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Dicot stem

The document summarizes the structure of a dicot stem. It has three main parts - the epidermis, cortex, and stele. The epidermis is a single layer of cells coated with a waxy cuticle. The cortex is made of parenchyma cells and can include collenchyma. The stele contains the vascular bundles in a ring, which include xylem and phloem. Between the bundles are medullary rays and a pith center. Cambium allows for the stem to secondarily thicken over time.

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Dicot Stem
Stem • Aerial part of plant body is called shoot system. • Stem is the axis of shoot system with branches, leaves, buds and flowers. • Stem is divided into nodes (from where leaves and axillary bud arise) and internodes.(portion of stem in between two adjacent nodes). Origin of stem • Short axis of embryo of endosperm hypocotyl with one or more cotyledons and shoot primodium at its upper end. • Is lower end bears the root primodium which is responsible for growth. • The embryonic shoot is called plumule and embryonic root is called radicle. • radicle forms the first shoot • plumule forms the shoot system
Anatomical features of stem • Epidermis is uniseriate with distinct cuticle • cortex heterogenous with a prominent hypodermis. • endodermis is not prominent as in roots. • vascular bundles are discrete with narrow or wide medullary rays. • Vascular bundles are conjoint, collateral , bicollateral and concentric. • Xylem is always endarch • Lateral branch from the cortex are exogenous in origin.
Primary structure of dicot stem • consists of 3 major parts epidermis, cortex and stele. Epidermis: outermost protective cellular layer • uniseriate formed of single layer of thin walled parenchyma • living, closely packed , coated with a fatty substance , called cutin. • this non living deposition of unsaturated fatty acids forms a separate layer over the epidermis called cuticle, which prevents the loss of water. • the epidermis of young stem possesses number of stomata, guard cells are kidney shaped. • In herbaceous stems the epidermis is retained through out the life of a plant , whereas in woody stems it get sloughed off during secondary thickening due to formation of periderm.
Cortex- multi layered , extra stellar ground tissue seen in between epidermis and pericycle. • homogenous in simple stems , but heterogeneous in others. • In hetrerogenous , it has 3 regions namely outer cortex, inner cortex and endodermis • Outer cortex seen just below the epidermis, also called hypodermis. It is a collenchymatous region , sclerenchymatous in Casuarina. • Hypodermis gives mechanical support. • During secondary growth , collenchyma acts as meristematic tissue and give rise to cork cambium. • Inner cortex differentiated into Outer chlorenchymatous and inner parenchymatous. • Chlorenchymatous part is photosynthetic where as parenchymatous part is concerned with food storage. • In hydrophytes , the parenchymatous has well developed air spaces for buoyancy and gas exchange.
Endodermis- innermost layer of the cortex • woody dicots and gymnosperms do not have a distinct endodermis. • endodermal cells contain starch grains. • Hence called starch sheath. • Endodermis is uniseriate or single layered, made of barrel shaped and closely packed parenchyma cells. • In dicots , endodermal cells have characteristic thickening on radial and inner tangential wall known as casperian strip, casperian band or casperian thickening. • It is formed of suberized or lignified material which is impervious to water. Casperian strip block the movement of water and solutes through the protoplasts of endodermal cells.This facilitates the selective filtering of the sap solutes and also control the rate of flow. • Food storage is another function attributed to endodermis in plants.
• Stele – is the central vascular cylinder, having 4 parts- pericycle, vascular bundles, medullary rays and pith. • Pericycle- outermost part of the stelar region , seen just below the endodermis. • If present most cases pericycle is multilayered formed of alternating patches of sclerenchyma and parenchyma. • The parenchymatous cells of the pericycle function as food storage whereas sclerenchyma cells form a cap over the vascular bundle called bundle cap and give mechanical strength.
Vascular bundles • compact, bundle like groups of vascular tissues. • vascular bundle are limited in numbers and are arranged in the form of a broken ring. • Each bundle is made up of xylem and phloem with intrafascicular cambium in between. • Vascular bundles are open, conjoint, and collateral or bicollateral with endarch xylem arrangement. • Conjoint bundles in dicot stem are of two types Collateral Bicollateral
Collaterall Bicollaterall
Xylem • Xylem arrangement is endarch, hence xylem development is centrifugal. • Xylem consists of tracheids, vessels, fibres and xylem parenchyma. • Primary xylem consists of protoxylem and metaxylem. • Protoxylem contains a small amount of tracheary elements and much parenchyma cells. • Tracheary elements are narrow with annular and spiral thickening. • Metaxylem contains much amount of tracheary elements. • tracheary elements or vessels have scalariform, reticulate or pitted thickening. • Xylem concerned with condution of water and dissolved inorganic subsances. • tracheary cells provide mechanical support and thin walled xylem parenchyma cells store food materials.
Phloem • complex food conducting tissue , composed of sieve elements , companion cells, parenchyma and fibres. • Primary phloem consists of protophloem and metaphloem. • When the plant body matures , the cells of the protophloem either disintegrate or differentiate into fibres. Cambium • is a layer of meristematic cells found in between xylem and phloem. • Being a part of a vascular bundle , it is called fascicular cambium or vascular cambium. • cells are thin walled with dense cytoplasm and prominent nucleus. • vascular bundles having cambium are called open bundles • Vascular bundles without cambium are called closed bundles.
Medullary rays • medullary rays or pith rays are radial bands of thin walled and radially elongated parenchyma cells occurring in between vascular bundles. • During secondary growth , the interfascicular parenchyma cells , lying in line with the fascicular cambium of adjacent vascular bundles become meristematic and function as interfascicular cambium . • These cambial segments cut off medullary rays on both sides. • Medullary rays are concerned with radial conduction
Pith or medulla • inner core of the stele. • mostly parenchymatous , formed of thin walled and loosely arranged cells with intercellular spaces. • Outer part of the pith is composed of smaller cells while central pith cell are large. • In most herbaceous plants , intermodal pith get disintegrated during development , forming a central cavity.