Anatomy of Flowering Plants

Specialized epidermal cells surrounding the guard cells are known as Subsidiary or accessory cell.

In monocot root, a large number of vascular bundles are arranged in the form of a ring around the central pith.

Vascular bundles are closed because there is no cambium present between the xylem and phloem.

Vascular bundles in monocotyledons (plants with a single seed leaf, like grasses and grains) are considered closed primarily due to Option D: Cambium is absent.

In botany, vascular bundles are the part of the plant that transport nutrients and water.

In monocotyledons, these bundles are typically scattered throughout the stem and do not have a cambium layer.

The cambium is a layer of actively dividing cells found in most dicotyledons (plants with two seed leaves) and is responsible for secondary growth, which includes the widening of the stems and roots.

This secondary growth is possible because the cambium adds layers of vascular tissue called secondary xylem (wood) and secondary phloem.

In monocotyledons, the absence of cambium means there's no secondary growth, and thus, the vascular bundles are termed closed.

This is different from open vascular bundles found in dicotyledons, where the presence of cambium allows for continuous growth and change in the arrangement of the xylem and phloem.

Options A, B, and C describe other aspects of vascular bundles but are not the reasons why monocotyledonous vascular bundles are considered closed.

For instance: Option A: The presence or absence of vessels with perforations is more about the type of vessels in the xylem and not directly related to the concept of open or closed vascular bundles.

Option B: While xylem surrounded by phloem is a characteristic of some vascular bundles, it doesn't define them as open or closed.

Option C: A bundle sheath may be present in both monocotyledons and dicotyledons and is not a defining factor for open or closed vascular bundles.

Therefore, Option D, Cambium is absent, is the correct reason for vascular bundles in monocotyledons being considered closed.

Anatomically fairly old dicotyledonous root is distinguished from the dicotyledonous stem by position of the protoxylem.

In dicot root, the protoxylem is located near the periphery of the vascular cylinder while in dicot stem the protoxylem is located near the centre of vascular bundle i.e., the xylem is endarch.

The walls of vessels (tracheary elements) are lignified and less thicker than tracheids.

The lumen is wider.

Vessels differ from tracheids in having cell fusions arising through the dissolution of end walls.

C 4 plants show kranz type of anatomy.

In kranz anatomy, the mesophyll is undifferentiated and its cells occur in concentric layers around vascular bundles.

The vascular bundles are surrounded by large sized bundle sheath cells which are arranged in wreath like manner in one to several layers.

In C 4 plants there are two carboxylation reactions, first in mesophyll chloroplast and second in bundle sheath chloroplast.

RuBP is present in bundle sheath chloroplasts where C 3 cycle takes place.

Lateral meristems are meristems which occur parallel to the circumference of the organs in which they develop.

They undergo periclinal divisions producing secondary tissues on the outer and inner sides and increase the girth of the plant organs.

Examples are vascular cambium and corkcambium.

Sometimes, a few epidermal cells in the vicinity of the guard cells become specialised in their shape and size and are known as subsidiary cells or accessory cells.

Lenticels are lens shaped openings formed in bark due to secondary growth.

They permit gaseous exchange in woody trees.

They also contribute to transpiration but in minute amounts because the suberised complementary cells present beneath the pore prevent excessive water loss.

At the time of secondary growth interfascicular cambium is formed by parenchymatous medullary rays.

Interfascicular cambium along with intrafascicular cambium (formed from cambium cells present between xylem and phloem) constitute continuous cambium ring.

If new cells are cut off in both directions it causes secondary growth in most dicotyledonous plants.