Anatomy of Plant Stems
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Anatomy of Dicot and Monocot Stem
Q.N. 1. Describe the anatomy or internal structure of the dicot stem.
Primary dicot stem consists of epidermis, hypodermis, cortex, endodermis, pericycle, vascular bundles, medullary rays or pith rays, and pith or medulla (see Figure 3).
1. Epidermis
Epidermis is the outermost layer of the stem. It is composed of elongated, barrel-shaped parenchymatous cells without intercellular spaces. It is covered externally by a cuticle. Stomata and several unbranched multicellular hairs or trichomes are also present. Each stoma has a pair of specialized kidney-shaped cells, guard cells. Epidermis helps in protection of internal tissues, exchange of gases through stomata, reduces the rate of surface transpiration by cuticle, prevents the entry of harmful organisms and heat insulation by epidermal hairs.
2. Hypodermis
Hypodermis lies below epidermis. It is made of 3- to 4-layered collenchyma. Collenchyma is green due to chloroplasts in it. It helps in mechanical support, cell elasticity, storage of food, and photosynthesis.
3. Cortex
Cortex lies below hypodermis. It is made up of a few to several layers of thin-walled, angular, oval or rounded parenchymatous cells with intercellular spaces. Its main function is to store food.
4. Endodermis
It is a single wavy innermost layer of the cortex. It is made of barrel-shaped, compactly arranged, parenchymatous cells without intercellular spaces. Cells in it contain a large number of starch grains. Due to the presence of starch grains in cells, endodermis is also called starch sheath.
5. Pericycle
The pericycle lies between the endodermis and vascular bundles. It is generally heterogeneous or rarely homogeneous. The heterogeneous pericycle contains both the parenchyma and sclerenchyma. Sclerenchymatous pericycle is just outside the vascular bundles in the form of semicircular patches, called bundle caps. The parenchymatous pericycle is made either of only parenchyma or only of sclerenchyma. The sclerenchymatous pericycle gives mechanical support while the parenchymatous pericycle stores food.
6. Vascular bundles
Vascular bundles in dicot stem are present as eustele form i.e. ring of vascular bundles present inner to the pericycle around the central pith. They are wedge-shaped structures. Each vascular bundle consists of phloem at the outer side, xylem towards inner side, and a strip of cambium in between phloem and xylem. So, the vascular is conjoint, collateral, and open.
Phloem
Phloem lies towards the pericycle. It consists of sieve tubes, companion cells, phloem parenchyma, and some phloem fibres. The sieve tubes are the main conducting element of phloem that transport food.
Xylem
Xylem lies towards the pith or inner side of the vascular bundle. It consists of tracheids, vessels, xylem parenchyma, and xylem fibres. Tracheids and vessels form small protoxylem and large metaxylem. Protoxylem is the first formed xylem and lies towards the centre whereas metaxylem is the later formed xylem and lies towards the periphery. This type of xylem is called endarch xylem. Protoxylem has annular thickenings whereas metaxylem has pitted thickenings. Vessels remain in chains or rows. They are with an angular outline. Xylem helps in conduction of sap and mechanical support.
Cambium
Cambium lies between xylem and phloem of the same vascular bundle. It is a narrow strip of primary meristematic tissue. It is called intrafascicular cambium or fascicular cambium. In the transverse section, cambial cells appear rectangular. Cambium helps in increasing the girth of the stem by producing secondary phloem towards outer side and secondary xylem towards inner side.
7. Medullary rays or pith rays
Medullary rays lie between two adjacent vascular bundles. They are parenchyma with radially elongated cells. There may or may not be intercellular spaces. Medullary rays help in radial conduction of food and water. They also transport gases from pith to cortex and vice-versa.
8. Pith or Medulla
Pith is present at the centre of the stem. It contains oval, rounded or polygonal parenchyma cells with or without intercellular spaces. Pith cells help in storage of food.
Q.N. 2. Describe the anatomy or internal structure of monocot stem.
A typical monocot stem consists of epidermis, hypodermis, ground tissues, and vascular bundles (see Figure 4).
1. Epidermis
Epidermis is the outermost layer of monocot stem. It is composed of compactly arranged, elongated, barrel-shaped parenchymatous cells without intercellular spaces. It contains stomata. Each stoma has a pair of specialized dumb-bell shaped cells, called guard cells. It helps in the protection of internal tissues, exchange of gases through stomata, minimize the surface transpiration by cuticle, and prevent the entry of harmful organisms.
2. Hypodermis
Hypodermis lies below epidermis. It is made up of 2- to 3-layered thick-walled dead lignified sclerenchyma. It helps in mechanical support.
3. Ground tissue
Ground tissue lies below the hypodermis. It is a homogenous mass of tissue that is not differentiated into cortex, endodermis, pericycle, pith, and medullary rays. It is made up of oval or rounded parenchymatous cells with intercellular spaces. It helps in storage of food.
4. Vascular bundles
The vascular bundle in monocot stem is atactostele i.e. vascular bundles are scattered throughout the ground tissue. Vascular bundles are small and numerous towards the periphery but they are large and few towards the centre. Vascular bundles are oval or rounded.
Each vascular bundle contains both phloem and xylem. Xylem lies towards the centre and phloem lies towards the periphery. Cambium is absent. This type of vascular bundle is called conjoint, collateral, and closed. Each vascular bundle is surrounded by a sheath of parenchyma, called bundle sheath.
Phloem
Phloem consists of sieve tubes, companion cells, and a few phloem fibres. Phloem parenchyma is absent. Sieve tubes conduct organic matters.
Xylem
Xylem consists of vessels, tracheids, xylem parenchyma, and a few xylem fibres. Protoxylem is smaller than metaxylem. Protoxylem is the first formed xylem whereas metaxylem is the later formed xylem. Protoxylem lies at the centre while metaxylem lies towards the periphery. This type of xylem is called endarch xylem. Vessels are arranged to form ‘V’ or ‘Y’. Some of the protoxylem vessels and xylem parenchyma cells are separated during rapid growth of the stem and form a cavity, called protoxylem cavity (schizo-lysigenous cavity). Protoxylem cavity stores water. Xylem helps in conduction of sap and provides mechanical support.
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