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.
Age of a tree can be estimated by number of annual rings.
Annual ring constitute alternate concentric rings of spring wood and autumn wood.
The vascular bundles are arranged in a loose circle inside the endodermis of a monocot root.
In a monocot root, more than six vascular bundles are present.
It shows polyarch condition.
The common bottle cork is the product of phellogen.
Phellogen produces cork or phellem on the outer side.
It consists of dead and compactly arranged rectangular cells that possess suberised cell walls.
The cork cells contain tannins.
Hence, they appear brown or dark brown in colour.
The cork cells of some plants are filled with air e.g., Quercus suber (Cork Oak or Bottle Cork).
Stem of maize has water containing cavities in vascular bundles.
Companion cell is a type of cell found within the phloem of flowering plants.
Each companion cell is usually closely associated with a sieve element.
They remain connected with sieve cells by plasmodesmata.
They help in loading of phloem sieve cells with sugars through active transport.
Xylem fibres provide rigidity to the plant.
Vascular bundle consists of complex tissues, the phloem and xylem.
In dicots, between xylem and phloem, cambium is present which helps in secondary growth.
This type of vascular bundle is called open.
While in monocots cambium is absent, so these are called closed vascular bundles.
