PLANT HISTOLOGY AND ANATOMY
PLANT TISSUES
Plant histology is a branch of
botany dealing with the study
of tissues.
The study of internal structure
of various parts of a plant is called plant anatomy.
There are two main groups of plant tissues:
I.Meristamatic tissues/embroyonic tissues/ undifferentieated tissues
II.Permanent tissues
Properties of meristamatic tissues:It is a formative tissue, made up immuture
1. They are actively dividing and
actively metabolizeing cells.
2. The cells are isodiametric in shape , some times may be polygonal
or fusiform,and are closely arranged
without intercellullar spaces.
3.The cells are small,immature,undifferentieted and embroyonic. The cell
wall is thin,homogeneous and made up of cellulose.
4. The eytoplasm is dense and
vacuoles are either small or absent, the nucleus is large and prominent.
5. the plastids are in the
proplastid stage, other cell organelles are less elaborate.
TYPES OF MERISTEMS:
Meristems are classified in three differant ways:
A) ACCORDING TO THEIR ORIGIN AND DEVELOPMENT
Are of 3 types.
i) PROMERISTEM(Primordial meristem):
Found at the tips of the root and stem and represents the earliest or youngest
stage of any devoloping organ.
ii) PRIMARY MERISTEM:
It is derived from promeristem and
on differentiation produces primary permanent tissues of the primary body of a
plant.
ex:-Vascular Cambium and apical meristems of root & stems.
iii) SECONDARY MERISTEM:-
Arise amongst non meristamatic tissues by a process of dedifferentiation
of permenant cells . They are
lateral in position position.ex:-
cambium of the root, interfascicular cambium and cork cambium.
B.ACC. TO THE POSITION THEY OCCUPY IN THE PLANTS:
i) APICAL MERISTEM:
Occur at the apices of root and stem.They are reffered as growing points
and responsible for increase in the length of roots and stems and bring about
primrary growth.
ii) INTERCALARY MERISTEMS:
They are intercalated or inserted i.e.held in between two mans of
permanent tissues. They are detached portions of the apical
meristems. This occurs at the base of internode and and leaf sheaths of
monocots and grasses. They are short lived.
iii) LATERAL MERISTEMS:
They occupy lateral or peripheral position, that is parallel to the sides
of the organs.They are responsible for increase in thikness or
diameter of stem and roots.
ex:- vascular cambium, cork cambium
C) BASED ON FUNCTION :- In the plant body.
i) PROTODERM:-(or protoderm
meristem) It is the outer most region
and differentiates into epidermal tissues system.
ii) PROCAMBIUM:- It develops into
primary vascular tissues- xylem, phloem and cambium.
iii) GROUND MERISTEM:-It gives rise to
fundamental or ground tissue system and
conxists of hypodermis,cortex, endodermis, pericycle,pith or medulla and
medullary rays
PERMANENT TISSUES
Are differentiated and spealized tissues ,with definite size, structure and
function. They are dirived from meristamatic tissues due to the process of
differentiation.
THERE ARE TWO TYPES
I. Simple permanent tissues-
are composed of similar cells .These are
of three types . Parenchyma,Collenchyma and Scelerenchyma.
II. Complex permanent tissues- are composed of
differant types of cells and are heterogenious, but their cells function as a
unit.
There are two types of:- 1. xylem 2. pholem
PARENCHYMA:-( Parenchein= to pour beside)
1. It is the most basic type and least spealized fundamental tissue found
in the plants, where cells are loosely aranged with abundent intercuelular spaces .
2.The parenchyma cells are isodiametric in shape ,or polygonal with many
sides .
3. Their cell walls are thin
,primary made up of cellulose only with simple pits. In storage tissues the
walls may become thick due to deposition of hemicellulose(ex: endosperm of date
palm).
4. They have active living protoplast with central vacuole and
peripheral nucleus.
5. A slightly thick walled,elongated parenchyma cells are called
prosenchyma.
FUNCTIONS
1.Hence it is the basic fundamental packing tissue,in which other
specialized tissues are embedded.
2.Parenchyma cells with chloroplast are called chlorenchyma & they
perform photosynthetic function,and are mainly found in the leaves.
3.Parenchyma cells can act as
storage cells and they can store various reserve materials in vacuoles in the
eyhplasm (such as strach grains).
specialized parenchyma cells which store engastic substances are called
idioblasts.
4.In some aquatic plants (hydrophytes), parenchyma cells
are modified, usually star shaped, with increased intercellular spaces.Thus
parenchyma with large air cavites is known as aerenchyma which help in the
circulation of air and provide buoyancy to aquatic plants (Nymphia, Hydrilla,
petiole Canna, banana etc.) are called aerenchyma.
5. Parenchyma cells occuring in xylem and pholem may help
in the conduction of water and food.
6. Thin walled parenchyma cells under fully turgid
(swollen)condition can provide support, especially in herbaceous plants.
7.Parenchyma that has dedifferentiated and reaquired the
power of division, functions as lateral meristem to produce secondary tissues.
8.Parenchyma cells that form the epidermis ,develop
cutinized outer walls & become protective in function.
TYPES OF PARENCHYMA
Simple parenchyma,Cholernchyma,Aerenchyma.
COLLENCHYMA(Gr.kolla=glue)
1.Collenchyma generally consists of living cells with vacuolated
chloroplast & peripheral nucleus. The cells are generally elongated
longitudinally with pointed or oblique ends.
2.They are extensible with some elasticity.Their cell
walls are thickend at the angles/ corners due to deposition of cellulose and pectin
with a high percentage of water. The cells are compactly aranged without
any intercullar space.
3)There are 3 types of collenchyma cells:
1.Angular collenchyma(annular)-the cells highly
thickened at the corners (ex:- Tagetes).
2.Lacunar collenchyma (tubular)-the alls are
irregularly arranged with intercullar spaces,and walls surrounding spaces are highly thickened and
appear as hollow cylinders.
3. Lamellar(plate)- the tangential walls (side
walls)of the cells are thickened
(ex:-sunflower
, Lycopodium)
Functions: Collenchyma generally occurs in the peripheral position in organs in
which it occurs. Hence its primary function is support, and is described as
living mechnical tissue.It also provides elasticity and tensile strength to
actively growing organs.
*In some tissues collenchyma posses chloroplasts,they
carry out photosynthetic function.
* Their cells may become meristamatic,and help in the
formation of secondary tissues.
* At the mangins of many leaves there are many patches of
collenchyma, which prevents cutting due
to action of wind and they also help to keep the leaf erect.
* Collenchyma cells are extensible wlith some
degree of elasticity, so tissues can provide tensile strength to the actively
growing organs.
SCLERENCHYMA (GR.skleros=hard)
*This tissue is made up of thick walled elements
(cells are uniformly thicknened)arranged without intercellullar spaces.
* The cells posses narrow lumen(cell cavity)without
living protoplast,hence at maturity it is a dead tissue.
*In the cell wall in addition to cellulose,there is
heavy deposition of lignin.Unthickened areas i.e. simple pits are seen in the cell walls.
THERE ARE TWO TYPES OF SCLERLENCHYMA CELLS
(1) Fibres (2)Sclereids
1)Fibres: are elongated
elements with pointed or oblique ends, with narrow lumen or cavity .
They are massed together in stands.
There are two types:
Intra
xylary/xylary fibres-associated with xylem
Extraxylary-not associated with xylem.
2)SCLEREIDS:- (sclerotic cells) Short variable in shape and size with very thick hard
strongly lignified walls with very
narrow lumen.They may occur singly it is called idioblasts and in a
groupscalled spicular cells or sclerites
THERE ARE 5 TYPES
1.BRACHY SCEREIDS:-(stone cells) cells are isodiametric in shape
& found in the soft parts of plants-like cortex,pith, pulp.
2.MACRO SCLEREIDS:-(Rodcells or malphigian cells)
are columnar (pillar like)forming palisade like layer in the seed coats of peas
beans etc.
3.OSTEO SCLEREIDS:-(PROP CELLS)-Bone shaped with dialatid ends-ex:-seed coats of many
monocoat seeds.
4.ASTROSCLEREIDS:-(STELLATE CELLS)-Star like
sclereids. seen in the intercullar spaces of leaves of stems of some
hydrophytes like Nymphaea.
5.TRICHO SCEREIDS:-(TRICHOBLASTS)-Are long
hair-like some times branched cells in the aerial roots of Monstera and
in the caves of Olea.
FUNCTIONS:-1.It provides mechanical
support ,and protection and rigidity
to the plant body.Hence it is described as deadmechanical
tissue.
2.Sclereids provide firmness and hardness to the plant body
ex:-endocarp of
coconut,shell of cashew nut,seed coats etc.
3.Sclerenchyma fibres occuring in the form of a sheath around vascular tissues in monocoats in the
pericylce of dicot
stems provide protection to the vascular bundles.
COMPLEX PERMANENT TISSUES
A complex (compound tissues)is a collection of different
types of cells,that help in the performance of a common function.There are two
types of complex permanent tissues in plants.
Xylem and Phoelem.
XYLEM:-(Water conducting tissue)(GR.xylos=wood)
This complex tissues is compared of both non-living and
living cells. The non-living elements
are of three types tracheids,tracheae, xylem fibres.The living cells
form the xylem parenchyma.
TRACHEIDS:-
1.A tracheid is a narrow elongated single cell with tapering (pointed/slanting/chisel-
like/oblique) end wall which are imperforated.(or closed)
2.Tracheids overlap one another at their(slanting)ends
and in the overlapping region pit pairs with pit membrane found .
3They do not have living proto plasm at maturity and are
dead cells,with a cavity( lumen) in the centre.
4. The cell walls are hard,thick and lignified.The secondary walls are provided with simple or bordred pits.Some exhibit various type of lignification such as annular (ring-like)spiral (spring like)reticulate(net-work-like)scalariform(ladder-like)and pitted(with depression)
4. The cell walls are hard,thick and lignified.The secondary walls are provided with simple or bordred pits.Some exhibit various type of lignification such as annular (ring-like)spiral (spring like)reticulate(net-work-like)scalariform(ladder-like)and pitted(with depression)
5. Tracheids occur in pteridophytes,gymnosperms &
angiosperms.
FUNCTIONS:-
1.The cells are perfectly adapted for cunduction of water
and mineral salts from the roots to the stem and leaves.
2.The thick and rigid walls of the tracheids help in
providing mechanical support to the plants.
TRACHEA/VESSELS
1.A vessel or trachea is a multicellular tube-like
structure.It consists of a series of short and broad individual cylindrical
cells called vessel members.(length is about 0.75-3.5 micron metres and width is about 300-700 micron metres )
2. These vessel members are joined end-to-end and
run parallel to the long axis of the plant body.
3. End walls of individual vessel members are either
perforated or absent .
4. Number of vessel members in a vessel varies from
2-indefinite.
5.Like tracheids, vessels are also dead at maturity with
lignified, thick secondary walls. Lignification is similar to that of
tracheids.
6.Hence lumen is wider than the lumen of
tracheids they are more efficient and advanced in water conduction than
tracheids.
XYLEM PARENCHYMA(WOOD PARENCHYMA)
Parenchyma cells anounciated with the xylem are called
xylem parenchyma,and is the living component
of xylem. They store water and food in the form of fat.
XYLEM FIBRE/WOOD FIBRES Are the sclerenchyma fibres assosiated with xylem.These provide additional
mechanical strength to the plant body.
[Depending on their origin and there are two types of
xylem in plants:
1. Primary xylem:-occur in the primary plant body and is
derived from procambium of apical
meristem.It is differrentiated into protoxylem(first to differentiated and
mature) and meta xylem(produced later)
Secondary xylem:-Develops from vascular cambium during secondary growth of the plant body].
PHLOEM (Phloos=bast): Is another vascular tissue which is specialized for conduction of
food(organic solutes), synthesized in the leaves to the other part of the plant
body.
Phloem is composed of sieve tubes,companion cells,phloem
parenchyma which are living and pholem fibres which is a non- living element.
SIEVE TUBES
1.Are slender tube like structures,(multicellular
in origin)made up of elongted elements, which are joined end to end are
called sieve tube elements/ members.(diametre of each elements is about
20-40 m & length is about
100-500 m)
2.They have thin walls made up of cellouse only,(but in some plants thickened with pectic
sustances).The end walls are thick and are called( 1-10
m in diametre) sieve plates. It has depressed wall areas called sieve
areas provided with groups of
perforations.
[such an end wall resembles a sieve and hence called sieve plate. Simple sieve plate has single
sieve area.Compound seive plate has many
sieve areas].
3.Sieve plate is not transerverse, but inclined
obliquiley.
4.Sieve tube members are living do not posses nucleus.
They have few mitochondria,SER,and plastids.They lack
vacuole,tonoplast,ribosomes and Golgibodies.Cytoplasm posses fibrillar protien
called P-protien. Cytoplasm also cotains amorphous bodies called slime
bodies.
5.In the sieve plates,each strand is enclosed in a
cylinder made up of special type of carbhohydrate called callose.
7.In dicots P-proteins of a sieve tube member,attached to
the sieve plate to form the slime plugs , generally absent in
monocots.
(sieve cells are narrow,elongated cells with
tapering ends.These cells have sieve areas throught the lateral walls & end
walls)
COMPANION CELLS:-
1.A companion cell is a thin walled elongated
parenchymatous cell having dense cytoplasm and a prominent elongated nucleus.
2.One or more companion cells may be associated with a
single seive tube member.
3.The cytoplasm of sieve tube member and that of
companion cell are connected by Plasmo desmata.
4.The sieve tube member is dependent on the companion
cell for its existence and the to form a
single functional unit, are derived from a single mother all.
PHLOEM FIBRE: Therse are sclerenchyma fibres associated with the phloem.
PHLOEM PARENCHYMA: These are living parenchyma cells associated with the phloem.
FUNCTIONS:-
1.Primary function is translocation (conduction) of
organic solutes.Hence it is a food conducting tissue.
2.Phloem fibres provide mechanichal support and
elasticity .
3.Phloem parenchyma stores organic foods like
strach,lipids etc.
TISSUES SYSTEMS:-All the tissues of a plant,which perform the same general function
irrespective of position and continuity,together form a tissue system.There are
3 types of tissues systems in plants.
1)EPEDERMAL/DERMAL TISSUE SYSTEM:
Forms the outer most covering layer of a plant organs.
2)VASCULAR TISSUE SYATEM:
Composed of xylem& phloem and is called vascular bundle(fascicular).
there are three types depending on their distribution.
i) CONJOINT:-Xylem&phloem are combined in the same bundle,along the same radius.
TWO TYPES:- A) Collateral bundle-xylem facing centre and
phloem
periphery.
Again two types a) Collateral & open- with
cambium(in between xylem &phloem).
b)Collateral
& closed -without cambium.
B)Bicollateral
bundle-with a layer of cambium
&phloem on each side of the xylem
ii) CONCENTRIC BUNDLE:
One vascular tissue surrounds the other.
If xylem is surrounded by the phloem - Hadro ecentric/Endroxylic/amphicribal
vascular bundle(e.g.ferns).
If phloem is surrounded by the xylem - lepto
centric/amphivasal/endiphloic (e.g.,Dracaena,Yucca )
iii) RADIAL:
Xylem & phloem lie on different radii alternating
with each other.
3)THE GROUND
TISSUES:-It forms the bulk of hte plant ground in between
epidermal & vascular tissue system.
Secondary growth in dicot stem
PRIMARY GROWTH:-Begins immediately after germination and involve formation of stem,roots
and leaves.Primary growth is due to activity of apical meristem and results in
the elongation of the plant body.
SECONDARY GROWTH:- It is increase in the girth or width in a woody plant,
due to production of secondary tissues,by the activity of lateral meristems.
These secondary tissues provide additional support to the
plant body.
MACHANISM OF SECONDARY GROWTH:-
Secondary growth is iniated and continued in two places
in the stem:-
1.Stelar region(vascular region) - due to activity of
vascular cambium
2.Extra stelar(cortical)region - cork cambium.
Secondary growth in the stelar
region
(1)Formation a cambial ring:-
The vascular or fasicular cambium is located
between xylem and phloem in the vascular bundle.This cambium is discontinous
because in between vascular bundles there are medullary rays. A complete ring of
cambium is produced,by the development of new interfasicular cambium.This
arises in line with the fasicular cambium by the meristamatic activity of some
of the medullary ray cells. This cambial ring is plenty fasicular &
partly interfasicular. After 2-3 years these two regions are
indistinguishable,and the term vascular cambium used.
(ii)Formation of secondary vascular tissues:-
(secondary xylem & secondary phloem)
Now cells of
the cambium divide anticlinally & periclinally to produce new cells
towards either side of the cambium.The cells produced on the innerside
soon mature and differentiate into secondary xylem(wood).This is produced
year after year during the life of the plant.It accumalates forming the bulk of
the plant body.The cells produced on the outside of the cambial ring
differentiate into the secondary Phloem.Less secondary phloem is produced
when compared to secondary xylem.
( iii) Formation of secondary medullary rays:-The secondary vascular tissues are interrupted by the radially arranged
ribbon-like parenchymotous rays,produced by ray initials.
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Annual rings:-The activity of the cambium is greatly influenced by seasons.This is
particularly so in the temperate countries,where the seasons are well marked.In
such a climate the cambium is more active in the favourable season that is spring,than
in the,summer and autumn.
The secondary xylem produced during spring is called
spring xylem/spring wood/early wood .This
has wider and more xylem elements(tracheids&trachea)with thin walls.
The secondary xylem produced during summer is called summerwood/autumn
wood/late wood.This has lesser number of narrow tracheary elements(xylem
elements) with thicker walls.
The spring wood and summer wood togther making up one
years growth constitute annual rings or growth ring.Such annual rings
are concentrically aranged in an old stem.
Heart Wood and SapWood:-
As a tree becomes old its trunk and lager branches show two zones,the central zone heart wood and periphiral zone sap wood.
As a tree becomes old its trunk and lager branches show two zones,the central zone heart wood and periphiral zone sap wood.
The heart wood is also called duramen,centrally
located darker in colour,due to presence of pigments. Xylem elements are
blocked by bladder like in growths
called tyloses.Some times heart wood rots away,leaving behind a hollow
cavity in the centre this is modified non-functional secondary xylem,does not
take part in conduction.
The sap wood is also called alburnum, is
periphiral in position, consists of younger rings of secondry xylem and still
retains conducting properties.It is lighter in colour when compared to heart
wood.
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Activity of the phellogen (cork cambium) :Extra stelar
secondary growth:-
1.Formation of cork cambium or phellogen:
Cells in the
cortical collenchyma or in the epidermis
on dedifferentiation, become meristematic and develop into the cork cambium.It
consists of a single layer of cells that can divide and produce new cells,
both towards inside and out side.
2.Formation of secondary cortex or phellordem:
Cells which are produced toward innerside become
parenchymatous, form phellodum or secondary cortex.
3.Formation of cork or phellem:
Cells which are produced on outerside soon die as there
is deposition of their primsuberin on their primary walls.These
suberized cells are compactly arranged without intercullar space.This tissue is
called cork or phellem.
These three new additional tissues in the cortex that is,
phellem, phellogen and phelloderm,together forms Periderm.
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Bark:-All the tissues lying outside the vascular cambium constitutes bark.It
includes periderm, pericycle,primary cortex, primary and secondary phloem.
*Lenticels:-Are lens-shaped raised spots developed on the stem.These are formed
just beneath th areas where stomata occur,and are formed during periderm
formation.(during secondary growth in dicot stems).
A lenticel constists of loose mass of colorless cells
called coplimentary cells. These are thin walled, (without
suberin)small, spherical produced from the phellogen.Spongy arenchymotous
nature of the lenticel readily allows an exchange of gases in between the
internal living cells and the atmospehere.
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[Bark and cork
of some plants are commercially important.
The bark of Cinchona is used for the extraction of
quinine which is used in treating malaria.Dried bark of cinnamom is used as a
spice.The cork of Quercus suber (mediterranean Oak)is used in making
bottle cork, cricket balls etc].
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