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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