Describe internal structure of Dicot root
Dicot root of
Gram consists of:
(1) Epiblema
or piliferous layer
This is single
outermost layer of thin walled cells; the outer walls of most of these cells extend
outwards and form unicellular rot hairs. This layer is used for absorption of
water and various numeral salts from the soil and therefore has no cuticle.
Root hairs increase the absorbing surface of the root.
(2) Cortex
This consists of
many layers of thin walled rounded cells with numerous intercellular spaces
between them. Cells of cortex contain leucoplasts and store starch grains.
(3) Endodermis
This is single
ring like layer of barrel shaped closely packed without intercellular spaces.
Radial walls of thin layer are thickened and thickness extends to the inner
walls also. Endodermis is innermost layer of cortex and surrounds the stele as
a cylinder.
(4) Pericycle
This lies
internal to endodermis and like it, is a single ring like layer; its cells are
much smaller and thin walled but with abundant protoplasm.
(5) Conjunctive
Tissue
The parenchyma
lying in between xylem and phloem bundles constitutes the conjunctive tissue.
(6) Pith
This occupies
only a small area in the centre of the root. Sometimes the pith is nearly
obliterated owing to the wood vessels meeting in the centre.
(7) Vascular
bundles
These are
arranged in a ring as in Dicot stem but here xylem and phloem form an equal
number of separate bundles and their arrangement is radial. The number of xylem
or phloem bundles varies from two to six. The cambium is absent in the young
root but soon makes its appearance. Phloem bundle consists of sieve tubes,
companion cells and phloem parenchyma. Xylem bundle consists of protoxylem
which lies towards the circumference on the pericycle and metaxylem towards the
centre. Xylem is exarch in roots and its development is centripetal.
Protoxylem is
composed of small vessels (annular and spiral) and metaxylem of bigger vessels
(reticulate and pitted). The metaxylem groups often meet in the centre and then
the pith gets broken.
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