Download Primary Structure of Stem

Primary Structure – Dicot Stem
I B.Sc Botany
Stem
• Important organ
• The shoot consists of axis (stem)
• Its lateral appendage (leaf)
Anatomy Dicotyledonous Stem
• Helianthus annus
• Young stem of Sunflower
Epidermis
• Outer most single layer
• It is composed of compactly arranged tubular
parenchymatous cells.
• The cells are attached end to end along their radial
walls without intercellular spaces & appear
rectangular in CS.
• Cells are living with vacuolated protoplasm usually
without chloroplasts.
• Outer wall is cuticularised
• A large number of multicellular epidermal hairs
• Occasionally few stomata are found on the epidermis.
• Stomata are ranunculaceous (Anomocyctic) type.
Ground tissues
• Ground tissue is differentiated in to two
distinct zones
• Hypodermis
• Cortex
Hypodermis
• It is found immediately below the epidermis.
• Which consists of a few layers of
collenchymatous cells showing angular
thickness.
• It forms a continuous band providing
mechanical support to the growing stem.
• Collenchymatous cells may have a few
chloroplasts.
Cortex
• It is found next to hypodermis
• It consists of loose parenchymatous cells with
fairly large number of intercellular spaces.
• The cells are thin walled, oval or rounded & living
with distinct nucleus & cytoplasm.
• Some isolated mucilaginous canals lined with
epithelial cells are found.
• The cells may contain some chloroplasts & may
function as assimilatory cells.
• Cortical cells mainly serve for food storage.
Endodermis
• The innermost layer of cortex
• Which separates cortex from stele
• It is a kind of wavy band made of compactlyset barrel-shaped parenchymatous cells.
• Since these cells are rich in starch grains, it is
referred to as starch sheath.
• Their radial & inner walls are thickened due
to the deposition of lignin & suberin forming
casparian strip.
Stele
• It includes pericycle, vascular bundles, pith &
medullary rays.
• The stele type – Polyfascicular siphonostele
or dissected siphonostele.
Pericycle
• It is found just below the endodermis
• It is composed partly of parenchymatous &
partly sclerenchymatous cells.
• Sclerenchymatous tissues are found in the
form of crescent shaped patches outside the
vascular bundles.
• It is also called as Bundle cap.
• They provide mechanical support to the plant
parts.
Vascular Bundles
•
•
•
•
The vascular bundles are arranged in a ring
Each vascular bundle being wedge-shaped
Conjoint, collateral & open type.
Each bundle has a patch of xylem towards
centre, a patch of phloem towards the
periphery & a strip of cambium in between
them.
Xylem
• It lies inner to cambium
• It consists of tracheids, vessels, fibres &
parenchyma.
• The tracheids are found around the vessels &
lying in between.
• Vessels are arranged in radial rows, with
protoxylem lying towards centre &
metaxylem towards cambium.
• Endarch arrangement of xylem.
• The protoxylem consists of annular, spiral &
reticulate vessels
• Metaxylem has reticulate & pitted vessels.
• Xylem fibres are irregularly intermixed.
Phloem
• It is found just below the sclerenchymatous
patches
• It consists of sieve elements, companion cells &
phloem parenchyma.
• The sieve elements appear as large cavities &
dispersed in isolated manner.
• Companion cells are relatively smaller in size &
found associated with sieve tubes.
• The phloem parenchyma & bast fibres are
absent.
Cambium
• It usually consists of number of layers of
meristematic cells.
• Lies inner to the phloem.
• The cells are thin walled & appear rectangular
or brick shaped (TS).
• This cambium is called intra-fascicular or
fascicular type.
• It is embedded in the vascular bundle.
Pith
• It occupies the centre part of stem.
• It consists of parenchymatous cells.
• The cells are rounded or polygonal, thin
walled with several inter-cellular spaces.
• Cells of pith store food material.
Medullary rays
• These are also called pith rays.
• The rays form elongated structure
• Which extends in between the two vascular
bundles.
• These are composed of thin walled, radially
elongated parenchymatous cells
• Involved in lateral translocation of food,
water & storage of food material.
Multiseriate Medullary ray
Monocot Stem
•
•
•
•
•
•
Zea mays (Maize)
Triticum aestivum (Wheat)
It shows three distinct regions
The epidermis
The cortex
The stele
Epidermis
• Single outermost layer
• Composed of small, compactly arranged, thin
walled somewhat barrel-shaped
parenchymatous cells
• The external surface of cells contain the
deposition of cutin, the layer is called cuticle
• Sometimes stomata are also found
• But epidermal hairs are absent.
Cortex
• Ground tissues do not show very distinct
demarcation of hypodermis & cortex.
• Hypodermis
• A few layers on the innerside of epidermis are
typically sclerenchymatous
• This region is called hypodermis
• The sclerenchymatous cells are compactly
arranged without intercellular spaces.
Ground Tissues
• The region lies below the hypodermis
• It consists of thin walled parenchymatous
tissues with a large number of intercellular
spaces.
• There is no differentiation of general cortex,
endodermis, pericycle, pith & pithrays.
stele
• Atactostelic type
• In which number vascular bundles are found
irregularly scattered.
• The vascular bundles are conjoint, collateral &
closed type.
• The peripheral bundles are relatively smaller
and compactly set
• But those of central region are larger in size &
widely placed.
Vascular Bundles
•
•
•
•
•
•
•
•
•
Oval in shape
Found surrounded by sclerenchymatous bundle sheath.
The xylem is Y-shaped.
The two metaxylem vessels with wider cavities and pitted
thickening lie at the two lateral arms
Protoxylem usually one or two with narrow cavities & spiral
or annular thickening at the base.
Below the protoxylem vessels, there lies a large water
containing cavity formed lysigenously by disintegration of
lowest protoxylem
It is called protoxylem cavity or lysigenous cavity.
Such a cavity is conspicuous in mature bundles.
Few tracheids & xylem parenchyma lie around protoxylem
vessels.
• Phloem lies outside the xylem & is rather
small.
• It is composed of only sieve tubes &
companion cells.
• Phloem parenchyma being absent.
• In a matured bundle, the protophloem cells
get crushed due to internal pressure
• So inner phloem is the metaphloem.