Introduction to Anatomy of Monocot and Dicot Stems
Primary structure of monocot stem - Maize stem
The sketch out of the maize stem in transverse section is more or less circular in shape. Inner structure of monocotyledonous stem shows hypodermis, ground tissue, epidermis, and vascular bundles.
Epidermis is the most outer layer of the stem. It is formed of single layer of very tightly packed parenchymatous cells. The outer walls of them are covered with thick cuticle. The stability of this layer might be broken here and there through the existence of a few stomata. There are no epidermal outgrowths.
Some layer of sclerenchymatous cells lying under the epidermis comprises the hypodermis. This layer provides mechanical power to the plant. It is broken up here and there through chlorenchyma cells.
There is no difference into endodermis, cortex, pericycle and pith. The whole mass of parenchymatous cells lying inside to the hypodermis creates the ground tissue. The cell wall is built up of cellulose. The cells consist of reserve food material such as starch. The ground tissue's cells next to the hypodermis are smaller in size, polygonal in shape and efficiently arranged. In the direction of the centre, the cells are arranged loosely, in shape rounded and bigger in size. The vascular bundles lie fixed in this type of tissue. The ground tissue stores food and carries out gaseous exchange.
Vascular bundles are spread in the parenchymatous ground tissue. Every vascular bundle is surrounded through a sheath of sclerenchymatous fibres termed as bundle sheath. The vascular bundles are conjoint, endarch, collateral, and closed. Vascular bundles are several, small and arranged closely in the peripheral portion. In the direction of the centre, the bundles are relatively large in size and loosely arranged. Vascular bundles are skull shaped.
Diagram: T.S. of a Maize stem
The phloem in the monocot stem contains companion cells and sieve tubes. Phloem parenchyma and phloem fibres are not present. It can be differentiated into an outer crushed protophloem and an inside metaphloem.
Xylem vessels are arranged in the shape of the alphabet letter 'Y'. The two metaxylem vessels are situated at the upper two arms and one / two protoxylem vessels at the bottom. The lowest protoxylem disintegrates and creates a cavity termed as protoxylem lacuna in a mature bundle.
Basic structure of dicotyledonous stem - Sunflower stem
Inner structure of dicotyledonous stem reveals epidermis, cortex and stele.
Epidermis is protective in function and creates the outermost layer of the stem. Epidermis is a single layer of parenchymatous rectangular cells. The cells are efficiently arranged with no intercellular spaces. The outer walls of the epidermal cells comprise a layer that is termed as cuticle. The cuticle tests the transpiration. The cuticle is built up of a waxy substance termed as cutin. Stomata might be exists here and there. Epidermal cells are living. Chloroplasts are generally not present. So many multicellular hairs take place on the epidermis.
Cortex takes place under the epidermis. The cortex is distinguished into 3 zones. Below the epidermis, there are some layers of collenchyma cells. This zone is termed as hypodermis. It provides mechanical power to the stem. At the corners these cells are living and thickened. Inside to the hypodermis, some layers of chlorenchyma cells are exists with conspicuous intercellular spaces. This area carries out photosynthesis. A number of resin ducts also take place here. The third zone is built up of parenchyma cells. These cells store food materials.
The most inner layer of the cortex is termed as endodermis. The cells of endodermis are barrel shaped and arranged efficiently without intercellular spaces.
Diagram: T.S. of Sunflower stem
Because starch grains are abundant in these cells, this layer is as well termed as starch sheath. This layer is homologous morphologically to the endodermis that found in the root. In several dicot stems, endodermis along with casparian strips is not developed.
The middle part of the stem inside to the endodermis is termed as stele. It contains pericycle, vascular bundles and pith. In dicot stem, vascular bundles are ordered in a ring around the pith. This kind of stele is termed as eustele.
Pericycle is the layers of cells which take place among the vascular and endodermis bundles. In the sunflower's stem (Helianthus), some layers of sclerenchyma cells take place in patches outside the phloem in every vascular bundle. This patch of sclerenchyma cells is termed as bundle cap / hard bast. The parenchyma and bundle caps the cells among them comprise the pericycle in the stem of sunflower.
The vascular bundles contain phloem, xylem, and cambium. Xylem and phloem in the stem take place jointly and form the vascular bundles. These vascular bundles are wedge in shape. They are ordered in the shape of a ring. Every vascular bundle is conjoint, collateral, endarch and open.
Primary phloem takes place in the direction of the periphery. It contains protophloem and metaphloem. Phloem contains companion cells, sieve tubes, and phloem parenchyma. In the primary phloem, Phloem fibres are not present. Phloem carries out organic food materials from the leaves to another parts of the plant body.
Cambium contains thin walled and brick shaped meristematic cells. It is 2 to 3 layers in thickness. These cells are able of creating new cells throughout secondary growth.
Xylem contains xylem parenchyma, xylem fibres, vessels and tracheids. Vessels are thick walled and organized in some rows. Xylem conducts minerals and water from the root to other parts of the body of the plant.
Pitch is the large central portion of the stem. It is formed of parenchyma cells along with intercellular spaces. The pith is as well termed as medulla. The pith expands among the vascular bundles. These extensions of the pith among the vascular bundles are termed as primary pith rays / primary medullary rays. Task of the pith is storage of food.
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