internal structure of Monocot stem. Also differentiate between Dicot and Monocot stem
Inter structure
of Monocot stem of maize consists of following parts.
(1) Epidermis
This is single
outermost layer with thick cuticle on outer surface. In epidermis few stomata
may be seen.
(2) Hypodermis
(sclerenchyma)
This forms
narrow zone of sclerenchyma two or three layers thick below the epidermis.
(3) Ground
tissue
It is continuous
mass of thin walled parenchyma arising below sclerenchyam to the centre. It is
not differentiated into cortex, endodermis, pericycle etc as in Dicot stem. The
cells of ground tissue enclose many intercellular spaces.
(4) Vascular
bundles
These are
collateral and closed and lie scattered in the ground tissue. They are more
numerous and lie close together near periphery than the centre. Peripheral ones
are also seen to be smaller in size than central ones. Each vascular bundle is
oval and surrounded by a sheath of sclerenchyma developed on two sides upper
and lower. The bundle consists of (a) Xylem (b) Phloem, but cambium is absent.
(a) Xylem: It
consists of four distinct vessels arranged in the form of Y and a small number
of tracheids arranged irregularly. Two smaller vessels (annular and spiral)
lying radially towards the centre constitute protoxylem and two bigger vessels
(pitted) laying laterally together with the small pitted tracheids lying in
between them form metaxylem.
Besides thin
walled wood (or xylem) parenchyma almost surrounding water containing cavity is
present in the proto xylem and few wood fibres occur associated with tracheids
in between two big pitted vessels.
(b) Phloem: It
consists of sieve tubes and companion cells. No phloem parenchyma is present in
Monocot stem. Outermost portion of phloem which is a broken mass is the
protophloem and the inner portion is metaxylem. The former soon gets
disorganized and the latter shows distinct sieve tubes and companion cells.
Dicot
Stem Monocot
Stem
(1) Hypodermis Collenchymatous Sclerenchymatous
(2) General
Cortex Few layers of
parenchyma Continuous
mass of parenchyma upto
centre.
(3) Endodermis A wavy layer Ground Tissue
(4) Pericycle a zone of
parenchyma and without
differentiation into distinct sclerenchyma.
tissues.
(5) Medullary
ray a strip of
parenchyma in between not marked
out.
vascular
bundles.
(6) Pith the central cylinder not marked out.
(7) Vascular
bundles (a) Collateral and
open (a) collateral and closed
(b)
Arranged in a ring (b)
scattered
(c)
Of uniform size (c)
larger towards centre
(d)
Phloem parenchyma present (d) absent
(e)
Usually wedge shaped (e)
usually oval
(f)
Bundle sheath absent (f)
strongly developed.
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