Epidermis:
It is the outermost layer and has a single layer of parenchymatous cells.
It possesses stomata and large number of multicellular hairs (trichomes).
The outer walls are greatly thickened and cutinized.
The cells are compactly arranged and do not possess intercellular space.
The epidermis has following functions:
Minimize the rate of transpiration owing thick cuticle
Protects the underlying tissues from mechanical injury
Prevents the entry of harmful organisms
Helps in the exchange of gases through stomata.
Hypodermis:
This layer lies below the epidermis and is composed of 4 or 5 layers of
collenchymatous cells.
These cells are specially thickened at the corners against the intercellular spaces due
to deposition of cellulose and pectin.
The cells are living in nature and may contain few chloroplasts.
It provides mechanical strength and elasticity to the peripheral portion of the stem
particularly the young and growing organs.
They perform photosynthesis and also acts as storage of food.
Cortex:
It lies below the hypodermis.
It consists of a few layers of thin-walled, large, rounded, or oval, living
parenchymatous cells, having intercellular spaces.
Cells of cortex may contain some chloroplasts which may function to manufacture of
food materials.
They serve for storage of food.
Endodermis:
It is the single innermost layer of the cortex which separates the cortex from vascular
bundles.
Cells are somewhat barrel shaped and compactly arranged, having no intercellular
spaces and are parenchymatous.
Usually, the cells contain starch grains and thus the endodermis maybe termed as
starch sheath.
They serve as food reserve.
The radial and the transverse walls are thickened due to the deposition of lignin
forming casparian strips.
Pericycle:
It lies in between the endodermis and vascular bundles.
It is generally composed of sclerenchymatous and parenchymatous cells.
The sclerenchyma is in the form of semilunar patches above the vascular bundles
which give mechanical support to the plant parts.
Similarly, parenchymatous pericycle is present outside the medullary rays which
serves to store food.
Vascular bundles:
These are arranged in a ring around the central pith and inner to the pericycle.
These are conjoint, collateral, open and wedge-shaped.
The size of the bundles varies in different species.
Each bundle has a patch of xylem towards the center, a patch of phloem towards the
periphery and a strip of cambium in between them.
Xylem:
It lies towards the pith of vascular bundles.
It consists of tracheids, vessels, xylem parenchyma, xylem fibers.
Tracheids and vessels consists of smaller protoxylem and larger metaxylem.
Protoxylem is first formed that lies towards the center but metaxylem is later
formed that lies towards the periphery.
This type of xylem is called endarch xylem. It helps in conduction of sap.
Phloem:
It lies just below the sclerenchymatous patch of pericycle and is composed of
following elements such as sieve tubes, companion cells, and phloem
parenchyma.
It conducts the foods.
Cambium:
It lies in between xylem and phloem.
It consists of a narrow strip of meristematic cells having large nuclei and dense
cytoplasm, called fascicular cambium.
It is responsible for secondary growth in thickness of the plant body.
Pith:
It occupies the central portion of the stem.
It is composed of thin walled parenchymatous cells which are rounded or polygonal,
with or without intercellular spaces.
Food is stored in this region.
Medullary rays:
These are the thin-walled, radially elongated parenchymatous cells present in
between vascular bundles.
These store food materials and help in internal translocation of water.
