vascular bundles.
On the other hand monocots do not have this cambium present between xylem and phloem so, they are incapable of secondary growth. Further there vascular bundles are scattered in the ground tissue and are of different sizes. The vascular bundle of monocots are surrounded by a sclerenchymatous sheath known as bundle sheath. They are also known as closed vascular bundles as they are incapable of secondary growth.

Note:
The vascular bundle is a representation of two vascular tissues arranged in a particular way. The phloem part is responsible for the translocation of the photoassimilates, proteins as well as the plant growth regulators.