NEET Biology Anatomy of Flowering Plants Notes

ANATOMY OF FLOWERING PLANTS

While going through the chapter pay special attention to the following –

Terms and definitions –

1. Promeristem
2. Dermatogen
3. Periblem
4. Plerome
5. Idioblast
6. Aerenchyma
7. Prosenchyma
8. Phellem
9. Phelloderm
10. Epiblema
11. Laticiferous tissue
12. Trichomes

Differences –

1. Meristematic tissue and permanent tissue
2. Promeristem, primary meristem and secondary meristems
3. Apical, intercalary and lateral meristems
4. Dermatogen, periblem and plerome
5. Parenchyma, collenchyma and sclerenchyma
6. Amphicribral and amphivasal vascular system
7. Dorsiventral and isobilateral leaf
8. Spring and autumn wood
9. Heart wood and sapwood

• N. Grew. Father of plant anatomy and used the term tissue for the first time.
• K.A. Chaudhary. Indian plant anatomist.
• Anatomy deals with internal organization of plants as observed after section cutting.
• Nageli is regarded as Father of Modern Anatomy.
• The internal structure of most plants (except some lower plants) is complex and different types of cells form the plant body.
• These cells are in groups to form tissue which, perform definite function.

THE TISSUE

• A group of structurally similar or dissimilar cells that perform a common function and have a common origin is called a tissue. A plant is made up of different kinds of tissues. The complex tissues are, however, composed of different kinds of cells and they perform different types of functions.
• Each tissue has a specific function to perform and different types of tissues of a plant body function in co-ordination with one another.

Kinds of Plant Tissues

Plant tissues are broadly classified on the basis of stages of development into two categories:

• Meristematic tissue : They have the ability to divide.
• Permanent tissue : They have lost the ability to divide.

MERISTEMATIC TISSUE

Cells which are capable of active cell division are called meristematic cells. These tissues are found in growing regions of plants. e.g., meristem at apex of stem, root, leaf primordia and vascular cambium, etc.

The chief characteristics of meristematic cells are:

• The cells of meristem are always living.
• Meristematic cells are simple, immature, and isodiametric (same length, width and height, mostly rounded/ oval/ polygonal or rectangular).
• They retain power of division.
• They are without intercellular spaces.
• They have dense cytoplasm with prominent nuclei.
• Only cellulosic primary cell wall is present.
• They have no reserve food material and further no ER and plastids in them.
• Vacuole is small or absent.

Fig. 6.1 L.S. of axis showing the positions of meristems (labels: Apical meristem, Intercalary meristem, lateral meristem).

Classification of Meristematic Tissues

Meristematic tissues may be classified on the basis of :

• Mode of origin and development and
• Position in the plant body.

Meristems based on origin and development

On this basis meristems can be divided into following three types.

• Promeristem (primordial meristem) : These are group of cells which represent primary stages of meristematic cells. They are present in a small region at the tips of radicle and plumule. They give rise to primary meristems.
• Primary meristem : Primary meristems originate from promeristem that continue to divide to form different tissues. These cells are always in active state of division and give rise to primary permanent tissues for growth in length as well as width e.g., apical meristems, intercalary meristems, lateral meristems (intra-fascicular cambium in the vascular bundle of dicot stem).
• Secondary meristems : Secondary meristems developed from primary permanent tissue. They are not present from the beginning of the formation of an organ but develop at a later stage by differentiation and acquire power of division. e.g., interfascicular cambium in stem, cambium in roots and also cork cambium (phellogen).

Meristematic tissues based on position in plant body

On the basis of position, the meristematic tissues can be divided into the following three types:

• Apical meristem
• Intercalary meristem
• Lateral meristem

Apical meristem. It is situated at the shoot apex and the root apex. It includes both the promeristem and the primary meristem. Apical meristems give rise to primary permanent tissues which bring about the elongation of the stem and the root. This meristem is apical in stem and subterminal in roots due to presence of root cap (calyptra). The cells of this region are small, unspecialized and divide to cause primary growth of stem and root. It is never used up during the growth. Leaf primordia grow into leaf by activity of apical meristems.

Connecting Concepts

• To propagate virus free plants through tissue culture techniques the apical meristem is used.

Intercalary meristem. These tissues are found in between the permanent tissues. These tissue are actually parts of the apical meristem which get detached from it during the growth of stem and root in length. Intercalary meristem is present at the base of internodes, e.g, in grasses (Gramineae) or at the base of leaves, e.g., in Pinus or at the base of nodes, e.g., mint or Mentha (Labiatae).

Lateral meristem. These meristems are present along the lateral sides of stem and roots. Lateral meristem divides only periclinally or radially and is responsible for increase in girth or diameter. It includes vascular cambium and cork-cambium.

Meristem on the basis of functions

The first formed meristem (promeristem) gets differentiated into following three regions –

Protoderm or Dermatogen: It is the outermost layer meant for producing the single layered epidermis, hairs, velamen, stomata i.e., epidermal tissue system.

Ground meristem or Periblem: It produces hypodermis, cortex and endodermis or ground tissue system.

Procambium or Plerome: It is the innermost part of the meristem. It gives rise to the stele which comprises primary vascular tissues and ground tissues like pith, medullary rays and the pericycle.

Fig. 6.3 L.S. of root apex (labels: plerome, periblem, dermatogen, calyptrogen, root cap).

PROMERISTEMS AND THE TISSUE SYSTEM

PERMANENT TISSUES

• Permanent tissues are formed as a result of division and differentiation in meristematic tissues.
• These tissues are composed of mature, living or dead cells that have lost their ability to divide.
• The cells of these tissue may be thin-walled or thick-walled. The thin walled tissues are generally living whereas the thick-walled tissues may be living or dead.
• They have undergone growth and differentiation to attain a permanent position, function, shape and size. They are called primary permanent tissues if they are derived from apical meristem and called secondary permanent tissues when derived from lateral meristem.
• Permanent tissues are of two types : (i) Simple tissue (ii) Complex tissue

Simple Permanent Tissue

These tissues are a collection of similar cells that perform a common function. These tissues are of 3 types:

1. Parenchyma
2. Collenchyma
3. Sclerenchyma

Parenchyma

• It is simplest, unspecialized, most primitive, most abundant permanent tissue (found in all organs of the plant).
• Parenchyma cells are living, thin-walled, isodiametric (all sides equal) and appear oval or spherical in shape. Sometimes they become polygonal as a result of pressure of the neighbouring cells.
• The spherical parenchyma cells are loosely arranged with large intercellular spaces.
• Entire mesophyll of the leaf, cortex and pith of the roots and the stems are parenchymatous.
• The cell wall is thin and made up of cellulose. It helps in storage of food, conduction of substances, provides turgidity to softer parts of plants.
• Chlorenchyma (parenchymatous cells having chloroplasts) participates in photosynthesis. It is present in mesophyll cells of leaves and differentiated into palisade parenchyma and spongy parenchyma. It is also present in cortex of young stem.
  • Palisade parenchyma: It is a type of chlorenchyma in which cells are elongated. In heliophytes (sun loving), more layers of palisade parenchyma are found as compared to sciophytes (shade loving).
  • Spongy parenchyma: It is also a type of chlorenchyma with cells of irregular shapes and sizes, thus leaving numerous intercellular spaces.
• Aerenchyma : A parenchyma with well developed intercellular spaces which forms a connected system throughout the entire plant is known as aerenchyma. It is common in submerged hydrophytes.
• Prosenchyma : Elongated fibre like thick walled parenchyma for rigidity and strength.
• Guard cells are modified parenchyma with chloroplasts.

Fig. 6.4 Parenchyma (labels: Nucleus, Intercellular space, Vacuoles, Cytoplasm).

Functions

• The main function of parenchyma is storage of food.
• Parenchyma always retains its meristematic character. It becomes meristematic during wound healing, formation of adventitious roots, grafting, etc.
• Photosynthesis, respiration, secretion, assimilation, etc are some of the important processes which occur in parenchymatous cells.
• In fleshy stems and leaves, parenchyma cells function as water storage tissue e.g. Opuntia, Euphorbia.
• In hydrophytes, aerenchyma makes the plants buoyant due to the presence of air and also gaseous exchange is facilitated.
• Chlorenchyma (parenchymatous cells that develop chloroplasts) participates in photosynthesis.
• Xylem and phloem parenchyma are meant for storage and lateral conduction of water/food.
• Sometimes such cells start to divide e.g., Cork cambium, root cambium, and help the plant in secondary growth and formation of cork.

Collenchyma

• It is living mechanical tissue, found beneath the epidermis (i.e., hypodermis) of herbaceous dicot stem.
• Collenchymatous cells are longer than parenchyma cell.
• The intercellular spaces in this tissue are absent because in intercellular spaces at the corner of cells thickenings of cellulose and pectin develop due to which the cell wall become rigid and thick at corners.
• They are absent in dicot roots, monocots and underground parts.

Fig. 6.5 Collenchyma (label: Thickening on corners due to deposition of cellulose and pectin).

Functions

• It provides mechanical support to the organs and due to its peripheral position in stems resists bending and pulling action of wind.
• Due to presence of pectin in cell wall, collenchyma can hold large amount of water.
• When chloroplasts are present it takes part in photosynthesis.

Sclerenchyma

• These are dead, mechanical tissue and act as skeleton in plants.
• It develops either from the procambium or secondarily from parenchyma due to secondary wall deposition.
• It is hardest plant tissue, made up of highly thick walled cells in which nucleus and cytoplasm are absent. This uniform thickening is made up of mainly lignin and cellulose or both. Cotton and flax fibres are rich in cellulose whereas jute and coir fibres are rich in lignin.
• Two types of cells are included in sclerenchyma i.e., sclerenchymatous fibres and stone cells (sclereids).

Fig. 6.6 Sclerenchyma (labels: A fibre with Lumen and Thick cell wall; A sclereid with Pits, Lumen and Thick cell wall).

Sclerenchymatous fibres:

• Fully developed fibres are long and tapering at the ends.
• In some cases the cell becomes so much lignified that the lumen is greatly reduced.
• They provide great strength and flexibility.
• The fibres are present in hypodermis of monocot stem, pericycle of many dicots, secondary wood, vascular bundle sheath of monocot stems, and hypodermis of many leaves.

Stone cells (Sclereids):

• They are lignified, extremely thick walled so that the lumen of the cells is almost obliterated.
• It provides grittiness to fleshy fruits.
• They form part of seed coat in some members of leguminoseae. They also provide mechanical strength to a softer tissue.

Functions

• The main function of sclerenchyma is to provide mechanical strength.
• Sclereids provide grittiness in the pulp of many fruits like guava, pear, sapota, stiffness and hardness to seedcoat, nut and endocarp of walnut and coconut.
• In leaves, sclerenchyma provides rigidity and saves it from collapsing.

COMPLEX PERMANENT TISSUES

A complex tissue can be defined as a collection of different types of cells performing a common function.

• Xylem and phloem are the complex permanent tissues found in vascular tissue of all vascular plants.
• Xylem is mainly responsible for conduction of water and minerals from the roots to the top of plants (unidirectional).
• It also provides mechanical support to the plant. Phloem mainly carries food. Both xylem and phloem are assemblage of living and dead cells and may be primary or secondary in origin.

Xylem (Wood)

• Xylem is water conducting tissue. It is also known as hadrome.
• Xylem is made of 4 types of cells i.e., tracheids, tracheae (vessels), xylem fibres and xylem parenchyma.
• Only xylem parenchyma is living and all other three are dead tissues.
• Both tracheids and vessels are thick-walled. Vessels and tracheids are called tracheary elements because they conduct sap.

Tracheids

• The tracheids are dead (without cytoplasm and nucleus) at maturity.
• The tracheids form a long row placed one above the other.
• The cells are long, tubular with tapering, chisel like ends. The cell wall is lignified. Thickening in wall may be scalariform, annular, reticulate and pitted. Tracheids are generally with bordered pits.
• The main function of tracheids is to conduct water but due to their thick and hard walls they also provide mechanical support.

Tracheae (Xylem vessels)

• Tracheae differ from tracheids in being cell fusions arising through the dissolution of end walls.
• Thus, vessel is syncytial (multicellular) in origin. Vessels are also called septaless tracheids.
• Vessels is an elongated tube like structure that is composed of row of cells placed one above the other with their transverse walls absent.
• The walls of vessels are lignified and less thicker than tracheids. The lumen is wider than tracheids.
• The thickenings may be annular, spiral, scalariform and reticulate etc.
• A wood with xylem vessels is called heteroxylous or porous wood.

Fig. 6.7 Xylem (labels: Tracheids, Vessels, Border pits, Simple perforation plate).

Connecting Concepts

• Vessels are generally upto 10 cm in length but longest vessels are found in Eucalyptus and oak (Quercus), which are from 2 metre to 5 metre in length.
• Vesselless angiosperms : Although vessels are characteristic features of angiosperms but there are certain angiosperms where vessels are absent. Such vesselless angiosperms belong to families :
  1. Winteraceae (e.g., Wintera).
  2. Tetracentraceae (e.g., Tetracentron).
  3. Trochodendraceae (e.g., Trochodendron).
• Besides, vessels are absent in stem and leaves of Yucca and Dracaena.
• There are some non-angiosperms where vessels are present, e.g., some species of Selaginella, two species of Pteridium, (pteridophytes) and order Gnetales of gymnosperms (e.g., Gnetum, Welwitschia and Ephedra).

Wood (Xylem) fibres

• They are long, narrow and tapering at both ends.
• The walls are lignified.
• They provide only mechanical support.

Wood (Xylem) parenchyma

• These are thin walled (cellulose) living cells.
• They are present in primary as well as secondary xylem.
• They store food material and also involved in short distance lateral transport of sap.

The first formed xylem is called protoxylem and the later formed xylem is called metaxylem.

The vessels and tracheids of protoxylem are small and have annular or spiral thickenings on their secondary walls. In protoxylem, lignification occurs before the completion of elongation.

Vessels and tracheids of metaxylem are bigger and have scalariform, reticulated and pitted thickenings on their secondary walls (pitted thickenings are common). In metaxylem, lignification occurs after the completion of elongation. Fibres often occurs in metaxylem.

Depending upon position of protoxylem, xylem can be exarch, endarch or mesarch

Exarch (centripetal): Xylem matures from outside to inside and protoxylem lies towards the outside of vascular bundle, e.g., roots.

Endarch (centrifugal): Xylem matures from inside to outside and protoxylem lies towards the innerside of vascular bundle, e.g., stem.

Mesarch : Protoxylem lies in the middle of metaxylem, e.g., ferns.

Secondary xylem is derived from cambium during secondary growth. It has axial or vertical system (tracheids, vessels, xylem fibres and xylem parenchyma) and ray or horizontal system (ray parenchyma). It is neither exarch or endarch and not differentiated into proto or metaxylem. That is why, it is difficult to distinguish secondary xylem of root from stem after secondary growth.

Phloem (Bast)

• Phloem is also a conducting tissue that transports food materials, usually from leaves to other part of the plants (bidirectional).
• It is a complex tissue made up of 4 kinds of cells in angiosperms, these are sieve elements, companion cells, phloem fibres, phloem parenchyma.
• Phloem of gymnosperms have albuminous cells and sieve cells. They lack sieve tubes and companion cells.

Sieve element

• The sieve elements in angiosperms are sieve tubes arranged one above the other in distinct linear rows and have sieve plates (oblique or transverse perforated septa) on their end walls. These are associated with companion cells.
• A mature sieve element possesses a peripheral cytoplasm and a large vacuole but lacks nucleus. The functions of sieve tubes are controlled by the nucleus of companion cells.
• In gymnosperms and pteridophytes, the sieve elements have sieve plates to their lateral walls. Companion cells are absent and there is no distinct arrangement of sieve cells in linear rows – such sieve elements are called sieve cells.
• During winter, in temperate plants the pores in sieve plate get temporary plugged by insoluble carbohydrate called callus to prevent transport of food. The callus is soluble and disappears when contents of the sieve tube are dilute. At the advance of spring, this substance dissolves and the pores open again. On injury, sieve tubes collapses and sealed off by callose and protein.

Companion cells

• Companion cell is a long, narrow, thin walled cell that is usually attached to the lateral side of sieve tube.
• It contains cytoplasmic contents and a large nucleus.
• They are supposed to help in maintaining pressure gradient in sieve tubes. They are absent in gymnosperms and pteridophytes.
• In gymnosperms, companion cells are replaced by parenchyma cells called albuminous cells (or strasburger cells). Companion cells are absent in protophloem.
• The companion cells and sieve tubes maintain close cytoplasmic connections with each other through fine pits which are present between their common longitudinal walls.

Connecting Concepts

• In primary phloem of herbaceous plants, a single companion cell is associated with a single sieve tube element whereas in phloem of woody plants, many companion cells are associated with a single sieve tube element.
• Austrobaileya is an angiosperm without companion cell.

Phloem fibres

• These are sclerenchymatous dead cells that provide mechanical strength. They have thick wall with simple pits.
• These are absent in leaves, roots and primary phloem.
• The fibres from secondary phloem are commercially important as they are used for making ropes and coarse textile.

Phloem parenchyma

• These cells are living, thin walled parenchyma cells that store food, latex, tannin, crystals, etc. They are intermingled with sieve tubes.
• They are absent in monocots and in some dicots (e.g., Ranunculus).

The first formed primary phloem consists of narrow sieve tubes and referred as protophloem and the later formed phloem has bigger sieve tubes and referred as metaphloem.

Fig. 6.8 Phloem. (A) T.S.: Sieve plate, Sieve pore, Sieve tube, Phloem fibres. (B) L.S.: Sieve tube, Phloem parenchyma cells, Companion cell, Callose, Sieve plate.

SPECIAL TISSUES

These tissues perform special functions i.e., secretion and excretion in plants. They are found in pith, cortex, xylem and phloem. These tissues are of two types i.e., glandular tissue and laticiferous tissue.

Glandular tissues

A gland is a specialized group of cells that are endowed with the capacity to secrete or excrete products, e.g. secretion of resin gum, oil and latex. These are of two types:

• External glands : These generally occur in the epidermis of stem and leaves as glandular outgrowth, e.g., glandular hair, as in Urtica dioica stinging hairs, nectar secreting and enzyme secreting glands e.g. Drosera, Nepenthes etc.
• Internal glands: These are of several types, e.g., oil glands in Citrus, Eucalyptus, resin glands in Pinus, Cedrus, water secreting glands.

Laticiferous tissues are mainly composed of thin walled, elongated, branched and multinucleate tube like structures that contain colourless milky or yellow coloured juice called latex.

They are scattered throughout the ground tissue of the plant and contain stored organic matter in the form of starch, rubber, tannins, alkaloids, mucilage enzymes, protein, etc.

Connecting Concepts

• Water-secreting glands hydathodes or Water-stomata : Many plants like Garden Nasturtium, Colocasia, Pistia and Eichhornia possess special structures, which exude impure water in the form of small drops (Guttation).
• This guttation is through certain pores called hydathodes, which are present at the margins of the leaf. These are always open and large in size.
• In Hevea (rubber), laticifers occur in secondary phloem and in papaya, both in xylem and phloem.

THE TISSUE SYSTEM

Each system usually consists of only one tissue or an association of tissues which perform a common function. There are three main tissue systems :

• Epidermal tissue system
• Fundamental or ground tissue system
• Vascular tissue system

The Epidermal Tissue System

• This forms the outer covering of plant. It is in direct contact with the external environment.
• It performs manifold function, e.g., protection, absorption, excretion, secretion, gaseous exchange and control of transpiration, etc.

Epidermal tissue system includes :

Epidermis: It is the outermost layer of all organs of primary plant body. It is made up of elongated, compactly arranged single layer of cells.

• However, in leaf of Nerium, it is multi layered.
• The outer wall of epidermal cell is thick, inner is thin and radial walls are thick above and thin below.
• Epidermis of root differs in origin, structure and function and called as epiblema or piliferous layer.
• Cells constituting epidermal cells are parenchymatous with little quantity of cytoplasm with large vacuole.
• The outer wall of epidermal cells contain a fatty substance called cutin and form cuticle on the epidermis. Cuticle prevents the loss of water. It is absent in roots.
• Epidermis is protective in function.

Stomata: Stomata are tiny pores that are found on the epidermis of leaves, stem and some fruits.

• Stomata are absent in roots.
• Stomata helps in gaseous exchange at the time of respiration and photosynthesis.
• Stomata are composed of two bean-shaped epidermal cell called guard cells which encloses stomatal pore.
• In grasses, guard cells are dumb bell shape. The two guard cells attached to each other at both ends.
• Unlike other epidermal cells, the guard cells have chloroplasts and the outer walls of guard cells (away from the stomatal pore) are thin and the inner walls (towards the stomatal pore) are highly thickened.
• The guard cells may be surrounded by varying number of specialized epidermal cells called subsidiary cells or accessory cells. The guard cells control the opening and closing of stomata, which in turn, regulate the loss of water from the leaves to the atmosphere through transpiration.
• The stomatal aperture, guard cells and the surrounding subsidiary cells are together called stomatal apparatus.
• In dicot leaves, stomata are scattered whereas in monocots, these are arranged in rows. Further in dicots these are present generally on lower epidermis whereas in monocots, on both surfaces.
• In free floating hydrophytes, stomata are present on upper epidermis, whereas in submerged hydrophytes, these are absent or non-functional (in algae and fungi stomata are totally absent.)
• In xerophytes, stomata are of sunken type so as to reduce water loss by transpiration.

Check Point

Give the alternate name for the following tissue –

1. Dermatogen
2. Periblem
3. Plerome
4. Piliferous layer
5. Bast

Epidermal appendages (trichomes and epidermal hairs):

• Numbers of hairs are found on epidermis.
• The root hairs are unicellular and represent the extention of epidermal cells.
• On stems, epidermal hairs are called trichomes.
• In shoot system, they are usually multicellular. They may be soft or stiff, branched or unbranched. Sometimes they are secretory.
• They help in retarding the rate of transpiration.
• Root hairs help in absorption of water and minerals from soil.

Ground (Fundamental) Tissue System

• All tissues except epidermis and vascular bundle constitute the ground tissue.
• It consists of simple tissues such as parenchyma, collenchyma and sclerenchyma.
• In dicot stems, roots and monocot roots (in which vascular bundles are in a ring), the ground tissues constitute the following parts :

Cortex : It is differentiated into the following tissues.

• Hypodermis : Generally one or two layers of continuous or discontinuous tissue present below epidermis, is called hypodermis.
  • In dicot stem, hypodermis is collenchymatous and in monocot stem, sclerenchymatous. It is protective in function.
• General cortex : It is few to many layered thick and in most of the cases, it is thin-walled and parenchymatous with intercellular spaces. The cells of cortex contain starch grains, oil, tannins and crystals of various types.
  • Sometimes in young stem, chloroplasts develop in cortical cells, which is called chlorenchyma. In hydrophytes, cortex is aerenchymatous (with air cavities).
  • It provides mechanical support and stores food material.
• Endodermis : Endodermis is single layered structure which separates cortex from stele.
  • It is innermost layer of the cortex.
  • The cells of endodermis are barrel-shaped without inter-cellular spaces, living and containing starch. (Hence it is known as starch sheath).
  • Radial and tangential walls of endodermal cells possess thickenings of lignin, suberin and cutin in the form of strips or bands, which are known as casparian bands or casparian strips (as it were discovered by Caspar).
  • A well defined endodermis is constant feature of roots (not in stem).
  • There are thick-walled and thin-walled cells in endodermis. Thin-walled cells are known as passage cells or transfusion cells, which are present opposite protoxylem.
  • It acts as a watertight dam between vascular and nonvascular regions, stores starch, serve as protective layer and maintains root pressure.

Connecting Concepts

• Orchids root : In the aerial roots of orchids, outer most layer is made up of multilayered velamen tissue, which are large, dead cells with fibrous thickenings. These contain air in dry conditions and absorb moisture when atmosphere is wet. Outermost layer of velamen is limiting membrane having cuticle and inner to velamen is exodermis (outermost layer of cortext), which is have thick and thin-walled cells. Thin-walled cells are called passage cells. The main function of aerial root of orchid is to absorb moisture and so they are known as hygroscopic roots.
• In Smilax root, pericycle is multilayered and sclerenchymatous. Pericycle is present in most of the roots except in roots of parasitic plants and hydrophytes.

Pericycle : It is single or few layered parenchymatous or rarely sclerenchymatous cells.

• In some cases like sunflower stem, pericycle is heterogenous, i.e., composed of parenchyma and sclerenchyma.
• Sclerenchymatous pericycle is mechanical in function whereas parenchymatous pericycle is storage in function.
• In angiosperms, pericycle gives rise to lateral roots.

The Vascular Tissue System

• The central cylinder of the shoot or root surrounded by cortex is called stele. The varying number of vascular bundles formed inside the stele constitutes vascular tissue system.
• Each vascular bundle is made up of xylem and phloem with or without cambium.
• The vascular bundles in dicot stems and roots as well as monocots roots are arranged in a ring while in monocot stem they are scattered in general ground tissue.

Elements of a vascular bundle

• Xylem : In dicot and monocot stem, the development of xylem takes place towards periphery i.e., protoxylem is formed towards center and metaxylem towards the periphery. This is called centrifugal xylem and this condition is called endarch condition.
• Phloem: In stem, the phloem is away from the center towards the periphery and situated above the xylem. The phloem parenchyma is absent in monocots.
• Cambium: A thin strip of primary meristem is present in between xylem and phloem in dicotyledonous stems. It is called vascular cambium. In monocot stem, cambium is absent.
  • In roots, the xylem and phloem are not aggregated in a common vascular bundle but form separate strips that alternate each other.

Types of Vascular Bundles

There are three types of vascular bundle i.e., radial, conjoint and concentric.

• Radial : It is the most primitive type of vascular bundle. In this type of vascular bundles, alternate strands of phloem and xylem are present on different radii.
  • This type of vascular bundle is found in roots.
• Conjoint : Here, xylem and phloem are present on the same radius. This type of vascular bundle is found in stems and leaves.
  • Such vascular bundles are of two types i.e., collateral and bicollateral.
  • Collateral : It has one patch of phloem on the outer side and one patch of xylem on the inner side. If cambium is present in between the phloem and the xylem, it is called open and if the cambium is absent, it is called closed.
  • Bicollateral : It has an outer and an inner phloem as well as an outer and inner cambium. The xylem occurs in the middle of the vascular bundle.
• Concentric : In this vascular bundle, either xylem surrounds the phloem or phloem surrounds the xylem. Concentric vascular bundles are of two types—
  • Amphicribral (hadrocentric or centroxylic) : The xylem is surrounded by phloem, e.g., Ferns.
  • Amphivasal (leptocentric or centrophloic) : The phloem is surrounded by xylem, e.g., Dracaena, Yucca.

Fig. 6.9 Types of Vascular Bundles

Connecting Concepts

• The root hairs are not permanent structures. Active life span is upto 7 days. Afterwards they shrivel. The older epiblema cells then become cutinised and suberised.
• Bicollateral vascular bundles are found in families Cucurbitaceae, Solanaceae and Myrtaceae.

ANATOMY OF DICOTYLEDONOUS AND MONOCOTYLEDONOUS PLANTS

Main characteristics of internal structure of root are :-

• On outer side, there is single layered epiblema or piliferous layer or rhizodermis, which bears unicellular root hairs.
  • Cuticle and stomata are absent. (Root hair arise from epiblema).
• Inner to epiblema is multilayered cortex.
• Inner to cortex is present single layered distinct endodermis or starch sheath with distinct casparian strips or bands.
• Inside the endodermis, there is present distinct pericycle.
• Vascular bundles in root are radial.
• Xylem is exarch or centripetal.
• Phloem is made of sieve tubes, companion cells and phloem parenchyma. (Phloem parenchyma is absent in monocot root).
• In the centre, pith is well developed or ill defined.

DICOTYLEDONOUS ROOTS

Internally, a dicotyledonous root exhibits the following five zones in a transverse section :

Epidermis or piliferous layer :

• It is the outer protective layer of very compact cells. It is single layered.
• The epidermis or epiblema of roots lack both the cuticle and guard cells. Stomata are completely absent.
• It produces a number of unicellular root hairs.

Cortex : The cortex is well developed in the roots. The cells of the cortex are multilayered parenchymatous with large intercellular spaces.

Endodermis :

• It is innermost layer of the cortex.
• The cells of the endodermis are barrel shaped.
• They have thickened strips on radial and transverse walls. The thickened strip has deposit of lignin, suberin or cutin known as casparian strip.
• Casparian strips do not allow water and mineral ions to pass through it.
• There are few thin-walled cells in the endodermal layer. They are called passage cells. They are present opposite to protoxylem elements to permit an easy passage of water.

Pericycle :

• It is outer boundary of stele.
• It is single-layered or multilayered and consists of parenchyma cells. It gives rise to lateral roots as well as cork cambium.
• It gets lignified in older roots in monocotyledons. The pericycle is absent in some aquatic plants and parasites.

Stele : All tissues on the innerside of the endodermis such as pericycle, vascular bundles and pith constitute the stele are usually two to four xylem and phloem patches constituting radial vascular bundle. Later, a cambium ring develops between the xylem and phloem. Xylem is exarch.

• The parenchymatous cells which lie between the xylem and the phloem are called conjuctive tissue.
• Pith : It is either absent or very small and centrally placed.

Fig. 6.10 (a) T.S. of young root of Gram (dicotyledonous root); (b) T.S. of Maize (monocotyledonous root). Labels: Root hair, Epiblema, Cortex, Endodermis, Pericycle, Phloem, Protoxylem, Metaxylem, Pith.

Connecting Concepts

Reasons for Identification of dicot root.

1. Presence of root hairs.
2. Endodermis with casparian strips.
3. Absence of pith.
4. Radial vascular bundles less than eight.
5. Exarch xylem.

MONOCOTYLEDONOUS ROOTS

Anatomy of monocot root is very much similar to dicot roots but they do not undergo any secondary growth.

• Epiblema or piliferous layer : It is a single layer of thin-walled parenchyma cells. It has several unicellular hairs.
• Cortex : It is similar to that of dicot root.
• Endodermis : It is similar to that of dicot root.
• Pericycle : It is also similar to that of dicot root. In old roots the cells become lignified.
• Vascular bundles : Like dicot roots they are also radial and exarch. The number of vascular bundles, however is more than six (polyarch). Phloem is made up of sieve tubes and companion cells. Phloem fibres are rare. Phloem parenchyma is absent. Protoxylem is in contact with pericycle. Metaxylem is towards the inner side. Xylem is formed of vessels and xylem parenchyma. Fibres and tracheids are very few.
• Pith : It is large and well developed.
• Monocotyledonous roots do not undergo any secondary growth.

Connecting Concepts

Reasons for Identification of monocot root.

1. Presence of root hairs.
2. Endodermis with passage cells.
3. Presence of pith.
4. Radial vascular bundles more than eight.
5. Exarch xylem.
6. Occurrence of exodermis.

Difference between dicot and monocot roots :

Fig. 6.11 Comparison of the T.S. of monocot root and dicot root (labels: Root hair, Cortex, Protoxylem, Phloem, Metaxylem, Pith, Pericycle, Epidermis, Epiblema).

Connecting Concepts

• In Maize and Allium the outermost layer of the cortex possesses casparian strips on the anticlinal walls of the cells. After the shrivelling of epiblema and root hairs, the outer one (Smilax) or many (Maize) layers of the cortex become suberised and thick walled and form special layer called exodermis. In Pineapple and some grasses., sclerenchyma also develops inner to exodermis to provide extra support.

Main features of internal structure of stem are :

• Cuticle is usually developed.
• Hairs are multicellular.
• Vascular bundles are conjoint, collateral or bicollateral or even concentric. Further vascular bundles may be open or closed.
• Xylem is endarch (centrifugal), i.e., protoxylem is towards pith and metaxylem towards periphery.

Table : Differences between internal structure of root and stem

DICOTYLEDONOUS STEM

A young stem of sunflower exhibits typical features of a dicotyledonous plant. Internally, it is differentiated into the following parts :

Epidermis: It is single layered and consists of thin-walled parenchyma cells. The cells are flattened and lack intercellular spaces. Epidermis produces a number of multicellular hairs. There is a thick cuticle above the epidermis that reduces loss of water from surface of stem.

Cortex: It lies immediately below epidermis. Cortex is several layered and is differentiated into three regions: an outer hypodermis, middle cortical layers and an inner endodermis.

• Hypodermis: The hypodermis is composed of 4 or 5 layers of collenchymatous cells. They are living cells and contain chloroplasts. Hypodermis provides mechanical strength to plants, stores food and helps in photosynthesis.
• Cortical layer: Cortical layer lies between hypodermis and endodermis. It is made up of loosely arranged parenchymatous cells with intercellular spaces. Inside cortex number of oil ducts are present which are surrounded by protoplasmic glandular cells. Main functions of cortical layers are storage of food and carry out photosynthesis.
• Endodermis: It is innermost layer of cortex that separates the vascular tissue from cortical cell. It is a single layer of barrel-shaped cells which have conspicuous casparian strips. The cells of the endodermis are rich in starch grains and the layer is also referred to as the starch sheath.

Pericycle : It lies between the cortex and the vascular tissue. It is made up of both sclerenchymatous and parenchymatous cells. It is homogeneous and sclerenchymatous in Cucurbita. Sclerenchymatous cell forms a semilunar patches and placed above the phloem. Sclerenchymatous cells provide mechanical support and parenchymatous cells store food.

Vascular bundles : The vascular bundles of dicotyledonous stems are always arranged in the form of a ring. They are conjoint and open. The structure of the phloem and the xylem is typical. The larger metaxylem is on the outer side and the smaller protoxylem elements to the inner side i.e. the development of the xylem is centrifugal (endarch).

Primary medullary rays : The ground tissue found in between the vascular bundles is called medullary ray. It is madeup of parenchymatous cells and involved in radial conduction of food and water.

Pith or medulla: Pith is the ground tissue present in the centre. It is made up of large parenchymatous cells with intercellular spaces. Its main function is storage of food.

Fig. 6.12 (a) Diagrammatic representation of T.S. of sunflower stem (labels: Hair, Epidermis, Hypodermis, Cortex, Endodermis, Pericycle, Phloem, Cambium, Primary xylem, Pith); (b) Detailed structure of a portion of T.S. of sunflower stem (labels: Shoot hair, Cuticle, Epidermis, Hypodermis, Cortex, Resin duct, Endodermis, Pericycle, Phloem, Cambium, Metaxylem, Protoxylem, Vascular bundle, Medullary ray, Pith).

Connecting Concepts

• In sunflower stem, sclerenchymatous patches of pericycle present outside the vascular bundle known as bundle caps.
• In some dicot stems, scattered vascular bundles are present (instead of in a ring). e.g., in Peperomia, Piper, Thalictrum, Anemone, Nymphaea, etc.
• In root, there is radial arrangement of xylem and phloem strands and further xylem in root is exarch (i.e., protoxylem towards periphery). In stem, vascular bundles are conjoint and collateral and xylem is endarch (i.e., protoxylem towards centre). Inspite of these differences in arrangement, vascular continuity is maintained between root and shoot. The change from exarch condition of xylem in root to endarch condition in stem occurs in a specific region of axis, called transition zone or transition region. It is generally hypocotyl region (at base, near its middle or in upper part) in which this change or transition occurs.

MONOCOTYLEDONOUS STEM

The monocotyledonous stems are characterized by lack of differentiation of the ground tissue into cortex, pericycle, pith and by presence of the scattered vascular bundles. The details of its anatomy are as follows :

• Epidermis : It is a single layer of thin-walled parenchyma cells. A thick cuticle is present on the outer surface of epidermis.
• Hypodermis : The hypodermis consists of two or three layers of thick-walled sclerenchyma cells.
• Ground tissue : The entire mass of parenchymatous tissue within the hypodermis is called the ground tissue which consists of thin-walled spherical cells. The cells enclose large intercellular spaces.
• Vascular bundles : They are scattered all over in the ground tissue. Vascular bundle is surrounded by sclerenchymatous bundle sheath. Vascular bundles are collateral and closed. Peripheral vascular bundles are generally smaller than the centrally located ones. The phloem parenchyma is absent. The xylem consists of tracheids and four large vessels which are arrange in the form of the letter Y. Examples are Oxalis, Marsilea, grasses, etc.

Fig. 6.13 (a) Diagrammatic representation of T.S. of maize stem (labels: Cuticle, Epidermis, Hypodermis, Vascular bundles, Ground tissue); (b) Detailed structure of a portion of T.S. of maize stem (labels: Cuticle, Epidermis, Sclerenchyma, Parenchyma, Ground tissue, Bundle sheath, Protophloem, Metaphloem, Metaxylem, Protoxylem, Lysigenous cavity, Vascular bundle).

Differences between internal structure of monocot stem and dicot stem

DORSIVENTRAL (DICOTYLEDONOUS) LEAF

These are found in dicots. These are generally horizontal and sunlight falls on their upper surface (ventral surface or adaxial surface).

The important anatomical features of dorsiventral leaves are :

• Upper epidermis : It is single-layered. It is covered with a thick cuticle. It has more stomata than the lower epidermis.
• Mesophyll : It is differentiated into an upper palisade parenchyma and spongy parenchyma. The cells of the palisade parenchyma are cylindrical in shape and are compactly arranged. They have chloroplasts and, therefore, photosynthesise. They also check transpiration. The spongy parenchyma are loosely arranged with large intercellular spaces. They also contain chloroplasts and carry out photosynthesis.
• Vascular bundle : There is one prominent vascular bundle and several small ones. The size of the vascular bundles are dependent on the size of the veins. The veins vary in thickness in the reticulate venation of the dicot leaves. It has phloem towards the lower (abaxial) side and xylem towards the upper (adaxial) side. The vascular bundle is enclosed by thick walled bundle sheath.
• Lower epidermis : It is single layered and has less stomata or may be absent.

Connecting Concepts

• In dorsiventral leaf, above and below the largest V.B., there are present parenchymatous or collenchymatous mass of cells which do not contain chloroplast.
• In some dicot leaves multilayered upper epidermis is present, e.g., in Ficus, Piper, Nerium as well as Begonia. Cuticle is well developed on upper epidermis.

Fig. 6.14 T.S. of dicot leaf of mango (labels: Parenchyma, Cuticle, Upper epidermis, Palisade parenchyma, Bundle sheath, Xylem, Phloem, Spongy parenchyma, Sub-stomatal cavity, Stoma).

ISOBILATERAL (MONOCOTYLEDONOUS) LEAF

These are found in monocots and are generally vertical. In transverse section, following structure is seen :

• Upper epidermis : It is composed of large-sized parenchyma cells. Stomata are also present in this layer. A thick cuticle is present outside the epidermis. Bulliform cells may be present in grasses. When the bulliform cells in the leaves have absorbed water and are turgid, the leaf surface is exposed. When they are flaccid due to water stress, they make the leaves curl inwards to minimise. There are also known as motor cells e.g., in Psamma, Poa, Agropyron, Amophilla.
• Mesophyll : It is composed of round, oval or cylindrical cells. There is no differentiation into palisade and spongy parenchyma.
• All the cells of the mesophyll are alike.
• They are loosely arranged. They have large number of chloroplasts. In sciophytes, there may be no spongy parenchyma and the entire mesophyll may consist of only palisade parenchyma.
• Vascular bundles : The vascular bundles are of uniform size. Like the dorsiventral leaves, they have phloem on the lower side and xylem on the upper side.
• Vascular bundles are conjoint, collateral and closed.
• Lower epidermis : Like the upper epidermis, the cells of the lower epidermis are single-layered and composed of compactly arranged parenchyma cells. Stomata are also present in the lower epidermis.

Fig. 6.15 T.S. of maize leaf (labels: Upper epidermis, Cuticle, Mesophyll tissue, Bulliform cells, Vein, Lower epidermis, Midrib, Sclerenchyma, Stoma, Sub-stomatal cavity).

Check Point

Fill in the blanks

1. The cambium ring of dicot stem cut off cells towards pith, mature into _________ while towards periphery mature into _________.
2. In sciophytes, there may be no _________ and entire mesophyll consists of _________.
3. Vascular bundles of isobilateral leaf are _________ , _________ and _________.
4. _________ cells of grasses check transpiration by rolling inwards.
5. The cambium of dicot stem are more active on _________ side.

Table : Differences between dicot and monocot leaf

SECONDARY GROWTH

• Growth of stems and roots in length due to activity of primary (apical) meristems is called as primary growth.
• In most dicot stems and roots, in later stage lateral meristems (vascular cambium and the cork-cambium) become active and cause increase in the thickness. This is called as secondary growth.
• Secondary growth occurs due to addition of secondary tissues (secondary cortex, secondary phloem and secondary xylem) formed by the activity of vascular cambium in the stelar region inside pericycle and cork cambium in the extrastelar regions (region outside pericycle i.e., cortex).

Connecting Concepts

• Anamolous or abnormal secondary growth in some monocot stems, e.g., in Yucca, Dracaena, Aloe, Sensiviera and Agave, etc.

Secondary Growth in dicot stem

• The vascular bundles in dicot stem are conjoint, collateral and open and are arranged in a ring.
• Secondary growth is initiated by vascular cambium. Initially cambium is limited only to the vascular bundles called as intra fascicular cambium (present between phloem and xylem).
• The parenchymatous cells of the medullary rays lying with the fascicular cambium become meristematic and give rise to the interfascicular cambium.
• The interfascicular and the fascicular cambium join with one another to form a complete cambium ring.
• The cambial ring because becomes active and begins to cut off new cells, both towards the inner and the outer sides. The cells cut off towards pith, mature into secondary xylem and the cells cut off towards periphery mature into secondary phloem.
• The cambium is generally more active on the inner side than on the outer. As a result, the amount of secondary xylem produced is more than secondary phloem and soon forms a compact mass.
• The primary and secondary phloems get gradually crushed due to the continued formation and accumulation of secondary xylem. The primary xylem however remains more or less intact, in or around the centre.
• At some places, the cambium forms a narrow band of parenchyma, which passes through the secondary xylem and the secondary phloem in the radial directions. These are the secondary medullary rays.

Fig. 6.16 Diagrams (A)-(D) showing the activity of cambium ring (labels: Epidermis, Cortex, Primary phloem, Fascicular cambium, Primary xylem, Secondary xylem, Secondary phloem, Cork, Secondary cortex, Phloem, Cambium, Xylem).

Connecting Concepts

• Rhytidome. The layer of cork cambium or phellogen becomes inactive after sometimes and successive phellogen layers develop in deeper tissues and all the dead cells outside the innermost layer of cork cambium or phellogen constitute rhytidome.
• Tyloses. These are balloon-like structures which are produced due to ingrowth of adjacent xylem parenchyma cells into the lumen of xylem vessels through pits.
• The age of tree can be determined by counting annual rings in oldest or basal portion of tree trunk. Calculation of age of the tree by counting annual rings is called dendrochronology.

Formation of Annual Rings or Growth Rings

• The activity of vascular cambium fluctuates with change in climate and under the control of a series of physiological and environmental factors.
• The cambium is more active during spring and summer season due to more requirement of food dissolved in water, high temperature, longer duration of sunshine, etc., hence cambium forms large-sized secondary xylem elements. This type of secondary xylem is called the spring wood or early wood.
• During autumn and winter season, the cambium is less active and it forms smaller, thicker and narrow xylem vessels. This is called the autumn wood or late wood. The spring wood is lighter in colour and has a lower density whereas the autumn wood is darker and has a higher density.
• The periodical activity of the cambium, thus results in distinct growth layers or rings of secondary xylem. These are called growth rings. The growth rings of spring wood and autumn wood produced in a year constitute an annual ring.

Wood

• Wood is cellular and contains all tissues inner to cambium i.e., xylem and pith.
• The secondary xylem is much larger in amount and constitutes the main bulk of woody stems. It is without phloem.
• Amount of cellulose in wood is 20-50% and remaining part is lignin and hemicellulose.
• Maximum part of dicot wood is made up of vessels and fibres.
• Wood of palms and bamboos (poorman wood) has no secondary xylem. It grows by primary meristem only.

Check Point

Name the cells/tissues which constitute the following

1. Phellem
2. Phelloderm
3. Phellogen
4. Inter fascicular cambium
5. Bark

• There are two types of wood i.e., soft wood and hard wood.
• The gymnospermic wood is called softwood and the angiospermic wood is called hardwood.
• Hardness and softness depend on the content of xylem fibres and xylem rays in the wood.
• Soft wood : It is nonporous, homoxylous gymnospermic wood rich in tracheids and xylem parenchyma, vessels and fibres are absent and resin canals are present. This wood is good for cabinet or fancy work, e.g, Deodar (Cedrus), Pine (Pinus).
• Hard wood : It is porous, heteroxylous angiospermic wood rich in vessels and fibres, tracheids and xylem parenchyma are fewer, xylem rays are multiseriate and resin canals are absent. This wood is good for construction work e.g., Teak (Tectona grandis), Sal (Shorea), Shisham (Dalbergia). Hard wood is of following two types:
  • Ring porous wood: The woods of plants in temperate (cold) region have distinct annual rings. The vessels of the spring wood elements are much larger than those of the autumn wood. This type of wood is called the ring porous. Ring porous wood is specialised and conduct more water as they are formed early during the development when need for water is great, e.g., Cassia, Dalbergia (Shisham), Quercus (oak).
  • Diffuse porous wood: In this type of wood, all the vessels are of uniform diameter and there is only a gradual change in the size of the elements the wood. This wood is found in plants of tropical (hot) climate e.g., Acer, Betul.

Heartwood and Sapwood

• In a very old stem the secondary xylem elements in the inner part turn darker in colour and is called the heartwood or duramen.
• It consists of dead elements which have a deposit of gums, resins, oils, tannins in their walls and lumens.
• The heartwood is, therefore, very strong and durable and imparts great amount of mechanical strength to the stem.
• The sapwood or alburnum is the light coloured peripheral part of the secondary xylem. It consists of dead tracheids, vessels, and fibres and some living cells.
• The sapwood elements continue to perform the function of conducting water and solutes etc. They also give mechanical support to the stem.

Fig. 6.17 Hard wood and Sap wood (labels: earlywood, growth ring, ray, pith, heartwood, sapwood, vascular cambium, inner bark, secondary phloem, cork cambium, outer bark).

Cork Cambium

• In hypodermis or outer cortical cells, a layer becomes meristematic which is known as cork cambium or phellogen.
• This phellogen also divides off cells both on its outer side and inner side.
• The cells cut off on outer side are phellem or cork cells and cells cut off on inner side are phelloderm or secondary cortex.
• The phellem or cork cells are dead and have deposition of a fatty substance called suberin.
• Phellem, phellogen and phelloderm collectively constitute periderm.
• The function of periderm is protective (because at maturity epidermis ruptures and hence protection function is performed by periderm).
• Bark is the loose term and is used to define all the tissues, outside vascular cambium. Bark = periderm + cortex + pericycle + primary and secondary phloem. Bark that is formed early in the season is called early or soft bark. Towards the end of the season, late or hard bark is formed.
• Lenticels are some loosely arranged areas in the periderm. Lenticels are characteristics of woody stem. They help in gaseous exchange and transpiration.

SECONDARY GROWTH IN DICOT ROOTS

Dicotyledonous roots like the dicotyledonous stems increase in thickness as a result of addition of secondary tissues cut off by the cambium and the cork cambium.

Origin and activity of the cambium ring

• The first event in secondary growth of a dicotyledonous root is the origin of the cambium.
• The cells of the conductive tissue just inside the phloem and just outside the xylem become meristematic. In order to make this cambium ring circular the wavy band of cambium becomes active only in those parts which lie on the inner side of the phloem patches.
• These cambium strips cut off secondary xylem elements only on the inner side as a result of which the primary phloem are gradually pushed on to the outer side.
• This sort of unequal behaviour of the cambium results in the conversion of its wavy band into a circular ring of cambium.
• Once the cambium assumes a circular outline it becomes uniformly meristematic and starts cutting off cells both on the inner and the outer sides.
• Except in the regions of the medullary rays the inner cells get modified into secondary xylem and the outer cells into those of the secondary phloem.

Origin and activity of the cork cambium

• Addition of secondary tissues in the central part of the root exerts lot of pressure on the peripheral tissues which get ruptured.
• A new meristem, known as cork-cambium, develops in the single-layered pericycle to replace the lost peripheral part of the root.
• The cork-cambium or the phellogen is few layers in thickness and composed of thin-walled rectangular cells. It cuts off cells on both the outer and the inner sides. The cells on the outer side are modified into brown-coloured suberized cork cells and on the inner side remain thin-walled and parenchymatous. It is termed secondary cortex.
• The cork and the secondary cortex are also known as phellem and phelloderm respectively. Due to activity of cork cambium, pressure builds up on the remaining layers of phellogen and ultimately these layers die and slough off.
• The bark of a root is comparatively thinner. Lenticels are fewer in number.

Fig. 6.18 (a)-(b) Diagrams showing secondary growth in dicot root (labels: Formation of cambium, Cortex, Periderm, Epidermis, Medullary ray, Primary phloem, Secondary phloem, Cambium, Secondary xylem, Primary xylem).