Types Of Transpiration

Transpiration in Plants and its types

The evaporation of water from the aerial part of plant is called transpiration. Transpiration is not a purely ph) sical process like evaporation. It is a vital phenomenon. It plays important role in the cell. 11e amount of water lost In transpiration is very large. It exceeds the final dry weight of the plant In se‘eral hundred times • A corn plant during its life of a fen necks transpires a large amount of water. I his water can till a barrel. Similarl . medium sited tree may lose more than [I ton of water every day. it has also been estimated that every ounce of dry matter in the aerial parts of a crop plant loses 20 —50 pounds of water by transpiration.

Demonstration of transpiration

A potted plant is placed under a bell jar. the soil in the pot is watered and the pot is covered with rubber sheet or the soil. The wails of the pot are coated %kith paraffin. After a short time. drops of water %% ill appear on the inner surface of the bell jar. Iliis water has come from the aerial parts oI the plant.


There are two type of transpiration: lot jar transpiration and lenticular transpiration.

  1. Foliar transpiration

the transpiration which takes place through the lea’ es is called foliar transpiration. The foliar transpiration may be stomata’ transpiration or cuticular transpiration.

(a) Stomata transpiration: The transpiration which takes place through he stomata is called stomata’ transpiration. The lower part of leaf has the loosely packed spongy mesophyll cells. Flicy have a number of large intercellular spaces. These air spaces communicate ith the outside atmosphere hy means of stomata in the leaf epidermis.

The xylem of the leaf vein suppls water to the cells of the mesophyll by osmotic diffusion. Mcsopbyll cells become turgid and saturated with water. Water evaporates from their moist walls into the internal atmosphere of the intercellular spaces of the mesophv II. It becomes saturated with water vapour. Finally water vapours move out of stomata.

(h) Cuticular transpiration: The transpiration which takes place though the cuticle of the leaf is called cuticular transpiration. Cuticle is present on outer walls of the epidermal cells. The cuticle is not permeable for water. Therefore. the amount of water lost through it is small. It is only 3 to 10% of the total transpiration. Cuticle

transpiration depends upon the thickness of the cuticle. The cuticle is poorlv or not at all developed in herbaceous plants growing in humid and shads places. Therelore. their cuticular transpiration becomes equals to the stomata! transpiration. Transpiration from stems, fruits and Bower parts is most!) cuticular.

  1. Lenticular transpiration

tenticels are pore like structures in the harks of the woodv stems. Some transpiration also takes place through the lent eels. It is called lenticular transpiration.

Environmental Factors That Affect The Rate Of Transpiration

Following factors affect the rate of transpiration:

(a) External factors

  1. Humidity of air

Water is evaporated through the stomata. This follows the simple law of diffusion. ‘Ibis diffusion can take place only if the water vapor content of the outer atmosphere is less than that of the inter-cellular spaces of the leaf. Transpiration is negligible in an atmosphere saturated with water. ‘Me drier the air. the more rapid is the transpiration. Following factors affect the humidity of air:

  • Saturation deficit: Saturation deficit is the difference between the

amount of Ysater vapor actuall) present in the air and the amount necessary to completely saturate it. The rate of transpiration depends upon the saturation deficit.

  • Absolute humidity: ‘Hu: amount of moisture actually present in the air is called its absolute humidity
  • Relative humidity: The percentage of the amount of moisture

necessary for saturation at a particular temperature is called relative humidity. The relatfte humidity decreases (or increases) with every rise in the temperature. But the absolute humidity remains unaffected. The rate of transpiration increases at lower relative humidity of the air and vice versa.

  1. Temperature

Temperature affects the saturation deficit of the air. So it has an indirect influence on the rate of transpiration. Rise in temperature decrease the relative humidity of the air. It increases the rate of transpiration. Low temperatures decrease the capacity (lithe air to hold moisture. Therefore, it increases the. relative humidity. It decreases the rate of transpiration. ligh temperatures also open the stomata widely. It increases the

  1. Wird

The moving air is called n intl. Wind has a [ton erlid effect on hum idit) Dry nind rea ‘es the moist air from the immediate %kinks of the

plants. It lour, • amount of air moisture near plant. As a result, the
humidity of the air isitinered. It promotes transpiration.

  1. Light

The rate of transpitation increases in light and decreases in the dark. Light affects transit ration in tuo nays. It raises the temperature of the lea es and increases the transpiration. Secondly. shun:Ha are opened in the presence of light. It increases transpiration.

  1. Atmospheric pressure

Low air pressure reduces the densitn of the air.’Ibtis it increases the rate of transpiration. Air pressure is lonered at high altitudes.

  1. Availability of Soil Water

‘there should be enough uater absorbed by the roots from the soil. It n ill compensate the stater lost by transpiration. When there is loner absorption of water. there n ill be to rate of transpiration. the factors hie!’ affect the absorption of %%titer also ailed the rate of transpiration.

(b) hrtentel hatees

I. Role of stomata: Most of the transpiration takes place through stomata. transpiration depends on the opening anti closing of stomata. Di Ilerent factor affect the opening and closing of stomata.

  1. Water relation of the Parenchyma cell: The parench) ma mesophyll cells also control the rate of iranspimtion. they become – saturated oith ;safer. Thus their nails easily • lose nater into the internal atmosphere of the leaf ibis loss of nater is compensated b) absorption of mato- from the root. If root cells do not absorb much %% titer then the %%titer content of the mesophyll cells decreases. .Fherelbre. mesophyll cells loss turgor. Their cell nails become tip. thus the evaporation trout their surfaces is reduced although the stomata remain open. As a result osmotic pressure of the memmhyll cells increases. thus they n ithdrann ater from the guard cells. the guard cells lose their turgor. Ilms stomata are closed Oen in the presence tn. HOB. Thus the inter nil %%Mel’ relation of the leaf are self regulating mechanism for the control of transpiration
  2. Number of stomata per unit area: the number of stomata per mitt area of leaf surlitce and their position also affect the rate of

transpiration. The stomata are sunken in depression bet. the epidermis in some plants like oleander and Pinus. It reduces the rate of transpiration.

4. Thickness of cuticle: Cuticular transpiration depends upon the thickness of cuticle. If cuticle is thin. there is more transpiration and vice versa.



The chromosomes which determine sex in the different organisms are called sex chromosomes. ‘Fhe term sex refers to the processes that enable species to exchange materials between homologous chromosomes. Ii causes recombination. Recombination is essential to their mechanism for reproduction.

Work on inheritance of sex started after discovery of Menders ork in 1900. The discovery of sex chromosomes revealed the genetic basis of sex determination.



Discovery of Sex chromosomes in drosophila

The fruit fly. Drosophila inelanogaster has eight chromosomes. These chromosomes are present in the form of four homologous pairs. T.H. Morgan in 1911 found difference in the chromosomes of male and female Drosophila. The chromosomes of the three homologous pairs were similar in both of the sexes. But the fourth was heteromorphic pair and it had different structures. The female has two similar rod shaped X-chromosomes in the fourth pair. But male has one rod shaped X-chromosome but the other a morphologically different. J-shaped Y chromosome in the ‘Mirth pair.

Sex chromosomes: X and V chromosomes are called sex-chromosomes. These chromosomes have genes for determination of sex.

Autosomes: Chromosomes of the other three pairs are autosomes. All chromosomes other than sex-chromosomes are called autosomes. Autosomes do not carry any sex determining gene.

Sex chromosomes in humans

I Ionians have 46 chromosomes. These are present in 23 pairs. 22 pairs are of autosomes and one pair is of sex-chromosomes. Autosome pairs are common in both the -sexes. But the 23 rd sex chromosome pair is different in males and females. A woman has two similar X chromosomes in her 23′ pair. But a man has an X chromosome and a much shorter Y chromosome. The 23rd pair in man is heteromorphic.

(a)      The female in human is XX

(b)      The male is XY

Sex chromosomes in grasshopper

Males and females have different number of chromosomes in some grasshoppers.

(a)         The female has 24 chromosomes. These chromosomes are present in the form of I I pairs of autosomes and a pair of X chromosomes. Female is XX.

(b)         The male grasshopper has 23 chromosomes. The male grasshopper has I I pairs of autosomes and only one X chromosome. The other sexchromosome is entirely missing in male. Thus male is X0.

Bentham and Hooker Classification system

1 his system was proposed by George Bentham and Joseph Dalton Hooker. They proposed natural classilication system. .[heir system as published ill Genera Plantarum. They divide the seed plants into 202 orders. .[heir system was chiefly based on De Condone. Advantage of Bentham and Booker Classification system:

  1. . It is oased on natural classification system. It changed the artificial classification system.
  2. . It differentiated between seed producing and non- seed producing plants.
  3. It mostly deals with flowering plants (Angiospermic plants).
  4. It covers a large number of plants.
  5. The criteria for. this system is vegetative characteristics (herbs. shrubs or trees), presence or pith. cambium, arrangement of leaves, leave venations, presence of corolla. calyx, perianth. stamens and carpels.

Disadvantages of Bentham and Hooker Classification system: I his system is based on natural classification system. It uses set of morphological characteristics. But this system failed to give concept of phylogenetic relationship. therefore. it is only a modified form of • Linnaeus system. Thus this system was not accepted by many taxonomists.

Scheme of Bentham and Hooker Classification system:

This classification system divided the plants into two major groups. Each group has sub-groups. Each sub-group is divided into divisions. Each division is di’ ided into series. Each series is divided into cohorts. Finally each cohort is di % ided into many orders.

Carolus Linnaeus Classification system

The classification system of Linnaeus was based on floral characteristics. He chiefly used characters of stamen’s. Therefore, his system is called sexual system. This system is very convenient for the identification of plants. I le presented his classification system in two books. These books are Species Plantarum and Genera Plantarum.

Advantages of Linnaeus system: Linnaeus was first to introduced some system of classification. It gave some basic knowledge of classification. So it was widely accepted by the taxonomists of that time. It provided a reference work. He gave clear cut concept of species. This concept is still used today.

Disadvantage of Linnaeus system: Linnaeus declared his system as artificial or temporary. He realized that that this system should be changed with some natural system. Linnaeus grouped some unrelated plants due to similarity in flower structure. Similarly. Linnaeus was a religious person.opposed the concept of evolution in classification.

Scheme of Linnaeus system: Linnaeus divided be plants into twenty four classes. He mostly used the characteristics of stamens to differentiate between classes. He also used tne conditions of perigynous, hypogynous, dynamous, filaments fused or not fused, number of fasicles anther attachment, presene of male and female flowers. These classes are:

I. Class Monandria: They have single stamen. Examples: Canna

  1. Class Diandria: They have two stamens. Example: Salva
  2. Class T Hand ria: They have three stamens. Example: Poa
  3. Class Tetrandria: They have four stamens. Example: Cuscuta
    1. Class Pentandira: They have five stamens. Example: Daucus
    2. Class Hexandria: They have six stamens. Example: Rumex
    3. Class Heptandria: They have seven stamens. Example: Aesculus

    4. Class Octandrit They have eight stamens. Example: Sapindus

    5. Class Enneandria: They have nine stamens. Example: Rheum

    6. Class Decandria: They have ten stamens. Example: Silene

    7. Class Dodecandria: They have 12-19 stamens. Example: Euphorbia

    8. Class Icosandria: They have 20 or more stamens and hypogynous condition. Example: Cactus.

    9. Class Polyandria: They have 20 or more stamens with hypogynous condition. Example: Ranunculus

    10. Class Didynamia: They have didynamous stamens. Example: • Bignonia

    11. Class Tetradynamia: They have tetradynamous stamens. Example: Trifolium.

    12. Class Monodelphia: They have fused filament with one fasicle. Example: Sida
    1. Class Diadelphia: They have fUsed filament with two fasieles. Example: Polygala

    2. Class Polydelphia: Their stamens are fused with several fasicles. Example: Citrus

    3. Class Syngenesia: They have anther connate. Example: Viola

    4. Class Gynandria: Their stamens are fused to their pistils. Example: Orchis

    5. ..Class Monoecia: They have male and female flowers on same plant. Example: Belida

    6. Class Dioecia: They lune male and female flowers on different plant. Example: Najas

    7. Class Polygamia: Their Ilmsers are polygamous. Example: Pupil

    8. Class Cryptogamia: Flowers are absent or not apparent. Example: Fucus

    Classification system of Jussieu

    This system was proposed by Bernard de Jussieu and Laurent Jussiett. They introduced Natural classification system. They divided the plants into 100 orders and 15 classes. They recognized the difference between Monocotyledons and Dicotyledons. They separated the cryptograms (non-seed producing) from seed producing plants. They divided the plants into three groups:

        I. Acotyledons: They are non-seed producing plants.

    1. Monocotyledons: They hat c single cotyledons.
    2. Dicotyledons: They have two cotyledons.

    Augustine Pyrame de Condone improved the Jussietis system. I le divided the plants into 213 orders. lie used morphological characteristics to differentiate between these orders.


Epidermis Diagram

Epidermis Diagram


Epidermis forms the outermost layer of cells on the primary plant body. It is present over stem. roots, leaves, flowers and fruits. In most plant. epidermis is composed of single layer of cells. But two or several layers are also found in different plants. When it is composed of single layer, it is called hypodermis. When it is composed of many layers. it is called multiseriate epidermis. The root epidermis is called rhizodermis or epihlema. The main function of epidermis is to check the transpiration. It also protects the plant parts. It also has stomata for gaseous exchange.

Cell structure in epidermis

Epidermis is composed of single layer of cells. These cells are tubular. They have different shapes. Their shape may be isodiametric. elongated, t‘avy or rectangular. The epidermal cells are compactly arranged. The epidermal cells are highly vacuolated. Some leucoplasts are also present in it. Chloroplasts are absent in epidermis except guard cells. The ep:dermal cells contain many mitochondria. dietyosomes and ER. :the cell sap of epidermal cells of many flowers contains pigments called anthocynins. The outer walls of the epidermal cells become thick by the development of secondary wall. The outer walls have remnants of plasmodesmata called ectodesniata. Ectodesmata allow the passage of certain substances that are discharged through the cuticle. The redial and inner tangential walls of epidermal cells are thin and possess plasmodesmata.

Wall of epidermal cells

The Ns al I of epidermal cells bear cuticle. A thin layer of cutin on the epidermal cells is called cuticle. Cutin is deposited on the outer walls of epidermal cells in aerial parts. Cutin is absent in underground parts. The protoplasts of the epidermal cells secrete cut in Clain makes the Nall of epidermal cells impermeable. It checks the transpiration. It also provides some support to cell. Sometimes. waxes are also deposited in the cutinited ‘vall. It makes the surface totally impermeable. Calcium salts and silica are also deposited in the walls of some plants. They make the plant surface rough. .•pidermal cells form different structures in seeds and fruits. They form hard shells in seed coats. In some cases, it forms mucilaginous layer for attachment. In some cases, it develops hairs for dispersal.

monocot throughout life. But in monocot. they disappear after secondary growth.

Types of Trichomes

Types of Trichomes


Frichomes are different types of appendages. Trichomes are commonly present on the surface of epidermis. They are present in the thrm of hairs, papillae and water absorbing roots, triehomes occur in all parts of the plant. Triehomes are often used as taxonomic character. The triehomes may be unicellular or multicellular. They may be glandular or nor-glandular. The scales or hairs may be peltate, tufted, stellate or branched. Hairs of the cotton seeds are unicellular.

1. Non-glandular trichomes: ‘FMy are simple hairs like. These hairs may be unicellular or multicellular. Such hairs are found in cotton.

2. Glandular trichomes: Glandular trichomes are called glands. They secrete different types of compounds. These compounds may be nectar (sugar solution), salt solution, gums. waxes etc. All glandular trichomes have endodennal cells below the secretory cells. Endodennal cells prevent the backflow of secretion. There are different types of glandular trichomes:

  1. Salt secreting trichomes: Examples: Atriplex. Cicer
  2. Mucilaginous secreting glands: Examples: Rheum •
  3. Nectar secretory glands: Example: rose
  4. Glandular trichomes in carnivorous plants: Example: Drosera

  5. Sticky substance secreting trichomes: Example: onion



Stomata are present in epidermis. Each stoma is bound by a pair of guard cells. These guard cells are bean shaped. The guard cells are

rich in chloroplast and starch grain. Stomatal aperture is simply a space between the two guard cells. The stomatal aperture, guard cells and subsidiary cells form the stomata] apparatus. The guard cells are produced by the vertical division of a single stomatal mother cell. The variation in turgidity in the wall of guard cells cause opening and closing of stomata. Exchange of gases takes place through stomata. When the walls of guard cells are fully turgid, the stomata open. When the walls of guard cells deflated, the stomata close.

In certain cases. the guard cells have one or more subsidiary cells. Each type of plant has characteristic number of subsidiary cells. Subsidiary cells are distinct from other epidermal cells. There are four main types of stomata:

  1. Anomocytic stomata: In this case, the guard cells are surrounded by certain number of cells. These cells have similar shapes. Example: buttercup.

  2. Anisocytic stomata: In this case, the guard cells are surrounded by three subsidiary cells. These cells have different sizes. Example: potato.

     3 Pai-acytic stomata: In this case. each guard cell is surrounded by one or more cells. The longitudinal axis of these cells is parallel to the guard cells and apertures. Example: onion

  1. Diactyic stomata: In this case. each guard is surrounded by two subsidiary cells. The common wall of these cells is at right angles to the longitudinal axis.

  2. Actimicytic stomata: In this case, the stomata are surrounded by circle of radiating cells.

Xylem and Phloem


Xylems are non-living conducting tissues. They conduct water and dissolved salts from root to different parts of plant. Xylem also forms \S ood in plants. It supports plant body. Xylem tissues are present  only in tracheophytes. Composition of XylemXylem is composed of thllow Mg types of tissues

only in tracheophytes.

Composition of Xylem

Xylem is composed of thllow Mg types of tissues:

I. Tracheary elements: The specialized water anti salt conducting cells of xylem are called tracheary elements

These cells are elongated and lignified. They have thick secondary wall with various types of pits. These cells are non­living at maturity. Tracheal,: elements are of two types:

(a) Vessel members: The tracheary elements which are

short, wide and with perforated end walls are called vessel members. These vessel members are united end to end to form long xylem vessels. The length of xylem vessels is variable in different plants. It varies from 2-15 Ii. I he perforated end plats of vessels arc called perforation plates. The perfiwation plates are simple or multiple.. Simple plates have only single aperture. Multiple plates !me many apertures. Different .vessels are connected to each other through pitted walls. Water c; n move through these pitted walls from one vessel to other. The secondary walls of

. vessels have different thicknesse. These may he annular (ringed), spiral. scalariform wwl reticulate. These thicknesses are found in newly forred xylem. The vessels of mature xylem have uniform waits. Vessels are prusent only in dicot angiosperms.

(h) Tracheids: The tracheary elements which are elongated tube like with tapering ends are called tracheids. They lack perforated plates in the end wall. Water moves from one tracheid to adjacent tracheids through pit membranes. Tracheids are present in all the vascular plants. The mature tracheids have characteristics thickenings like annular. spiral. reticulate and pitted. The two adjacent traeheids have bordered pits in the common wall. But the tracheal walls have simple pits.

2. Fibers: Fibers commonly _occur in xylem. The fibers are

elongated thick \vaned structures. They perform the supporting functions. Fibers have thick wall. Mature fibers are dead cells. The fibers in the xylem are dix ided into two ty pes:

(a) Fiber tracheids: These are like tracheids. But they have %cry thick wall. These walls ha e some remnant of bordered pit.

(b) Libriform tracheids: These fibers are narrower. They have remnant of simple pits in their %A ails.

3. Parenchyma: Parenchyma is present in both primary and secondary xylems. They form vertical rows in primary xylem. They are parallel to tracheary elements. Parenchymal cells are present in both vertical rims and transverse rows in secondary xylem. The xylem parenchymal cells are living. They retain protoplast. The parenchyma cells store food in the form of

starch. The all of parenchyma cells may remain thin.
Sometimes. they develop secondary wall with simple pits. The amount of parenchyma determines the softness and hardness of wood. Soft wood has a large number of parenchyma. Hard wood have tin+ er parenchyma. ‘Hie old tracheal elements become Wil­la net iona L The contents of the adjecent parenchyma In ‘grate into the tracheal elements through pits forming

Arrangement of xylem

Arrangement of xylem

Types of xylem

There are two types of xylems:

I. Primar xylem: The xylcms produced ns a result of primary growth are called primary xylem. The xylem components are arranged vertically only in prim’ y xylem. In this case, xylem elements are arranged to parallel axis of plant and plant form the . axial system. Primary items are produced procambium during primary growth. Tiere are two forms of primary xylems:

  • Protoxylem: The early primary xylems are called protoxylem. Protoxylem appears at the beginning of differentiation. Protoxylem mostly becomes mature before elongation phase. They have annular and spiral thickenings. Protoxylems have few treachery elements. But they have a large amount of parenchyma cells. They are mostly present near the pith in stern. In root, they are present away from centre.

  • Metaxylem: The xylems appear later during differentiation are called metaxylem, Metaxylem matures after elongation phase. Metaxylem has spiral. reticulate and pitted walls. Metaxylems are composed of tracheids. vessel. parenchyma and fibers. Metaxylems remians functional only in plants in which secondary growth does not occur (grasses). In other plants they become non-functiolial.

    2. Secondary Xylem: The xylems which are produced as a result of secondary growth are called secondary xylem. Secondary xylems are produced by the activity of cambium. They are found in only those plants in which secondary growth occurs. Secondary xylems form axial system and ray systems. Secondary xylems are composed of two types of systems:

    • Axial or vertical systems: In this case, xylem elements are present parallel to vertical axis. They mostly composed of dead tracheary elements (tracheid and vessels), fibers and parenchyma.

    • Radial or transverse system: This system is composed of parenchyma cells only. Their long axes are at right angles to the long axis of organs. They form xylem rays. They are mostly composed of living cell.




      Phloem are living conducting tissues. They conduct prepared food form leaves to different part of the plant. They are also involved in storage of food and mechanical support. They are always associated with xylem to form aseular Ii .

      Components of phloem

      Phloem is composed of sieve elements. companion cells, phloem parenchyma and phloem litters.

      I. Sieve elements: The elongated cells with characteristic sieve

      areas in their walls are called sieve elements. Sieve areas arc lbrmed by modified pits. I he protoplasts of the adjoining cells are continuous through the pores of sieve areas. this elements have primary thin walls. The sieve areas have pores cytoplasmic connecting strands. These strands may be thin like plasmodesmata: These strands are surrounded In callose carbohydrate. The cal lose layer becomes thick in mature sieve elements. The cadose completely closes the dormant sieve elements like a pad. The callose disappears from the acticale sieve elements. ‘Some sie e elements arc oblique. Such sieve elements have sescrai prominent  areas. Some sieve elements bine trans’, ertu end walls. Such sieve elements only single sic

      area with large pores. The sieve elements are divided into sieve tube members and sieve cells.

      1. Sieve cells: The sieve areas are not specialized in sieve cells.

      These areas are not restricted to some specific part of the wall of the cell. The sieve cells are arranged in longitudinal files. The sieve areas are present mostly in the adjacent walls of the sieve cells. The phloem of gymnosperms and lower vascular plants contain sieve cell only.

      1. Sieve tube members: Sieve areas are well developed in

      sieve tuber members. Sieve areas are confined only to end walls of cells. They form sieve plates at end walls. The sieve tube members join end to end to form long tubes called sieve tubes. The phloem of angiosperms is composed of mainly sieve tube members.

      1. Companion cells: The thin walled parenchyma cells closely associated with the sieve elements are called companion cells. Companion cells cut off from the cells initial which later form sieve element. Companion cells are living. They arc physiologically active. They have a prominent nucleus. The siese tube element and companion cell have close contact. Companion cells are absent in gymnosperms and loser vascular plants. But certain parenchyma cells called albuminous cells are closely’ associated with sieve elements in these plants. These albuminous cells are similar 10 the companion cells in their function.

      2. Sclerenchyma: Sclerenchymatous fibers are commonly Ibund in both primary and secondary phloem. The fibers occupy the outer portion in the primary phloem. They are used as commercial Fibers. e.g. Hibiscus. Phloem fibers are elongated. They have thick secondary wall. They may be living or dead at maturity. the phloem fibers may be septate in certain cases. The phloem% are called bast due to presence of these fibers. The thick ssall of fibers is not lignified. It is composed of only cellulose. The fibers are also arraiiged axially (vertically ) in secondary ph loem s.

      3. Parenchyma: Small parenchymas are present in both primary and secondary’ phloems. These parenchyma cells are arranged vertically in primary phloem. But these are arranged venically and radially in secondary phloem. These cells become thick wall in older portion of phloem and change into sclerenchyma cells. Phloem parenchyma cells are thin walled. They are living and physiologically active. They store different compounds like starch, tannins, and different crystals.

        Types of phloem

        1. Phloems may he -primary or secondary.

          1.  Primary phloem: The phloem funned as a result of primary crmult is called primary phloem. Primary phloem is initiated in the embryo. It develops Man procambium. Primary phloem has two types.
          • Protophloem: Protophloem is composed of elongated sieve elements only. They lack companion cells. Sieve tube elements lack nucleus. So they remain active only for a short time. Thus they soon disappear.

          • Metaphloem: Metaphloem mature later than protophloem. So they remain active for longer time. Secondary growth does not occur. in monocot plants. So they remain active in

        2. Secondary phloem: The phloem formed as a result of secondary growth is called secondary phloem. It has two types:

        1. Vertical system: Vertical system is composed of sieve elements, companion cells, phloem fibers and phloem parenchyma.

        2. Ray system: Ray system is composed of ray parenchyma only.

Sclerenchyma Tissues

The lignified tisries .vhich lack protoplast at maturity are called sclerenchyma. Hey have thick secondary wall. These are main strengthening tissues of plant. Sclerenehyma cells are found in all parts of plant. They: are present in both ground tissues and vascular tissues. Sclerenchyma has two types: Fibers and sclereids

(a) Fibers

Very long and narrow sclerenchymatous cells with tapering ends are fibers. The length of the fibers varies greatly. Fibers are found in all parts of the plants. They are present in stern, roots and leaves. Fibers are also present in xylem and phloem. They may form component of vascular tissues. Or they are present outside the vascular tissues.

Types of fibers

There are three types of fibers: Xy Ian. bast and septate fibers.

I.  Xylary fibres or wood fibers: They are present in xylem. They form major pall of the xylem. Xvlary fibers have three main types:

  • Libiform fibers: These fibers have very thick Y all and simple pits. They are longer than the tracheids of that plant.

  • Fihro-tracheids: They have intermediate thickness between tracheids and libilorm fibers. They have bordered pits.

  • (e) Mucilaginous fibers: The innermost laver of these fibers is rich in cellulose. Ii has greater Nater holding capacity. Therefore, these fibers absorb %%ate’: and swell. These fibers are common in elastic Yood.

2.  Bast fibers: These litsers are present in phloem and cortex tissues. In cortex, they form uninterrupted hollov. cylinders in !tic ground tissues. In phloem. the) form fiber sheath.

3   Septate fibers: These arc found in both xy lem and phloem. Ii hers have interim I septa. These are found in septate yood ti hers.

Development of fibers

Fibers develop from different meristems like procambium, cambium and ground meristem. Fibers also develop from parenchyma cells. The initials of the primary fiber appear earlier. They grow in length and form fibers. The secondary •fibers develop in the fully grown tissues.

Commercial fibers

Phloem or bast fibers have great economic importance. These fibers give commercial fiber. These Fibers are used in rope and cloth making. Some important plants which give commercial fibers are: Hemp. Jute, Kenaf, Flax etc. Sonic commercial fibers are obtained from the leaf of monocot plants. These plants are: Manila hemp, Bowstring hemp, New Zealand hemp and pineapple fibers.

(b) Sclereids

The variable shaped sclerenchymatous cells with strongly lignified wall having simple pits are called sclereids. Any non — fibrous sclerenchymatous cell is sclercid. Sclereids are found in different parts of plants. 1 hey are present in epidermis, ground tissues and vascular tissues. The are present in the form of hard mass of cells. The seed coats of many seeds are entirely composed of sclereids. Sonic non-functional parenchyma cells are present in kascular tissues. The walls of these parenchyma cells become thick and they become sclereids.

Sclerenchyma Tissues Diagram

Sclerenchyma Tissues Diagram

Types of Sclereids

There are four types of sclereids:

  1. I. Brachysclereids or stone cells: They have isodiametric shape. They are found in phloem. cortex, bark of stem and fruits.
  2. Macrosclereids: They are rod shaped. They are found in testa of seeds.
  3. Astrosclereids: They are star shaped. They are mainly found in leaves.
  4. Osteosclereids: They are hone or spoon shaped. They are present in seed coat.

Development of Sclereids

The development of sclereid is coordinate and intrusive.

I. Coordinated: In this ease, the cell wall continues to grow uniformly. This uniform growth on all sides without separation from neighbouring cells is called coordinated growth.

2. Intrusive: Later the growth becomes localized at certain points. It produces several processes. These processes grow outward and form branches. These branches penetrate into the middle lamella of the neighbouring cells. This type of growth is called intrusive growth.

Types of Sclereids

Types of Sclereids



The group of cells performing some collective function is called tissue. Tissues organize the body of plant for performing different function. Many types of tissues associate with each other to form tissue system. Vascular system is an example of tissue system. The tissues have following characteristics:

  1. Tissues are composed of many cells. These cells may be of one type or different types.

  2. The cells in a tissue are held by tight junctions.

  3. All the cells in a tissue have similar organization.

  4. All cells in the tissue are involved in same activities. Classification of tissues ‘there are two types of tissues:

  • Simple tissues: The tissues with similar type of cells are called simple tissues. Examples: parenchyma..collenchyma and sc lerenchyma.

  • Compound tissues: The tissues with different types of cell are called compound tissues. Example: Xylem and phloem. Xylem is composed of tracheal cell, vessel cell and parenchymatous cells. Phloem is composed of sieve cells. companion cells and parenchymatous cells.


    The simple tissues with cells having thin and elastic Walls are called parenchyma. Parenchyma forms the main ground tissues. They are continuous throughout the body. They are present in root, stem and leaves.



    Cell structure of parenchymatous cell

    The parenchyma cells have active protoplast. They have prominent nucleus. These cells have thin primary and secondary elastic walls. Their %sans are chiefly composed of cellulose. These cells are closely packed. But intercellular spaces are produced by the dissolution of middle lamella. These air spaces are common for exchange of gases.

    Aerenchymas have prominent intercellular spaces. The protoplast of the adjacent cells is connected by plasmodesmata.

    Shapes of parenchyma cells

    The parenchyma has different shapes.

    1. Polyhedral: These cells are mostly polyhedral in shape.
    2. Stellate: These parenchyma are kund in the stems of plants. They have well developed air spaces between them.

    3. Elongated: Elongated parenchymas are found in the palisade tissue of leaf.

    4. Lobed: Lobed parenchymas are found in spongy and palisade mesophyll tissues of some plants.

    Origin of parenchyma

    Parenchyma are mostly primary in origin. But they are also produced as a result of secondary gro>>th.

    Functions of parenchyma

    Parenchyma cells are less specialized. But it performs major activities in the cell.

    1. Meristematic: They resume meristematic activity under certain special conditions. These conditions are wound healing. regeneration, formation of adventitious roots and union of grans
    2. Photosynthesis: Some parenchymas are involved in photosynthesis. The parenchyma cells of mesophyll tissues of leaves are rich in chloroplast. They are called assimilatory parenchyma or chlorenchyma.

    3. Storage: Parenck ma also store food in fruits and roots etc. They are called storage parenchyma. These parenchymas have many leucoplasts in their cells. These parenchyma stores starch. fats, oils and other granules. Storage parenchymas are common in the cortex of stem, root and seeds.

    4. Ground tissues: Parenchyma acts as ground tissues in most of the tissues.

    5. Secretory and excretory structure: In certain cases. parenchyma acts are secretor> and excretory structure.

    6. Pigmented cells: Parench> ma cells of flowers and fruits contain chromoplast. It gives colours to petals and fruits.

    7. Storage of water: The parenchyma cells of succulent plants store water. These cells are large with thin cell wall. They have thin layer of cytoplasm. These cells have large vacuoles. These vacuoles contain mucilaginous sap. This sap increases the water holding capacity of the cell.


    The parenchyma cells with uneven angular thickening which support young growing part of plant are called collenchyman. Collenchyma tissues form separate strands or continuous cylinder near the surface of the stem cortex, in petioles and along the vein of the leaves. Collenchyma tissues are absent in roots. Collenchyma, tissues are also absent in stem and leaves of many monocot plants.



    Collenchyma cells have living protoplasts. They are simple tissues and contain single type of cells. The cells are elongated with pointed and prismatic ends. They are capable of further growth and divisions. Collenchyma tissues also contain chloroplasts in green organs. Collenchyma form complete cylinder in stem. Collenchyma tissues are present in the margin of leaves.

    Cell wall in collenchymatous cells

    Their thickened primary wall is non-lignified. It is mostly composed of cellulose and poor in pectic substance. Therefore, it is elastic in nature. The angular thickenings are rich in cellulose. An additional layer of microfibrils is present inside the %all. In some cases, the cell N% all of collenchyma is sclerified. Sclerification occurs by the formation of lamella. The lamellae are rich in cellulose. Sometimes. these lamellae become lignified. Additional lamellae appear inside. It decreases the lumen of collenchymatous cells.

    Size and shapes

    Different collenchyma tissues have diflerent sizes and shapes. Mostly these cells are elongated with tapering ends. The longest collenchyma tissues are present in the central position. The shorter collenchyma tissues are present in the periphery.

    Types of collenchyma

    1. Angular collenchyma: The thickenings in the wall of these cells are present in the angles of the cells. They are found in the petioles of some plants.
    2. Lamellar collenchyma: The thickenings in these tissues are – present on the tangential walls of the cell. These are present in the stem cortex of some plants.
    3. Lacu ar collenchyma: In these tissues thickenings are present in with faces intercellular spaces. Lacunar collenchymas are prex, In the petiole of composite species.


Water Absorption

WATER Absorption: of water*, root hairs: .[he absorption of \safer from the soil takes place tlitimigh the root hairs. Ihe root hairs are in contact with the wateriilms on the soil particles. ‘[here is a thin lining of cytoplasm inside the root hair. This cytoplasm encloses a large vacuole filled with cell sap. The Cytoplasm and cell sap of the root hair are continuous with the root cell. The cell %\ all of the toot hair is a permeable membrane.

lite cell sap is aqueous solution of mineral salts and organic acid. herefbre. it exerts a high pressure. The soil solution is dilute. So it • has a low osmotic pressure (less than I atm). .1 he cell \van of the root hair contains pectic substances. So it imbibes soil water. [hen this

water passes into semi permeable cytoplasniic membrane by osmosis. Thus it removes film of capillary water from soil particle. It draws films of water from the adjacent soil particles. This process continues and water moves from considerable distances to the root hairs. This movement of water takes place due to cohesive forces between water molecules.

The root hair cell absorbs water and becomes fully turgid. Therefore. its osmotic pressure fall and its turgor pressure increases. So the suction pressure of adjacent conical cell increases. Thus it pushes water into cortical ccIlS.

  1. Passage 9f wafer through endodermal cells: Water passes from root hair cells to the cortical cells. The endodermal cells have Caspariait. strips. They offer resistance in the flow of water. Theregte. water pissds through endodermis by symplast pathway. Secondly. :the endodermal cells opposite To the protoxylem do not have thickenings. These are known as passage cells. These cells allow the movement of %sitter across the endodermis from the cortex to the protoSylem. Then water passes through pericycle and it finally’ reached at elements of the xn lent.
  2. Reachingofwaterin the xylem: The walls of the .xylem vessels are inclaStie)* it has fl titrgor pressure and the whole of the osmotic pressnres of the xylem forms suction pressure. Thereliwe. it draws water from the pericycle cell. I lins gradient of suction pressure (diffusion pressure deficit) from the root hair to the xylem vessel

helps in the absorption of water. I he force which water is drawn. in from the soil depends. upon the difference between the osmotic pressure of xnlein sap and the soil solution.

  1. Root pressure: Water is Forced into the xylem vessels by the surrounding cortical cells w ith a certain force. this induces a pressure w Inch raises the water to many feet in the xylem. This pressure is called root pressure. Root pressure is responsible for the phenomena of bleeding and guttation.