Unit IV: Anatomy

Meristematic Tissues and their Functions

Meristematic tissues are a group of young cells that are in a continuous state of division.

These tissues are mostly found at the apices of root and shoot.

Characteristics of meristematic tissues are:

(i) They are living and thin walled

(ii) Vacuoles are few and small in size

(iii) The cells contain a dense protoplasm and conspicuous nuclei

(iv) The cells are spherical, oval or polygonal in shape

(v) They do not store reserve food material and are in an active state of metabolism.

Types of Meristems:

I. Classification based on origin and development:

(i) Promeristem or priordial meristem:

A group of young meristematic cells of a growing organ. It is the early embryonic meristem from which other advanced meristems are derived. In a plant, it occupies a small area at the tip of stem and root. It further divides to form primary meristem.

(ii) Primary meristem:

These are derived from promeristem. They are present below the promeristem at shoot and root apices. These cells divide and form permanent tissues.

(iii) Secondary meristem:

It is derived from primary permanent tissues which have the capacity of division e.g. Cork-cambium, cambium of roots and inter fascicular cambium of stem.

II. Classification on basis of position:

(i) Apical meristem:

These are found at the apices or growing points of root and shoot and bring about increase in length. It includes both pro-meristem as well as primary meristem.

Several theories have been put forward to explain the activity of apical meristem:

Apical cell theory:

The theory was first proposed by Hofmeister (1857) and advanced by Nageli (1878). According to this theory, a single apical cell is the structural and functional unit of apical meristem which governs the entire process of apical growth. However, such organization has been found only in cryptogams.

Histogen cell theory:

This theory was given by Hanstein (1868). According to this theory root and shoot apices consists of the central or inner mass called Plerome surrounded by the middle region composed of isodiametric cells called periblem and the outermost uniseriate layer of Dermatogen. Dermatogen gives rise to epidermis, periblem to cortex and endodermis and plerome to vascular bundle and pith. These three layers were called Histogen by Hanstein.

Tunica-corpus theory:

This theory was proposed by Schmidt (1924). According to this theory, mass of dividing cells are of two types: Tunica, the outer consisting of one position of different meristems or more peripheral layers of cells, forming the outer region and Corpus, the central undifferentiated multilayered mass of cell. Epidermis is derived from outer layer of tunica and other tissues from remaining layer of tunica and corpus.

(ii) Intercalary meristem:

It lies between the region of permanent tissues and is considered| as a part of primary meristem which has become detached due to formation of intermediate permanent tissues. It is found either at the base of leaf e.g. Pinus or at the base of internodes e.g. grasses.

(iii) Lateral Meristem:

These are arranged parallel to the sides of origin and normally divide periclinally or radially and give rise to secondary permanent tissues. These increase the thickness of the plant part.

III. Classification on basis of function:

(i) Protoderm meristem:

It is the outermost layer of the young growing region which develops to form epidermal tissue system.

(ii) Procambium meristem:

It is composed of narrow, elongated, prosenchymatous, meristematic cells that gives rise to the vascular tissues system.

(iii) Ground Meristen:

It is composed of large, thick-walled cells which develop to for ground tissue system, i.e. hypodermis, cortex and pith.

Simple Permanent Tissues

The tissues that have lost the capacity of growth and division temporarily or permanently are known as permanent tissue. It is formed as a result of division and differentiation of meristematic tissue.

This tissue maybe living or dead thin (living tissue) or thick walled (dead tissue).

Types of Permanent Tissue:

1. Simple tissue

2. Complex tissue

1. Simple Tissue:

Simple tissues called homogenous tissues. A simple tissue may be defined as a group of similar cells that perform a common function.

These are classified into three groups:

(a) Parenchyma

(b) Collenchyma

(c) Sclerenchyma.

(a) Parenchyma: (Para= Soft, Enchyma = Tissue)

i. These are composed of isodiametric living cells which may be oval or rounded.

ii. Cells have intercellular spaces.

iii. Cell wall is thin and made up of hemicellulose and cellulose.

iv. Vacuole is present and nucleus size is small.

v. Cells have dense and living protoplasm.

vi. Cells may show meristematic activity. So these are also called as potential meristematic tissue.

vii. It is found in cortex, pith, mesophyll tissue, and endosperm and also associated with xylem and phloem.

According to specific function, parenchyma is classified into four groups:

1. Aerenchyma:

Parenchyma having large air spaces is known as aerenchyma. It is found in hydro­phytes and it gives buoyancy to plant.

2. Chlorenchyma:

Parenchyma with large amount of chloroplast is known as chlorenchyma. It is found in mesophyll of leaf and helps in photosynthesis.

3. Prosenchyma:

Parenchymatous cell becoming long and tapper at both ends, without intercellular space is known as prosenchyma. It helps in mechanical support.

4. Idioblast:

Parenchyma that stores unwanted or ergastic substances is known as Idioblast.

Functions of Parenchyma:

1. Parenchyma mainly serves as storage tissue.

2. Being meristematic in nature, parenchyma is associated with regeneration, formation of adventitious root, graft union, wound healing.

3. Aerenchyma renders buoyancy, prosenchyma gives mechanical support, chlorenchyma helps in photosynthesis and idioblast stores ergastic substances.

(b) Collenchyma: (Colla = Give, Enchyma = Tissue):

i. Collenchyma term was coined by Schlieden.

ii. It is living mechanical dicot tissue.

iii. Cells are longer than parenchyma cells.

iv. Cell wall is thick and made up of cellulose and pectin.

v. Cell carries less amount of chloroplast.

vi. Due to presence of pectin it is the tissue with highest refractive index.

vii. Intercellular spaces are absent, because at the corner of the cells, secondary cell wall substance develops due to which the cell wall become rigid and thick at corners.

viii. It is found in hypodermis of dicot stem and upper and lower portion of vascular bundle of dicot leaf.

ix. On the basis of distribution of wall thickening chloenchyma can be classified into

1. Angular collenchymas:

It is most common type of collenchyma, where thickening occurs only at corners of the cells and side walls remain thin, e.g., – Vitis. Ficus.

2. Lamellar/Plate collenchyma:

Thickenings are plate like and occur only in tangential wall, e.g., – Rheum.

3. Tubular/Lacunar collenchyma:

Thickenings are distributed all around the intercellular spaces, e.g., – Malva, Salvia.

Functions:

1. It provides mechanical support to the young and growing organs of the plant in the form of elasticity.

2. It helps in photosynthesis.

(c) Sclerenchyma (Secros = hard, Enchyma = Tissue):

i. Sclerenchyma term was coined by Mettenius.

ii. These are hard, rigid and dead tissues.

iii. Cell wall is very thick due to deposition of lignin.

iv. These are of various shapes and size.

v. It provides mechanical support as well as rigidity to plant body.

Sclerenchyma is of two types:

(1) Fibre;

(2) Sclereids

1. Fibre:

i. These are very much elongated cells with tapering or pointed ends.

ii. At maturity it losses it protoplasm and becomes dead.

iii. Presence of wider lumen at middle.

iv. Wall is made up with lignin.

v. Fibres are present in pericycle of many dicots and secondary xylem and phloem tissue.

vi. Fibre is the longest cell (Boehmeria fibre, length is upto 55 cm) in plant.

vii. According to position, fibres are of two types:

1. Xylary fibres – Fibres associated with xylem is known as Xylary fibres.

2. Extra Xyfary fibres – Fibre present in any other part of the plant except xylem. e.g., Phloem fibres, cortical fibres, Perivascular fibres, etc.

Functions:

1. It mainly helps in mechanical support to plant body in the form of plasticity.

2. Commercially important fibres are Hemp (Cannabis sativa), Jute (Corchorus capsularis), Flex (Linuin usitatissimum), and Ramie (Boehmeria nivea).

2. Sclereids:

i. Sclereid term was coined by Tschierch.

ii. They have extremely thick wall of lignin with narrow leumen.

iii. These are generally found in hard part of the plant and gives rigidity.

Complex Permanent Tissues

The tissues that have lost the capacity of growth and division temporarily or permanently are known as permanent tissue. It is formed as a result of division and differentiation of meristematic tissue.

This tissue maybe living or dead thin (living tissue) or thick walled (dead tissue).

Types of Permanent Tissue:

1. Simple tissue

2. Complex tissue

2. Complex Tissue:

i. A group of different types of cells performing same function is known as complex tissue.

ii. It is also known as vascular tissue.

It is classified into two groups:

1. Xylem

2. Phloem

1. Xylem:

The term xylem was introduced by Nageli. It is the main sap (water and minerals) conduction complex tissue of the plant. It is associated with phloem and form the vascular bundles. It consists of both parenchymatous as well as sclerenchymatous cells. On the basis of components xylem can be classified into primary xylem and secondary xylem. Primaryxylem is present in primary growth of the plant body and derived from procambial cells. It is differentiated into protoxylem and metaxylem. Secondary xylem is present in secondary plant body. It is made up with four components.

(a) Tracheids

(b) Vessels

(c) Xylem fibre

(d) Xylem parenchyma

Out of four components only xylem parenchyma is living and rest of the components are dead.

(a) Tracheids:

1. These are elongated, narrow lumen cells with tapering ends.

2. Cell wall is made up of lignin and thickening may be scalariform, annular, and reticulate or pitted (generally bordered pit).

3. Tracheids are found in Pteridophytes, Gymnosperms and Angiosperms.

4. It develops softwood (Nonporous wood).

5. Tracheids form a long row placed one above the other.

6. It helps in conduction of water, mechanical support and formation of wood.

(b) Vessels:

1. These are elongated, broad ended, cylindrical elements with wider lumen.

2. These are commonly found in angiosperm (except family – winter- aceae, Tetracentraceae, Trochodendraceae) and also in advanced Gymnosperm i.e., Order-Gnetales (Gnetum, Welwitschia and Ephe­dra). Vessels are absent in stem and leafs of Yucca and Dracaena.

3. Walls of vessels are lignified and thickening may be annular, spiral, scalariform or reticulate.

4. Cell wall possesses many bordered pits but the pits are smaller than those found in tracheids.

5. The end wall of vessels is perforated known as perforated plates. It may be simple (with single opening) or multiple (with two or more openings).

6. Vessels develop hard wood or porous wood. Vessels with narrow lumen are protoxylem and wider lumen is known as metaxylem.

7. It also helps in conduction of water, mechanical support and formation of wood.

(c) Xylem fibre:

1. It is dead sclerenchymatous cell found in xylem.

2. These are long, narrow with thick lignified wall and tappering ends.

3. It is of two types (i) Fibre tracheids (it is similar to tracheids with bordered pit); (ii) Libriform fibre (with simple pit).

4. It helps in mechanical support.

(d) Xylem parenchyma:

1. Xylem or wood parenchyma is only living component of xylem.

2. These are parenchymatous, thin walled, oval or elongated and present in both primary and secondary xylem.

3. It mainly helps to stores starch and fats.

4. It is responsible for the formation of medullary rays.

Functions of Xylem:

1. Tracheids and vessels help in conduction of water and minerals from base to top of the plants. They also help in formation of soft and hard wood, respectively.

2. Xylem parenchyma stores food.

3. Xylem fibre helps in mechanical support.

2. Phloem:

The term phloem was coined by Nageli (1858). It is a permanent living complex tissue.lt helps in translocation of organic food (in the form of sucrose) from leaves to various parts of the plant. It is also called bast or leptome. On the basis of origin phloem is classified as primary phloem (in primary growth) and secondary phloem (in secondary plant body).

On the basis of position it is distinguished into:

(1) External phloem. Present outside the xylem in vascular bundle.

(2) Internal phloem. Present inner to the xylem along with outer phloem – e.g., Cucurbita and

(3) Included phloem, embedded in the secondary xylem e.g., Salvadora. Phloem is made up of four components.

(a) Sieve tubes

(b) Companion cells

(c) Phloem parenchyma

(d) Phloem fibres

Out of four components only phloem fibre is dead and rests are living.

(a) Sieve tubes:

1. These are elongated cylindrical, tube like living cells.

2. Presence of thin cellulosic wall and are placed end to end.

3. They are non-nucleated at maturity but protoplasm possesses large vacuole.

4. In Gymnosperm, sieve cells are present in place of sieve tubes.

5. Between two sieve tubes there are present perforated sieve plates with many sieve pores.

6. During winter season, polysaccharide substances, callose, get deposited on sieve plates. These calluse pads inhibit conduction of food. On return of spring, callose pads get dissolved with the help of enzyme callose and conduction of food restarts and growth of plant starts.

(b) Companion cells:

1. These are elongated, narrow, thin walled living cells associated with seive tubes and are placed side by side with them.

2. The sieve tubes and companion cells are connected through pits.

3. In Gymnosperm, albuminous cells are present in place of companion cells.

4. Companion cells help in transport of food along with sieve tubes.

(c) Phloem parenchyma:

1. It is elongated, pointed, living, thin-walled parenchymatous cell inter mixed with sieve tube elements.

2. It is absent in monocot plants (also in dicot Ranunculus)

3. It helps in conduction and stores food. It also contains oil, starch, mucilage and latex.

(d) Phloem fibre:

1. These are dead elongated sclerenchymatous cells having lignified walls with pits.

2. These are found both in primary and secondary phloem.

3. It serves to provide mechanical support.

Functions of Phloem:

1. It helps in translocation of solute from apex to base of the plants.

2. Secondary bast fibres (jute fibre) are of economic value.

Secondary Growth in Dicot Stem

Primary growth produces growth in length and development of lateral appendages. Secondary growth is the formation of secondary tissues from lateral meristems. It increases the diameter of the stem. In woody plants, secondary tissues constitute the bulk of the plant. They take part in providing protection, support and conduction of water and nutrients.

Secondary tissues are formed by two types of lateral meristems, vascular cambium and cork cambium or phellogen. Vascular cambium produces secondary vascular tissues while phellogen forms periderm.

Secondary growth occurs in perennial gymnosperms and dicots such as trees and shrubs. It is also found in the woody stems of some herbs. In such cases, the secondary growth is equivalent to one annual ring, e.g., Sunflower.

A. Formation of Secondary Vascular Tissues:

They are formed by the vascular cambium. Vascular cambium is produced by two types of meristems, fascicular or intra-fascicular and inter-fascicular cambium. Intra-fascicular cambium is a primary meristem which occurs as strips in vascular bundles. Inter-fascicular cambium arises secondarily from the cells of medullary rays which occur at the level of intra-fascicular strips.

These two types of meristematic tissues get connected to form a ring of vascular cambium. Vascular cam­bium is truly single layered but appears to be a few layers (2-5) in thickness due to presence of its immediate derivatives. Cells of vascular cambium divide periclinally both on the outer and inner sides (bipolar divisions) to form secondary permanent tissues.

The cells of vascular cambium are of two types, elongated spindle-shaped fusiform initials and shorter isodiametric ray initials. Both appear rectangular in T.S. Ray initials give rise to vascular rays.

Fusiform initials divide to form secondary phloem on the outer side and secondary xylem on the inner side. With the formation of secondary xylem on the inner side, the vascular cambium moves gradually to the outside by adding new cells.

The phenomenon is called dilation. New ray cells are also added. They form additional rays every year. The vascular cambium undergoes two types of divisions— additive (periclinal divisions for formation of secondary tissues) and multi­plicative (anticlinal divisions for dilation).

Ray initials produce radial system (= horizontal or transverse system) while fusiform initials form axial system (= vertical system) of sec­ondary vascular tissues.

1. Vascular Rays:

The vascular rays or secondary medullary rays are rows of radially arranged cells which are formed in the secondary vascular tissues. They are a few cells in height.

Depending upon their breadth, the vascular rays are uniseriate (one cell in breadth) or multiseriate (two or more cells in breadth). Vascular rays may be homo-cellular (having one type of cells) or hetero-cellular (with more than one type of cells). The cells of the vascular rays enclose intercellular spaces.

The part of the vascular ray present in the secondary xylem is called wood or xylem ray while the part present in the secondary phloem is known as phloem ray. The vascular rays conduct water and or­ganic food and permit diffusion of gases in the radial direction. Besides, their cells store food.

2. Secondary Phloem (Bast):

It forms a narrow circle on the outer side of vascular cam­bium. Secondary phloem does not grow in thickness because the primary and the older sec­ondary phloem present on the outer side gets crushed with the development of new functional phloem. Therefore, rings (annual rings) are not produced in secondary phloem. The crushed or non-functioning phloem may, however, have fibres and sclereids.

Secondary phloem is made up of the same type of cells as are found in the primary phloem (metaphloem)— sieve tubes, companion cells, phloem fibres and phloem paren­chyma.

Phloem pairenchyma is of two types— axial phloem parenchyma made up of longitudinally arranged cells and phloem ray parenchyma formed of radially arranged parenchyma cells that constitute the part of the vascular ray present in the phloem.

Elements of secondary phloem show a more regular arrangement. Sieve tubes are comparatively more numerous but are shorter and broader. Sclerenchyma fibres occur either in patches or bands. Sclereids are found in many cases. In such cases secondary phloem is differentiated into soft bast (secondary phloem without fibres) and hard bast (part of phloem with abundant fibres).

3. Secondary Xylem:

It forms the bulk of the stem and is commonly called wood. The secondary xylem consists of vessels, tracheids (both tracheary elements), wood fibres and wood parenchyma.

Wood parenchyma may contain tannins and crystals besides storing food. It is of two types— axial parenchyma cells arranged longitudinally and radial ray parenchyma cells arranged in radial or horizontal fashion. The latter is part of vascular ray present in secondary xylem.

Secondary xylem does not show distinction into protoxylem and meta-xylem elements. Therefore, vessels and tracheids with annular and spiral thicken­ings are absent. The tracheary elements of secondary xylem are similar to those of meta-xylem of the primary xylem with minor differences. They are comparatively shorter and more thick-walled. Pitted thickenings are more common. Fibres are abundant.

Width of secondary xylem grows with the age of the plant. The primary xylem persists as conical projection on its inner side. Pith may become narrow and ultimately get crushed. The yearly growth of secondary xylem is distinct in the areas which expe­rience two seasons, one favourable spring or rainy season) and the other un-favourable (autumn, winter or dry summer).

In favourable season the temperature is optimum. There is a good sunshine and humidity. At this time the newly formed leaves produce hormones which stimulate cambial activity. The activity decreases and stops towards the approach of un-favourable sea­son. Hence the annual or yearly growth appears in the form of distinct rings which are called annual rings.

Annual rings are formed due to sequence of rapid growth (favourable season, e.g., spring), slow growth (before the onset of un-favourable period, e.g., autumn) and no growth (un-favourable season, e.g., winter). Annual rings are not distinct in tropical areas which do not have long dry periods.

Annual Rings (Growth Rings). It is the wood formed in a single year. It consists of two types of wood, spring wood and autumn wood. The spring or early wood is much wider than the autumn or late wood. It is lighter in colour and of lower density. Spring wood consists of larger and wider xylem elements.

The autumn or late wood is dark coloured and of higher density. It contains compactly arranged smaller and narrower elements which have comparatively thicker walls. In autumn wood, tracheids and fibres are more abundant than those found in the spring wood.

The transition from spring to autumn wood in an annual ring is gradual but the transition from autumn wood to the spring wood of the next year is sudden. Therefore, each year’s growth is quite distinct. The number of annual rings corresponds to the age of that part of the stem. (They can be counted by increment borer).

Besides giving the age of the plant, the annual rings also give some clue about the climatic conditions of the past through which the plant has passed. Dendrochronology is the science of counting and analysing annual growth rings of trees.

Softwood and Hardwood:

Softwood is the technical name of gymnosperm wood be­cause it is devoid of vessels. Several of the softwoods are very easy to work with (e.g., Cedrus, Pinus species). However, all of them are not ‘soft’. The softness depends upon the content of fibres and vascular rays. 90-95% of wood is made of tracheids and fibres. Vascular rays constitute 5-10% of the wood.

Hardwood is the name of dicot wood which possesses abundant vessels. Due to the presence of vessels, the hardwoods are also called porous woods. In Cassia fistula and Dalbergia sisso the vessels are comparatively very broad in the spring wood while they are quite narrow in the autumn wood. Such a secondary xylem or wood is called ring porous.

In others (e.g., Syzygium cumini) larger sized vessels are distributed throughout spring wood and autumn wood. This type of secondary xylem or wood is known as diffuse porous. Ring porous wood is more advanced than diffuse porous wood as it provides for better translocation when the requirement of the plant is high.

Sapwood and Heartwood:

The wood of the older stems (Dalbergia, Acacia) gets differentiated into two zones, the outer light coloured and functional sapwood or alburnum and the inner darker and nonfunctional heartwood or duramen. The tracheids and vessels of the heart wood get plugged by the in growth of the adjacent parenchyma cells into their cavities through the pits. These ingrowths are called tyloses.

Ultimately, the parenchyma cells become lignified and dead. Various types of plant products like oils, resins, gums, aromatic substances, essential oils and tannins are deposited in the cells of the heartwood. These substances are collectively called extractives. They provide colour to the heartwood. They are also antiseptic. The heartwood is, therefore, stronger and more durable than the sapwood.

It is resistant to attack of insects and microbes. Heart wood is commercial source of Cutch (Acacia catechu), Haematoxylin (Haematoxylon campechianum), Brasilin (Caesalpinia sappan) and Santalin (Pterocarpus santalinus). Heart­wood is, however, liable to be attacked by wood rotting fungi. Hollow tree trunks are due to their activity.

B. Formation of Periderm:

In order to provide for increase in girth and prevent harm on the rupturing of the outer ground tissues due to the formation of secondary vascular tissues, dicot stems produce a cork cambium or phellogen in the outer cortical cells. Rarely it may arise from the epidermis (e.g., Teak, Oleander), hypodermis (e.g., Pear) or phloem parenchyma.

Phellogen cells divide on both the outer side as well as the inner side (bipolar) to form secondary tissues. The secondary tissue produced on the inner side of the phellogen is parenchymatous or collenchymatous. It is called secondary cortex or phelloderm. Its cells show radial arrangement.

Phellogen produces cork or phellem on the outer side. It consists of dead and com­pactly arranged rectangular cells that possess suberised cell walls. The cork cells contain tannins. Hence, they appear brown or dark brown in colour. The cork cells of some plants are filled with air e.g., Quercus suber (Cork Oak or Bottle Cork). The phelloderm, phellogen and phellem together constitute the periderm.

Cork prevents the loss of water by evaporation. It also protects the interior against entry of harmful micro-organisms, mechanical injury and extremes of temperature. Cork is light, compressible, nonreactive and sufficiently resistant to fire.

It is used as stopper for bottles, shock absorption and insulation. At places phellogen produces aerating pores instead of cork. These pores are called lenticels. Each lenticel is filled by a mass of somewhat loosely arranged suberised cells called complementary cells.

Lenticels:

Lenticels are aerating pores in the bark of plants. They appear on the surface of the bark as raised scars containing oval, rounded or oblong depressions. They occur in woody trees but not in climbers. Normally they are formed in areas with underlying rays for facilitating gas exchange. Lenticels may occur scattered or form longi­tudinal rows.

A lenticel is commonly produced beneath a former stomate or stoma of the epidermis. Its margin is raised and is formed by surrounding cork cells. The lenticel is filled up by loosely arranged thin walled rounded and suberised (e.g., Prunus) or un-suberised cells called comple­mentary cells.

They enclose intercellular spaces for gaseous exchange. The complementary cells are formed from loosely arranged phellogen cells and division of sub-stomatal parenchyma cells. The suberised nature of complementary cells checks excessive evaporation of water.

In temperate plants the lenticels get closed during the winter by the formation of com­pactly arranged closing cells over the complementary cells.

Bark:

In common language and economic botany, all the dead cells lying outside phello­gen are collectively called bark. The outer layers of the bark are being constantly peeled off on account of the formation of new secondary vascular tissues in the interior. The peeling of the bark may occur in sheets (sheets or ring bark, e.g., Eucalyptus) or in irregular strips (scaly bark).

The scaly bark is formed when the phellogen arises in strips instead of rings, e.g., Acacia (vem. Kikar). Bark formed in early growing season is early or soft bark. The one formed towards end of growing season is late or hard bark.

Bark is insect repellent, decay proof, fire-proof and acts as a heat screen. Commercially it is employed in tanning (e.g., Acacia), drugs (e.g., Cinchona— quinine) or as spice (e.g., Cannamon, vem. Dalchini). The cork of Quercus suber is employed in the manufacture of bottle stoppers, insulators, floats, sound proofing and linoleum.

Significance of Secondary Growth:

1. Secondary growth adds to the girth of the plant. It provides support to increasing weight of the aerial growth.

2. Secondary growth produces a corky bark around the tree trunk that protects the interior from abrasion, heat, cold and infection.

3. It adds new conducting tissues for replacing old non-functioning ones as well as for meeting increased demand for long distance transport of sap and organic nutrients.

Primary Structure in Monocot and Dicot Roots

I. Cicer (Dicot Root):

It is circular in outline and reveals following tissues from outside with-in:

Epiblema:

1. It is the outermost layer consisting of many thin-walled cells.

2. From some of its cells arise unicellular hair.

3. Cuticle is absent.

Cortex:

4. It is very large, parenchymatous and well- developed occupying the large part of the section.

5. In this region there are present many intercellular spaces.

6. Cortical cells are filled with starch grains.

7. In older roots, few-layered exodermis, consisting of thin-walled compact cells, is present just below the epiblema.

8. Endodermis is the ring like innermost layer of cortex made up of barrel-shaped cells.

9. Casparian strips are present in the endodermal cells.

10. Some of the endodermal cells, particularly those opposite to the protoxylem, are thin-walled and have been termed as passage cells.

Pericycle:

11. Single-layered, ring-like pericycle is present close to the endodermis on its inner side.

12. It is also a compact layer of thin-walled cells.

Vascular Bundles:

13. The vascular bundles are 2 to 6 and radial, i.e., xylem and phloem present on different radii alternating with each other.

14. Xylem and phloem patches are equal in number.

15. Xylem consists of protoxylem and metaxylem.

16. Protoxylem is exarch and consists of small annular and spiral vessels.

17. Metaxylem strands are big, present towards the centre and are made up of large reticulate and pitted vessels.

18. In some cases the metaxylem meet in the centre and thus obliterate the pith.

19. Phloem is made up of sieve tubes, companion cells and phloem parenchyma.

20. In mature roots, cambium also appears cutting the secondary structures.

21. The parenchymatous cells in between xylem and phloem strands form conjunctive tissue.

Pith:

22. It is very small, parenchymatous and without any intercellular spaces. It gets reduced after the formation of secondary structures.

II. Zea mays (Monocot Root):

It is circular in outline and reveals the following tissues from outside with-in:

Epiblema:

1. Single-layered epiblema consists of barrel- shaped or rounded cells.

2. From some cells arise unicellular hair.

Cortex:

3. It is well-developed, several cells deep and parenchymatous.

4. The cells are thin-walled, rounded in shape and leave many intercellular spaces.

5. Just below the epiblema are present 2 to 6 layers of collenchyma in old roots. This represents exodermis.

6. Remaining part of the cortex is parenchymatous.

7. Endodermis is the innermost layer of cortex. It consists of many compactly arranged, barrel- shaped cells.

8. Casparian strips are present on the radial and transverse walls of the endodermal cells.

9. Thin-walled endodermal cells are known as passage cells. They lie opposite to protoxylem.

Pericycle:

10. Single-layered pericycle consists of thin-walled cells and present inner to the endodermis.

Vascular Tissue:

11. It is composed of alternating strands of phloem and xylem.

12. Vascular bundles are radial, exarch and polyarch. Cambium is absent.

13. Xylem consists of vessels, tracheids and xylem parenchyma.

14. Protoxylem elements are towards the outer side, i.e., exarch, small in diameter and their walls have thickenings.

15. Metaxylem vessels face towards the centre and have larger diameter. Innermost metaxylem vessel is very large and spherical or oval.

16. Phloem consists of sieve tubes, companion cells and phloem parenchyma. It exhibits exarch condition with its protophloem towards the periphery and metaphloem towards the centre.

17. Thick-walled, sclerenchymatous conjunctive tissue is present in between the vascular bundles.

Pith:

18. It is well-developed and parenchymatous.

19. The cells are round in shape and leave many intercellular spaces.