DICOTYLEDONOUS ROOT
The primary internal structure of dicot root can be
studied from the Transverse Section (TS) of a young root
Of sunflower, pea or gram. The primary root is the one
which has only primary permanent tissues that are
formed from vegetative shoot apex. Secondary tissues are
absent.
The following structures can be seen from periphery
towards the centre
1. Epidermis
It forms the outermost layer in young root. It is
equivalent to epidermis of stem. The stomata and cuticle
are not present in it. The cells are thin-walled and
tubular. Some of the epidermis cells are prolonged to
form thin-walled tubular structures called root hairs.
The cells which produce root hair are called root hair
cells or trichoblasts. Due to the presence of root hairs,
epiblema is also called piliferous layer (Pilus — hair; ferre
— to carry) and rhizodermis (Rhiza — root; derma — skin).
Root hairs have pectose layer on the outside, this is to
help them to pass into the soil spaces for absorption of
water and mineral salts. The active lifespan of root hairs
is upto 7 days and die off in older parts of the root.
The cells of older epidermis shrivel afterwards and
become cutinised and suberised.
2. Cortex
It lies beneath the epiblema. It consists of several layers of
thin-walled parenchymatous cells with conspicuous
intercellular spaces. The cells of cortex store food. It also
conducts water from the epiblema to the inner tissues.
3. Endodermis
The innermost layer of the cortex is endodermis. It comprises of
a single layer of barrel-shaped cells without any intercellular
spaces. The endodermal cells are living and are rich in starch
grains.
They have characteristic bands of thickenings along their
radial and tangential walls. These are called casparian bands or
casparian strips.
The casparian strips are made up of suberin and lignin. These
strips prevent plasmolysis of endodermal cells and do not
allow wall to wall movement of substances, between cortex
and pericycle.
The cells of endodermis lying opposite to the protoxylem are
thin-walled to permit free passage of water and minerals from
cortex into the xylem. These are called passage cells.
4. Stele
All tissues on the innerside of  the endodermis such
pericycle, vascular strand and pith constitute the stele.
i. Pericycle
The next to endodermis lies a layer of thick-walled
parenchymatous cells referred to as pericycle. The initiation
of lateral roots and vascular cambium during the secondary
growth takes place in these cells.
ii. Vascular Strand
The vascular strand consists of separate bundles of xylem and
phloem arranged alternately inner to the pericycle. Hence, the
xylem and phloem bundles are equal in number and lie on
different radii. Such vascular bundles are called radial bundles.
On the basis of number of xylem bundles, the root may be
diarch (with two xylem bundles), triarch, tetrarch, pentarch
and polyarch (with more than five xylem bundles).
When the protoxylem is towards the periphery and the later
formed xylem (metaxylem) is towards the centre of the root.
This kind of xylem condition is called exarch and is
characteristic of root.
The phloem and xylem bundles are separated from each
other by one or more layers of small thin-walled cells called
conjuctive tissue.
Later, it becomes meristematic and forms vascular
cambium. The phloem tissues conducts organic food from
leafto the other parts of the plant. Secondary growth occurs
in dicot roots.
iii. Pith
It is generally absent in dicot roots. If present, it is small and
consists of parenchyma cells. These function to store food
and waste products.
Features for Identification of
Dicotyledonous Root
Dicotyledonous root can be easily identified with the
following features
(i) Presence of root hairs.
(ii) Endodermis with casparian strips.
(iii) Absence of pith.
(iv) Radial bundles less than eight.
(v) Presence of exarch xylem.