Concentric Rings The concentric growth rings are called annual rings. These rings are formed due to the secondary growth. Secondary growth occurs in dicot trees due to the activity of cambium which is a meristematic tissue. The rate of activity of cambium is more in spring so wood formed has larger wider xylem cells, whereas wood formed in winter has narrower and smaller xylem elements. This results in the formation of two rings called growth rings.

By counting these rings, age of the tree can be determined. This branch of science is known as dendrochronology or growth ring analysis.

It is anatomical abnormality. It is an abnormal type of secondary growth, where a regular vascular cambium or cork cambium is not formed in its normal position. In case of old tree trunks, anomalous secondary growth produces cortical and medullary vascular bundles. Thus, the additional or accessory vascular bundles given appearance of several fused trunks.

Difference between lenticels and stomata is as follows

Lenticels	Stomata
Lenticels are formed due to loosening of the epidermal and cortical tissues.	Stomata are specialised epidermal structure.
These are mostly found on the stem region. Lenticels does not have guard cells.	Mostly found on lower surface of leaves. Stoma has guard cells.
These openings are not regulated. They are used for removal of waste.	Opening and closing is regulated mechanism. They are involved in gaseous exchange, removal of extra water and waste.

Sieve tube

It is present in the phloem tissue. It's function is the transportation of synthesised food through out the plant.

Interfascicular Cambium

It's function is to bring about secondary growth in the dicot stem and root. It is a kind of secondary meristematic tissue present in between two vascular bundles.

Collenchyma

It's function is to provide mechanical support to young growing herbaceous stem. It's cells have angular thickening at corners.

Aerenchyma

It provides buyoncy to the hydrophytic plants. It is a specialised parenchyma having large air spaces.

The epidermal cells surrounding the guard cells of stomata are called subsidiary cells. Differences between guard cells and epidermal calls are

Guard cells	Epidermal cells
They are bean or kidney shaped.	They are barrel shaped.
They possess chloroplasts.	They ladk chloroplasts.
They are smaller.	They are bigger.
Cell walls of guard cells are not uniform and thicker.	Epidermal cells are uniformly thin.