(I) PRIMARY STRUCTURE OF PLANTS
PLANT ANATOMY :
The branch of botany which deals with study of internal structures and organization of plants or plant organs.
N.Grew known as father of plant anatomy.
K.A. Chaudhary known as father of Indian plant anatomy.
Note : The anatomy of seed plants by Katherine Esau was published in 1960.
A group of cells which is similar or dissimilar in shape, having a common origin and usually perform a common function is called tissue.
The term tissue was coined by Nehemiah Grew.
The tissues are divided into two groups by Karl Nageli :-
CHARACTERISTICS OF MERISTEMATIC TISSUES
It is an undifferentiated tissue.
Cell cycle of meristem is in continuous state of division. Thus, meristematic tissue is composed of immature cells.
Meristematic cells have only primary cell wall which is thin and flexible ( elastic) and made up of cellulose. Secondary cell wall is absent.
Cells of meristem are small and isodiametric.
They have dense cytoplasm
They have prominent and large nucleus.
Normally vacuoles are absent in meristematic cells if present then they are small in size and Meristematic cells aremetabolically highly active so lack of reserve food in these cells.
Plastids are absent in meristems. If they are present, then only in the proplastid stage.
They do not have intercellular spaces. Cells are closely fitted ( packed) together. so it is a compact tissue
Ergastic substances are absent.
CLASSIFICATION OF MERISTEMATIC TISSUE :
[A] MERISTEMATIC TISSUE BASED ON ORIGIN AND DEVELOPMENT
On the basis of origin and development meristems can be divided into following three types :-
(I) PROMERISTEM/EMBRYONIC MERISTEM/PRIMORDIAL MERISTEM/URMERISTEM
This meristem develops in beginning during embryonic stage. They develop primary meristem.
(i) Primary meristem
Meristematic cells developed from promeristem are known as primary meristem.
These cells are always in division phase and form primary permanent tissue.
(ii) Secondary Meristem
These are the meristems developed from primary permanent tissues.
Some of the cells of primary permanent tissues become meristematic and constitute secondary meristem.
By the activity of secondary meristems, secondary growth takes place.
Note : Formation of meristem from any permanent tissue is called dedifferentiation
[B] MERISTEMATIC TISSUES BASED ON LOCATION (POSITION) IN PLANT BODY
On the basis of position, meristematic tissues are divided into three types :-
(a) Apical Meristem : The meristems which occur at the tips of roots and shoots and produce primary tissues are called apical meristems.They are responsible for increase in the length of plant organs. Example :- Root apex, shoot apex. They are responsible for primary growth.
Note : In embryonic stage, apex has two regions (i) Promeristem (ii) Eumeristem
Eumeristem is derived from promeristem.
According to Haberlandt , eumeristem (It is a primary meristem) is divided into three regions on the basis of function.
(i) Protoderm : It is the outer most layer of eumeristem. By the activity of protoderm epidermal tissue system is formed. It includes epidermis, root hair, stem hair etc.
(ii) Procambium : These cells are long and it gives rise to the vascular tissue system.
(iii) Ground Meristem : The cells of this region are large, thin walled and isodiametric. Ground tissue systemis formed by the activity of these cells. It includes hypodermis, cortex, endodermis, pericycle, pith-rays and pith.
(b) Intercalary Meristem :
The meristem which occurs between mature tissues.
This is the separated region of apical meristem.
By the activity of this meristem length of the plant organs increases.
They are present in some plants stem.
They may be present either at the base of internode e.g. grasses, bamboo and Equisetum etc, or at the base of node e.g. Mint. They are also present at the base of leaves e.g. Pinus. By the activity of this meristem, length of leaves increases.
Note :They are short lived and convert into permanent tissue.
(c) Lateral Meristem :
Lateral meristem occurs in lateral side of plant organ or parallel to thelongitudinal axis/Tangential plane of plant organ.
Activity of lateral meristem increases the girth of plant organ.
Lateral meristems are both primary and secondary in origin (mostly secondary in origin).
Primary lateral meristem : 1 . Marginal meristem 2 . Intra fascicular cambium
1. Marginal meristem :- It occurs at the margin of leaf. Its activity increases the width of leaf.
2. Intra fascicular cambium or fascicular cambium : This cambium occurs inside the vascular bundle. Except intra fascicular cambium all cambia are secondary in origin.
Secondary lateral meristems :– Cork cambium,vascular cambium and inter fascicular cambium.
Note : Lateral meristems are cylindrical.
[C] CLASSIFICATION BASED ON PLANE OF DIVISION
(I) RIB-MERISTEM / FILE MERISTEM :
Meristem in which anticlinal division occurs in one plane. For example, tunica is a type of rib-meristem.
Note : Formation of some cells of cortex and pith takes place by this meristem.
(II) PLATE-MERISTEM :
The meristem which divides anticlinally intotwo planes at right angle to each other. By this division a plate like structure is formed. Formation ofleaf blade takes place by the activity of this meristem.
(III) MASS-MERISTEM :
Meristem which divides in all possible planes resulting it the increase in the volume of plant body (organ).
Example : The formation of embryo and endosperm takes place by the activity of this meristem.
[D] CLASSIFICATION BASED ON RATE OF DIVISION
Cytohistological zonation theory :- According toFoster, meristem is classified into two region on the basis of rate of division :-
2. Flank Vegetative shoot apex :-
(I) SUMMIT :
The rate of division is slow in this region. This region is located at the apex.
(II) FLANK :
The rate of division is very fast in this region. This region lies behind the summit and leaf primordia are formed by this region.
Time period between initiation of two successive lea f p ri mordia is c al led "P la s toc hron".
Shape of vegetative shoot apex ® Conical or dome shaped
Shape of Reproductive shoot apex ® Broad & flat
Shoot apex isterminalin position
Growth of leaf primordium ® First apical then marginal.
Function of leaf primordium is to provide protection to shoot apex.
REPRODUCTIVE SHOOT APEX :
During reproductive phase i.e., at the time of flowering vegetative shoot apex transforms into reproductive shoot apex.This change of shoot apex is induced by florigen & light.
In reproductive shoot apex, Summit zone is more active and it forms stamen & Carpel and flank zone is less active and it forms sepal and petal.
COMPOSITION OF APICAL MERISTEM IN DIFFERENT PLANTS
Apical meristem is absent in lower Algae and Fungi. All the cells of these plants are divisible, so they do not show apical growth. Thus such type of growth in these plants, is called diffused growth. Diffused growth also occur in animals.
In some algae (eg. Fucus, Dictyota & Sargassum), Bryophytes and Some Pteridophytes (eg. Selaginella) apical meristem is consist ofsingle cell. This cell is known asapical cell. This apical cell is pyramid in shape and divide into two
In Ferns, Gymnosperms and Angiosperms apical meristem consist of many cells.
Several views have been available in relation to structure and growth habit of apical meristem.
 Apical Cell theory :
This theory proposed by Karl Nageli and Hofmeister and supported by wolff
According to Nageli and Hofmeister (mainly), the apical meristem is composed of single apical cell. This view is applicable on Bryophytes and some Pteridophytes and some algae (Ex. Fucus, Dictyota & Sargassum)
 Histogen Theory : It was proposed by Hanstein (1870).
According to him, the root and shoot apices are distinguished into three meristematic regions or three layers of histogen cells. These are as follows. (i) Dermatogen :- This is the outermost single layer of cells. These cells forms uniseriate epidermis by anticlinal divisions.
(ii) Periblem :- This region is situated just below the dermatogen. It forms cortex (Hypodermis, general cortex and endodermis).
(iii) Plerome :- This is the innermost region. Stele formation takes place by division of these cells. It means formation of pericycle, vascular bundles, pith rays or medullary rays and pith .
This theory is only true for root apex. It is not applicable for shoot apex of higher plants because in most of the gymnosperms and angiosperms, shoot apex is not differentiated into three histogens.
Including above described three histogens, a fourth type of histogen is also present in monocotyledon root apex. This is known as Calyptrogen. Root cap is produced by Calyptrogen in monocots. Root cap & epiblema is produced by dermatogen in dicotyledons.
Exception : One histogen present in Ranunculus and two histogens occurs in Casuarina.
Due to presence of root cap, position of root apex is sub terminal. so maximum growth in root takes place behind the apex.
1. In hydrophytes root cap is absent eg. Pistia
2. Generally, root cap is single layered but in pandanus (screw pine) root cap is multilayered.
3. Root cap contain more amount of golgi body which secrete mucilage which makes the root slimy.
A group of inactive or less active cells present between the dermatogen and calyptrogen or between active meristem and rootcap is called quiescent Centre. These cells contain less amount of DNA, light Cytoplasm, small nuclei and synthesis of protein is also less. Quiescent centre name coined by "Clowes". Quiescent centre was discovered in Maize root with the help of autoradiography.
Calyptrogen name proposed by Janczewski.
Note: The quiescent centre in the root meristem serves as a reserve for replenishment of damaged cells of the meristem
 TUNICA CORPUS THEORY
This theory was proposed by Schmidt (1924). This theory is applicable on shoot apex. This theory is based planes of division. According to this theory two types of layers are found in the shoot apex :-
(I) TUNICA :
This is peripheral layer. Epidermis is formed by this layer. In tunica cells,Anticlinal divisiontakes place only in one plane.
Surface growth of the plant organs increases due to anticlinal divisions.
Anticlinal division occurs at right angle to longitudinal axis (tangential plane) of cell.
When anticlinal division occurs in one plane, number of layers are not increased.
Generally, tunica is single layered, but some times it is multilayered, then the outer most layer forms the epidermis and remaining layers form rest types of the tissue system with the association of corpus. (II) CORPUS : The mass of cells present below the tunica is called Corpus. The cells of this zone divides in all directions (many planes) due to whichvolume increases. The cells of corpus usually larger than the cells of tunica.
Function : Formation of ground tissue system and vascular tissue system.
 MANTLE CORE THEORY
This theory was proposed by "Popham and Chan"for shoot apex. They compared mantle with the tunica and core with the corpus. Mantle forms epidermis. According to them corpus or core is distinguished into three zones –
(I) Sub-Apical Meristem :- This is present just below the mantle. It's function is re-establishment of mantle if mantle gets damaged.
(II) Central-zone Meristem :- This is the inner most zone. This zone is responsible only for formation of pith.
(III) Peripheral Meristem :- This region is only responsible for the formation of cortex, pericycle and vascular tissues.
 NEWMAN'S THEORY :
According to this theory meristematic tissues of shoot apex are three types -
(I) Monoplex :- Such type of shoot apex in which meristematic cells are arranged in groups in place of meristematic layers. Such type of shoot apex is found in Ferns.
(II) Simplex :- Such type of shoot apex is formed by single layer of meristematic cells. It is found in Gymnosperms.(Except-Gnetales)
(III) Duplex :- Such type of shoot apex is formed by two layers of meristematic cells. Peripheral layer is called tunica and inner layer is called corpus. Such types of shoot apex is found in Gnetales and Angiosperms. Tunica-corpus organization occurs in duplex type of shoot apex.
Note - According to Newman, the tunica and Corpus of shoot apex is not distinguished in Ferns and Gymnosperm (except Gnetales).
 KORPER-KAPPE THEORY :
It was proposed by Schuepp (1917). According to this theory, the cells of central and peripheral part of the root apex exhibit differences in planes of cell divisions.
In peripheral region each cell first divides transversely and there after the lower daughter cell divides longitudinally thus forming the shape of 'T'. Such divisions are called the Kappe divisions. In the central region T is inverted (^) as the second division takes place in the upper daughter cell. Such divisions are called the Korper divisions.
As a result of these 'T' or '^' divisions, the cells in root apex remain arranged in rows.
By Kappe divisions, the number of rows increases downwardly and by Korper divisions upwardly.
Permanent tissues are composed of cells which have lost the power of division temporarily or permanently.
They are formed by division and differentiation of meristematic tissues.
Their cells may be living or dead. Permanent tissues are of three types :-
A. Simple tissue (Homogenous tissue)
B. Complex tissues (Heterogenous tissue)
C. Special tissue
[A] Simple tissues These tissues are made up of similar type of cells or only one type of cells that perform a common function.
Simple tissues are of three types :-I. Parenchyma II. Collenchyma III. Sclerenchyma
I. Parenchyma : It is very primitive type of tissue. It is first evolved tissue. Remaining all different types of tissuesare derived from this tissue. so it is also called as fundamental tissue l Parenchyma name coined by Grew.
CHARACTERISTIC FEATURES :
It is a living tissue.
It is first differentiated tissue derived from meristem.
It is a universal tissue.
Flesh of a fruit is mainly composed of parenchyma.
Body of bryophyte plants is mainly composed of parenchyma.
Parenchyma is most common tissue which is morphologically and physiologically unspecialized.
The cells of parenchyma are thin walled. Cell wall is made up of pectocellulose (mainly cellulose). So parenchyma is a soft tissue.
Each cell containing large central vacuole. So the main function of a parenchyma cell is storage of food.
Usually Inter cellular spaces are present between cells of this tissue, it is a loose tissue. Intercellular spaces are usuallyschizogenous in origin.
The cells are usually isodiametric.
The cells of parenchyma are spherical, oval or polygonal in shape. Each parenchymatous cell contains 14 planes of lateral line, which are maximum possible plane in a cell. These are known as tetrakaidecahedron.
MODIFICATIONS OF PARENCHYMA :
1. Prosenchyma :- The cells of this parenchyma are long with pointed ends. This parenchyma forms the pericyle of roots. Function : Provide support to plant organ.
2. Aerenchyma :- This parenchyma is made up of rounded cells. These cells surrounds the large air chambers.
Air chambers are lysigenous in origin. Aerenchyma is found in cortex region. It provide buoyancy to hydrophyte plants.
3. Stellate parenchyma :
The cells of this tissue are stellate and branched. Air spaces are also present but they are less developed. Main function of this parenchyma is to provide mechanical support/provide mechinical strength to leaf bases of banana.
4. Chlorenchyma : Such type of parenchyma in which abundant quantity of chloroplasts are found. Two types of chlorenchyma are present in dorsiventral leaves :-
(a) Palisade tissues :- Inter cellular spaces are absent. Their cells are tightly fitted together. They are present towards adaxial/ ventral/upper side of leaf. Number of chloroplasts are more in palisade tissue as compare to spongy tissue. So upper surface of a leaf appears more green as compared to lower surface.
(b) Spongy tissues :- Large intercellular spaces are present. So they facilitates transpiration and gaseous exchange.They are present towards abaxial/dorsal/lower side of leaf.
5. Mucilage Parenchyma : In the Mucilage parenchyma large vacuoles and Mucilage will be found. e.g. Succulent xerophytic plants. e.g. Aloe. Function -storage of water.
Functions of parenchyma :
1. The main function of this tissue is storage of food.
2. Some cells of parenchyma store waste materials. They are called "idioblast cells". Idioblast cells store oils, tannin and crystal of calcium oxalate .
II COLLENCHYMA : Term coined by Schleiden.
Main characteristics :
Collenchyma is a living mechanical tissue. Lignin is absent.
It is made up of elongated (In transverse section cells are oval, spherical or polygonal in shape) cells.
localized deposition of pectocellulose (mainly pectin) & hemi cellulose is the characteristics feature of collenchyma.
Usually intercellular spaces are absent.
Vacuolated cytoplasm is found in the cells of collenchyma * Chloroplast may be found in the cells of collenchyma.
Collenchyma is not a universal tissue. It is found in the stems of herbaceous dicotyledons below epidermis either as a homogenous layer or in patches.
Collenchyma is absent in woody plant parts (After secondary growth in plant parts), roots and monocotyledons.
Collenchyma forms the hypodermis of dicotyledon stems. Cells of collenchyma are flexible due to hydrophilic nature ofpectocelluloseso flexibility occurs in dicotyledonae stems.
Lamina margins of leaves also bears collenchyma. This protects the cracking of lamina margin due to the action of wind.
TYPES OF COLLENCHYMA :
On the basis of place of deposition, it is classify into three types by majumdar :-
1. Lamellar/plate collenchyma :- The cells of collenchyma arranged in lamellar forms. The cell have thickening on the tangential walls. Due to such type of deposition, cell looks like a lamellar or plates. Ex. Sunflower stem.
2. Angular collenchyma :- This type of collenchyma abundantly found in plants. The cells of this tissue are angular. The deposition of pectocellulose at the angles of cell wall. e.g., Stem of Datura, Solanum and tomato
3. Lacunar collenchyma/tubular collenchyma :- Large intercellular spaces are present in the cells of this tissue. Deposition of pectocellulose on the walls of intercellular spaces. e.g. Cucurbita stem and aerial roots of Monstera.
Mechanical as well as biological. It provides tensile strength for extensibility of various growing plant organs/provide tensile strength/provide tensile strength against bending & swaying.
Due to the presence of chloroplast, photosynthesis process takes place in collenchyma.
III SCLERE NCHYMA
Name coined by Mettenius. – Main features :-
Sclerenchyma is the main mechanical tissue.
These cells are long, narrow, thick walled, lignified and dead.
It provide mechanical support/mechanical strength to plants.
Various types of pits are formed due to the deposition of lignin on hard wall.
Types of sclerenchyma On the basis of length of cells and amount of deposition of wall materials (Lignin). Sclerenchyma cells are of two types.
(i) Sclereids (ii) Sclerenchymatous fibres
I. Sclereids : These cells are small, extremely thick walled and their ends are not pointed. Sclereids are isodiametricorirregular in shape. Sclereids cells have more pits and lumen is almost very small.
Their pit cavity isbranched.
Sclereids are classify by Tschierch, on the basis of their shapes :-
(A) Stone cells or Brachy- sclereids or Grit cells:-These cells are spherical or oval in shape. They are found in endocarp of drupe fruits, so endocarp becomes hard. *
They are present in endocarp of Coconut, Mango, Almond, Walnut etc.
Brachysclereids are also present in fleshy (edible) part of pear (Pyrus). Grittness in pear fruit is due to these sclereids.
Brachysclereids are also found in pulp of guava' and sapota.
(B) Trichosclereids : These are also known as internal hairs. They are spines like, bifurcated cells. These are found in floating leaves of hydrophytes.
(C) Astro Sclereids or Stellate sclerenchyma : These cells are stellate (star) shaped. They are found in floating leaves.
Example :- Both Astro and Tricho sclereids are present in floating leaves.
Note – Astro sclercids are also found in tea leaves.
(D) Macro-sclereids or Rod cells or Malpighi cells :-They are small and rod like cells. They are present in seed coats. Ex. legume seeds.
In leguminous plants, hardest seed coat is found in french bean (Phaseolus vulgaris).
(E) Osteo- Sclereids :-These are known asprop-cells. These arepillar like cells. Both end of pillar like cells spreads to form bone like structure.
Example :- These cells are found in leaves of Hakea and Osmanthus.
II SCLERENCHYMATOUS FIBRES
These cells are fibrous. They are longest cells in plant body. Their both ends are pointed (tapering). Due to thick cell wall, lumen is reduced.
Their cell wall contains simple and bordered pits.
On the basis of structure fibres are classified into two groups :-
(a) Libriform fibres :- They are highly thickened long fibres. They posses simple pits and narrow lumen.
Libriform fibres are found in phloem, xylem, pericycle and hypodermis (Maximum in phloem).
(b) Fibre Tracheids :- They are also highly thickened. Bordered pits are present in these fibres and lumen is broad. They are only found in xylem.
On the basis of position, fibres are divided into three types -
A. Surface fibres :- They are present on the surface of plant. These fibres also called as filling fibres.
(i) Seed surface fibre
-Example 1 : Cotton fibres - Cotton fibres are out growth of seed coat/out growth of testa. Cotton fibres are composed of cellulose. They are non-lignified. So cotton fibres are not true fibres. Two types of fibres are found in cotton. Long fibres are called 'lint' and small fibres are known as'fuzz'. Lint fibres are used in cloth industry. Fuzz are filling fibre.
Example 2 : Red silk cotton (Semal fibre) - Obtained from Salmalia malabaricum
Example 3 : White silk cotton (Kapok) - Obtained from Ceiba pentendra (Both red and white silk cotton fibres are not true fibres and they are also an example of seed surface fibre.)
(ii) Coir of coconut is also a type of surface fibre. They are derived from the mesocarp. These are true fibres, becouse they are lignified.
B. Xylary or wood fibres :- These are hard fibres. They fibres are not flexibe. They can not be knitted (weaved) easily so they are not useful. These are obtained from xylem.
Ex. Munj fibre (Saccharum munja)
C. Bast fibres/Extra xylary fibres/Phloem fibre :- These are also known as commercial fibres. These fibres are flexible and can be knitted (weaved) easily. They have great economic value.
These fibres are obtained from the phloem and pericycle of plants.
The bast fibres of Corchorus capsularis.(Jute),Crotalaria juncea (Sunn hemp) and Hibiscus sabdariffa (patua) are obtained from the secondary phloem of stem.
The bast fibres of hemp (Cannabis sativa) and Linum usitatissimum(flax) are obtained from the pericycle.
Fibres which are obtained from pericycle are called perivascular fibres.
Leaf fibres Þ Manila hemp (Musa textilis) and agave hemp (Agave sislana):- These are obtained from sclerenchymatous bundle sheath.
[B] Complex Permanent Tissue
The complex tissues are made of more than one type of cells and these work as a unit. Complex tissue are heterogenous.
Complex tissues are absent in gametophytes.
During vascularisation in plants differentiation of procambium followed by the formation of primary phloem and primary xylem simulta neously.
Complex tissues are of two types - (a) Xylem (b) Phloem
The term 'Xylem' is coined by Nageli.
The function of xylem is to conduct water & minerals salts upwards from the roots to stem & leaves and to give mechanical strength to the plant body.
For conduction of water death of protoplasm is must. Dead tissues are more develop in water scares condition.
On the basis of origin, xylem is divided into primary xylem and secondary xylem.
1. Primary xylem originates from procambium during vascularisation. On the basis of development primary xylem divided into two parts (1) Protoxylem (2) Metaxylem
Cells of protoxylem are small as compare to metaxylem
2. Secondary xylem originates from vascular cambium during secondary growth.
The elements of xylem are
1. Tracheids :
Tracheids are primitive conduting elements of xylem.
A single tracheid is a highly elongated cell with hard, thick and lignified wall and a narrow lumen. The ends of tracheids are tapering or chisel like.
The tracheids found one above, the other are separated by cross wall which bear bordered pits.
Tracheids are dead and lignified cells. The deposition of lignin on cell wall is responsible to form a different type of thickenings.i.e., annular (primitive type), spiral, scalariform, reticulate and pitted.
1. Usually bordered pits are present at the end wall of tracheids.
The maximum bordered pits are found in the tracheids of Gymnosperm plants.
2. Maximum deposition of lignin is found in pitted type of thickening and pits are formed in this type of thickening.
3. Annular and Spiral type of thickening of lignin is found in protoxylem.
4. Reticulate and Pitted (mainly) type of thickening of lignin is found in metaxylem.
5. Scalariform type of thickening is found in metaxylem tracheids of pteridophytes and in metaxylem tracheids of Cycas.
6. End wall of tracheids are imperforated/pitted.
7. Tracheids are unicellular.
Note: Pits are unlignified areas on lignified wall
(2) Vessels :
It is an advance conducting element of xylem. Vessels is a long cylindrical, tube like structure with lignified walls and, a wide central lumen.
The end wall is perforated (Transverse septum is absent between two vessel elements. If present then porous.) Thus vessels are more capable for conduction of water than tracheids. Due to presence of perforated end wall, vessels work as a pipe line during conduction of water.
The perforation may be simple (only one pore) or multiple (several pores). Vessels contain usually simple pits on their lateral walls. Types of thickening on the walls of vessels is the same as tracheids.
1. Vessels are found in most of the angiosperm but also present in some gymnosperms like Ephedra, Gnetum and Welwitschia.
2. Vessels are absent in some Angiospermic plants such as Dracaena, Yucca, Dagenaria, Drimys.
There are some angiosperm families in which vessel less angiosperms are included. e.g.Winteraceae, tetracentraceae and trochodendraceae.
3. Vessels are example of dead syncyte.
4. Vessels are multicellular.
Syncyte : Cell which is formed by fusion of cells, called as syncyte
3. Xylem fibres :
Xylem fibres provides strength to the tracheids and vessels. Mainly these fibres provide strength to the vessels.
They may be either septate or aseptate. They have obliterated central lumen.
They are abundantly found in secondary xylem.
4. Xylem Parenchyma :
It's cell wall is made up of cellulose.
Function : Radial conduction of water and store food material in the form of starch or fats and tannin etc.
Their walls possess pits.
Note :Function of ray paranchymatus cells - radial conduction of water.
Hadrom : – Tracheids and Vessels are collectively known as water conducting elements or "Hadrom". Hadrom term was proposed by Haberlandt.
(b) PHLOEM :
The term 'Phloem' is coined by Nageli.
The main function of the phloem is to conduct of food materials, usually from the leaf to other plant parts (eg. storage organ and growing regions) On the basis of origin, phloem is classified into two categories primary and secondary phloem.
Primary phloem originates from procambium during vascularisation and secondary phloem originates from vascular cambium during secondary growth.
On the basis of development primary phloem categorised into protophloem and metaphloem.
The protophloem has narrow sieve tubes whereas metaphloem has bigger sieve tubes.
Phloem remains active for less duration as compared to xylem.
Phloem consist of4 typesof cells :-
1. Sieve cell / Sieve tube
In Gymnosperms and pteridophytes In Angiosperms
Sieve element was discovered by Hartig.
Sieve cells/sieve tube element are living and thin walled.
Mature sieve tube elements are enucleated living cells.
Central vacuole is present in each sieve cells/sieve tube element and around the central vacuole thin layer of cytoplasm is present.
In Angiosperm plants, sieve tube elements are joined their ends and form sieve tube. Their end walls are perforated (Sieve Pores) in a sieve like manner to form the sieve plate. Translocation of food material takes place through these pores.
Sieve elements contain special type of protein-P-protein (p-phloem). Most likely function of p-protein is sealing mechanism on wounding and it is aslo related with conduction of food.
1. Food conduction is bidirectional in sieve tube.
2. In sieve tube, oblique transverse perforated septa (sieve plates) are present at their end wall.
3. Sieve tube is an example of living syncyte.
2. Companion cells : These are thin walled living cells. The sieve tube elements and companion cells are connected by pit fields present in their longitudinal walls, which is common wall for both and, with the death of one, other cell also dies.
A companion cell is laterally associated with each sieve tube element in Angiospermic plants.(In carrot more than one)
Sieve tube element and companion cell originates together. Both of them originates froma singlemother cell. So called as sister cells.
The companion cells and sieve tube elements maintain close cytoplasmic connections with each other through plasmodesmata.
Companion cell is a living cell with large nucleus. This nucleus also controls the activity of cytoplasm of sieve tube element.
Companion cells are only found in Angiosperms. (exception - Austrobaileya is angiosperm plant but companion cells are absent).
Special type of cells are attached with the sieve cells in gymnosperm (mainly) and in pteridophytes in place of companion cells. These cells are called as albuminous cells/ strasburger cell.
(1) Albuminous cells in conifers are analogous to companion cells of angiosperms. They are modified phloem parenchyma cells.
(2) The companion cells play an important role in the maintenance of a pressure gradient in the sieve tubes.
(3) In phloem of pteridophytes and gymnosperms sieve cell are present which are comparable to the tracheids.
3. Phloem fibres / Bast fibres : These are made up of sclerenchymatous cells.
These fibres are generally not found in primary phloem.
These fibres provide mechanical support to sieve elements (sieve cells and sieve tube).
4. Phloem Parenchyma :
It's cells are living and thin walled. It store various materials. eg. Resin, Latex, Mucilage etc.
The main function of phloem parenchyma is conduction of food in radial direction and storage of food. The conducting element of pholem is called Leptom.
Leptom term was proposed by Haberlandt.
(1) Phloem parenchyma is absent in the stems of monocotyledon plants.
(2) Phloem Parenchyma is absent in the stems of Ranunculaceae plants (dicot family).e.g. Thallictrum
SPECIAL TISSUE OR SECRETORY TISSUE
I. Laticiferous tissue :
These are made up of long, highly branched and thin walled cells. These cells are filled with milky juice, called as Latex.
Latex is the mixture of saccharides, starch granules, alkaloids, minerals and waste materials.
Starch granules present in latex are dumble shaped.
(1) Latex provide protection to the plant.
(2) It prevents the plants from infection of bacteria and fungus. In laticiferous tissue there are two types of cells1. Latex vessels and 2. Latex cells
1. Latex vessels :
These are articulated.
These are syncytic cells and coenocytic (multinucleated).
Example :- Latex vessels are present in Hevea, Ficus, Papaver, papaya, Argemone and Sonchus.
Highly developed latex vessels are found in the fruit wall of unripe fruit (capsule) of Poppy
Opium is obtained from the latex of Papaver somniferum. It contains an alkaloid named as morphine.
An enzyme papain is obtained from the latex of papaya (Carica papaya).
Para rubber/Commercial rubber is obtained from Hevea bransiliensis and Indian rubber is obtained from Ficus elastica Mostly latex is white in colour but in some plants latex is coloured.
Ex. Papaver – Dark brown
Argemone Yellow colour
In some plants latex is colourless Ex. Banana (Family - musaceae).
2. Latex cells :
These are non articulated. These are long, branched and multinucleated cells (coenocytic cells).
Example - Latex cells are found in Calotropis, Euphorbia and Nerium (Apocyanaceae), Ficus religiosa Note : Latex vessels and latex cells are found in cortex.
II. Glandular tissue :
As the name indicates that this tissue is made up of glands. These glands contain secretory or excretory materials.
Glandular tissues have two types of glands :-
(1) Unicellular. Ex. - Urtica-dioica. These cells are present on the surface of the leaves. These are spiny glands.
(2) Multicellular : Multicellular glands are of two types.