The document provides an overview of plant anatomy, focusing on the classification and functions of various plant tissues, which are categorized into meristematic and permanent tissues. Meristematic tissues are responsible for plant growth and consist of actively dividing cells, while permanent tissues have differentiated cells that perform specific functions, such as transporting water and nutrients or providing structural support. The text details specific types of tissues, including parenchyma, collenchyma, sclerenchyma, xylem, and phloem, along with their characteristics and roles in the plant structure.

1. STD 11 BIOLOGY
CHAPTER 06
ANATOMY OF
FLOWERING PLANTS
SANJAY SIDDHAPURA
M.Sc., B.Ed., GSET, GPSC
Ph.D. (Continue)

2. INTRODUCTION
• Branch of Botany that deals with study of internal structures and organization of plants is
called plant anatomy.
• Tissue is group of cells similar in origin, structure and function.
• The term tissue was coined by N. Grew, he is known as “Father of plant anatomy”.
• Tissue is a group of cells with common origin, structure and function that work together to
perform a particular function.
• Example :- xylem, phloem, parenchyma etc. are different types of tissues found in plants.
• Based on cell’s capability to divide, tissues are classified into two types by Carl Nageli.
• A. Meristematic tissue B. Permanent tissue
• 1. Meristematic tissues (Dividing tissues): It consists of undifferentiated actively dividing
cells.
• 2. Permanent tissues (Non-dividing tissues): It consists of differentiated cells that have
lost the ability to divide.

4. MERISTEMATIC TISSUES
• Growth in plants is largely restricted to specialised regions of active cell division called
meristems (Gk. meristos: divided).
• Meristematic tissues are thin-walled compactly arranged, immature cells that keep on dividing
continuously.
• The new cells produced are initially meristematic.
• Slowly, they grow, differentiate and mature into permanent tissues.
• Meristems are region within plant body that is involved in formation of new meristematic cells.
• Primary meristems appear early in the life of a plant and are responsible for the formation of
primary plant body, specific regions of the apical meristem produce dermal tissues, ground
tissues and vascular tissues.
• Primary growth involves development of new parts of a plant and growth in length of a
particular part.
• It includes apical meristem and intercalary meristem.

5. • Axillary bud: These are the buds constituted by some apical meristem left behind during the
elongation of stems and leaves.
• Axillary buds are capable of forming branches and flowers.
• Characteristic features of Meristematic tissues:
• The meristematic cells are spherical, or polygonal in shape.
• The cells are compactly arranged without inter-cellular spaces.
• The cell wall is thin, elastic and is made of cellulose.
• Each cell has abundant cytoplasm and prominent nuclei.
• Vacuoles may or may not be present.
• Functions: Meristematic tissue divides continuously to form a number of new cells and help
in growth of tissue.

6. TYPES OF MERISTEMATIC TISSUES
• Location: Meristematic tissues are
found in those regions which need to
grow continuously. For example, root
tips and stem tips contain meristematic
tissues.
• (a) Apical Meristems:
• They are the primary meristem.
• They are found at the growing tips of
roots and stems.
• The shoot apical meristem is present
at the tip of the shoots and its active
division results in the elongation of the
stem and formation of new leaves.
• The root apical meristem helps in root
elongation.
Shoot apical meristem
Root apical meristem

7. TYPES OF MERISTEMATIC TISSUES
• (b) Intercalary Meristems:
• They are primary meristem.
• It is present between the masses of mature
tissues present at the bases of the leaves
of grasses.
• It is present on base of leaves and
internodes.
• It helps in the growth of branches and
leaves.
• regeneration of grasses after they have
been grazed by herbivores.
• Since the intercalary meristem and the
apical meristem appear early in a plant’s life
also known as primary meristem.

8. TYPES OF MERISTEMATIC TISSUES
• (c) Lateral Meristems:
• They occur in the mature region of roots and shoots.
• They are called secondary meristems as they appear later in a plant’s life.
• It helps in adding secondary tissues to the plant body and in increasing the girth of plants.
• E.g. Fascicular cambium, inter fascicular cambium and cork cambium.
Shoot apical meristem
Intercalary meristem
Lateral meristem

10. PERMANENT TISSUES
• Permanent tissues are tissues that have lost the ability to divide, and have attained a definite
form and size.
• They are actually derived from newly formed cells from Primary and secondary meristematic
cells.
• Different type of permanent tissues is formed due to differences in their specialization.
• Characteristic features of Permanent tissue:
• The cells of permanent tissues normally do not divide.
• The cells may be thin walled (living) or thick walled (dead).
• Permanent cells are specialized to perform a particular function.
• The cells have attained definite shape and size.

12. TYPES OF PERMANENT TISSUES
• Depending on the structural differences (type of cells), permanent tissues are divided into two
types:
• (A) Simple tissue
• Parenchyma
• Collenchyma
• Sclerenchyma
• (B) Complex tissue.
• (a) Xylem
• (i) Tracheids (ii) Vessels (iii) Xylem fibers (iv) Xylem parenchyma
• (b) Phloem
• (i) Sieve tubes (ii) Companion cells (iii) Phloem Parenchyma (iv) Phloem fibers

13. Simple Tissues
• Simple permanent tissue is made up of only one kind of cells forming a uniform mass.
• The cells are similar in structure, origin and function.
• Simple permanent tissues are of three types:
• Parenchyma, Collenchyma and Sclerenchyma.
• 1. Parenchyma:
• Location :
• Parenchyma is widely distributed in plant body
such as stem, roots, leaves and flower.
• They are found in the cortex of root, ground tissue
in stems and mesophyll of leaves.

14. • CHARACTERISTIC FEATURES OF PARENCHYMA TISSUE
• Cells are isodiametric i.e. equally expanded on all sides.
• Cells may be oval, round or polygonal in outline.
• Nucleus is present and hence, living.
• The cell walls are thin and made of cellulose.
• Cytoplasm is dense with a central vacuole.
• Cells are loosely packed with large intercellular spaces between the cells.
• It may contain chlorophyll. Parenchyma containing
• chlorophyll is called chlorenchyma. It is the seat of photosynthesis.
• Parenchyma that encloses large air cavities is known as aerenchyma.
• Aerenchyma provides buoyancy to aquatic plants.

15. • FUNCTIONS OF PARENCHYMA TISSUE
• Parenchyma store and assimilate food.
• They give mechanical support to the plant body by maintaining turgidity.
• Presence of intercellular spaces in between parenchyma cells helps in exchange of gases.
• It prepares food if chlorophyll is present.
• It stores waste products like gum, crystal, tannin and resins.

16. COLLENCHYMA• Collenchyma is a strong and flexible tissue that provides flexibility to soft aerial parts.
• They are found below the epidermis in leaf stalks, leaf mid-ribs, and herbaceous dicot stems.
• Characteristics features
• Collenchyma cells are elongated cells with thick primary walls.
• Cell wall is unevenly thickened with cellulose at the corners.
• Intercellular spaces are absent.
• Nucleus is present and hence the tissue is living.
• Few chloroplasts may be present in the cells.
• Functions
• Collenchyma provides mechanical support to the growing parts of the plant such as young
stem and petiole of a leaf.
• It provides flexibility to soft aerial parts so that they can bend without breaking.
• Collenchyma cells may contain chloroplasts and thus take part in photosynthesis.

17. SCLERENCHYMA
• Sclerenchyma is strengthening tissue.
• It is found in and around the vascular
tissue, under the skin i.e. the epidermis
in dicot stems.
• Characteristics feature
• Cells are long, narrow, thick and lignified
usually pointed at both ends.
• The cell wall is evenly thickened with
lignin. Lignin is a water proof material.
• Intercellular spaces are absent.
• Nucleus is absent and hence the tissue is
made up of dead cells.
• Middle lamella i.e. the wall between
adjacent cells is conspicuous.

18. TYPES OF SCLERENCHYMA
• On the basis of variation in form, structure, origin and development, Sclerenchyma cells are of two types:
• (i) Fibers: They are long, narrow, thick walled and pointed cells.
• Location:
• Fibers are found in and around the vascular tissue.
• It may also occur below the epidermis.
• Fibers help in transportation of water in plant.
• (ii) Sclereids:
• They are spherical, oval or cylindrical.
• Highly thickened dead cells with very narrow cavities.
• The cells are hard and strongly lignified.
• They are shorter, iso-diametric or irregular cells.
• Location: These are commonly found in the fruit walls of nuts;
• pulp of fruits like guava, pear and sapota;
• seed coats of legumes and leaves of tea.
• Sclereids give firmness and hardness to the part concerned.

19. SCLERENCHYMA
• Functions
• Sclerenchyma gives mechanical support to the plant by giving rigidity, flexibility and elasticity to
the plant body.
• It forms a protective covering around seeds and nuts.

21. COMPLEX PERMANENT TISSUES
• Complex tissue is made up of more than one type of cells that work together to perform a
particular function.
• Complex tissues are of two types: Xylem and Phloem.
• (a) Xylem (Greek xylo= wood):
• Xylem is a complex permanent tissue that conducts water and mineral upward from root to the
plant.
• It is also known as wood.
• Xylem is composed of four different kinds of elements, namely,
• tracheids,
• vessels,
• xylem fibres and
• xylem parenchyma.

22. • (i) Tracheids: Tracheids are long, tubular dead cells with wide lumen and
tapering ends. The cell wall is thick with lignin. They have pores in their
walls.
• (ii) Vessels: Vessel is a long, cylindrical tube like structure that are placed
one above the other end to end. It is a non-living cell with lignified walls.
They generally possess pits. Vessel members are interconnected through
perforations in their common walls. The presence of vessels is a
characteristic feature of angiosperms.
• Function: Tracheids and vessels both are main water transporting
elements. Vessels are more efficient than tracheids.
• (iii) Xylem fibers: They are long, non-living cells with very thick lignin
deposition on the walls. They have narrow lumen and tapering ends.
• Function: Xylem fibers provide mechanical support to the plant.
• (iv) Xylem parenchyma: They are living cells with cellulosic cell wall.

23. TYPES OF XYLEM
• Primary xylem is of two types: protoxylem and metaxylem.
• The first formed primary xylem elements are called protoxylem and
the later formed primary xylem is called metaxylem.
• In stems, the protoxylem lies towards the centre (pith) and the
metaxylem lies towards the periphery of the organ. This type of
primary xylem is called endarch.
• In roots, the protoxylem lies towards periphery and metaxylem lies
towards the centre. Such arrangement of primary xylem is called
exarch.
• Function:
• They help in storage of starch or fats and other materials like
tannins.
• They also help in lateral conduction of water by the ray
parenchymatous cells.

24. PHLOEM
• Phloem is a complex permanent tissue that conducts food
synthesized in the leaves to different parts of the plant
body.
• Unlike, xylem, conduction of food occur both in upward
and downward directions (From leaves to storage organs
and from storage organs to growing organs).
• Phloem in angiosperms is composed of sieve tube
elements, companion cells, phloem parenchyma and
phloem fibres.
• Gymnosperms have albuminous cells and sieve cells. They
lack sieve tubes and companion cells.

25. • (i) Sieve tubes:
• Sieve tubes are elongated, cylindrical tubes with perforated end walls between
adjacent sieve tube cells.
• Sieve tube cells are placed end to end in a linear row.
• The perforated end walls are called as sieve plates.
• Sieve tube cells have vacuolated cytoplasm and lacks nucleus.
• The functions of sieve tubes are controlled by the nucleus of
companion cells.
• (ii) Companion cells:
• Companion cells are associated with sieve tubes.
• They are thin walled cells which lie on the sides of sieve tube cells.
• Companion cells have dense cytoplasm and prominent nucleus.
• Functions: They help sieve tubes in the conduction of
• food material by maintaining a proper pressure gradient
• in the sieve tube cells.

26. • (iii) Phloem Parenchyma:
• The phloem parenchyma cells are thin-walled and living.
• Phloem parenchyma is absent in most of the monocotyledons.
• Functions: They help in storage of food material and other substances
like resins, latex and mucilage.
• (iv) Phloem fibers:
• They are the only non-living (dead) component of phloem.
• They are thick-walled elongated and unbranched and have pointed, needle
like apices (spindle shaped) cells with narrow lumen.
• At maturity, these fibres lose their protoplasm and become dead.
• Functions:
• Phloem fibers provide mechanical support to the tissue.
• Phloem fibers are source of commercial fibers. E.g. Jute, hemp, flax etc.
• The first formed primary phloem consists of narrow sieve tubes and is referred
to as protophloem and the later formed phloem has bigger sieve tubes and is
referred to as metaphloem.

27. THE TISSUE SYSTEM
• Tissue structure and function would also be dependent on location.
• On the basis of their structure and location, there are three types of tissue systems.
• (A) Epidermal tissue system,
• (B) Ground tissue system or Fundamental tissue system
• (C) Vascular or conducting tissue system.

28. EPIDERMAL TISSUE SYSTEM
• Epidermal tissue system forms the outer-most covering of the whole plant body and
comprises epidermal cells, stomata and the epidermal appendages such as the trichomes and
hairs.
• (a) Epidermis:
• Epidermis is the outermost protective layer of plant body.
• It is usually single layer.
• Epidermal cells are parenchymatous with a small amount of cytoplasm lining the cell wall and
a large vacuole.
• The cells are elongated and closely packed without any intercellular spaces between them.
• The outermost layer of cell is covered with a water proof coating or layer called cuticle.
• Cuticle is made up of a waxy substance called cutin, which prevents the loss of water.
• Cuticle is absent in roots.

29. Stomata
• Stomata are structures present in the epidermis of leaves.
• Each stoma is composed of two bean shaped cells known as guard
cells which enclose stomatal pore.
• In grasses, the guard cells are dumb-bell shaped.
• 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 possess chloroplasts and regulate the opening and
closing of stomata.
• Sometimes, a few epidermal cells, in the vicinity of the guard cells
become specialised in their shape and size and are known as
subsidiary cells.
• The stomatal aperture, guard cells and the surrounding subsidiary
cells are together called stomatal apparatus.
• Stomata regulate the process of transpiration and gaseous
exchange.
• Stomata regulate the opening and closing of stomata.

30. • The cells of epidermis bear a number of
hairs.
• The root hairs are unicellular elongations of
the epidermal cells
• The root hairs help absorb water and
minerals from the soil.
• On the stem the epidermal hairs are called
trichomes.
• The trichomes in the shoot system are usually
multicellular.
• They may be branched or unbranched and
soft or stiff.
• They may even be secretory.
• The trichomes help in preventing water loss
due to transpiration.

31. THE GROUND TISSUE SYSTEM
• All tissues except epidermis and vascular bundles constitute the ground tissue.
• It consists of simple tissues such as parenchyma, collenchyma and sclerenchyma.
• Parenchymatous cells are usually present in cortex, pericycle, pith and medullary rays, in the
primary stems and roots.
• In leaves, the ground tissue consists of thin-walled chloroplast containing cells and is called
mesophyll.

32. THE VASCULAR TISSUE SYSTEM
• The vascular system consists of vascular bundles, the
phloem and the xylem.
• There are two types of vascular bundles:
• (i) Open vascular bundles: In open vascular bundles,
cambium is present between phloem and xylem. Such
vascular bundles possess the ability to form secondary
xylem and phloem tissues because of the presence of
cambium. It is found in dicotyledonous plants.
• (ii) Closed vascular bundles: Closed vascular
bundles lack cambium. Hence, since they do not form
secondary tissues they are referred to as closed. It is
found in the monocotyledons plants.

33. • Xylem and phloem can be arranged in two
different kinds of arrangement within a vascular
bundle.
• (i) Radial arrangement: When xylem and
phloem within a vascular bundle are arranged in
an alternate manner on different radii, the
arrangement is called radial. Such vascular
bundles are common in roots.
• (ii) Conjoint arrangement: When the xylem
and phloem are situated at the same radius of
vascular bundles, the arrangement is called
conjoint. Such vascular bundles are common in
stems and leaves. The conjoint vascular bundles
usually have the phloem located only on the
outer side of xylem.
Radial
Conjoint

34. ANATOMY OF DICOT ROOT
• Epidermis: It has a single layer of epidermal cells, many of which
protrude in the form of unicellular root hairs.
• Cortex: The cortex consists of several layers of thin-walled
parenchyma cells with intercellular spaces.
• Endodermis:
• The innermost layer of the cortex is called endodermis.
• It comprises a single layer of barrel-shaped cells without any
intercellular spaces.
• The tangential as well as radial walls of the endodermal cells have
a deposition of water impermeable, waxy material suberin in the
form of casparian strips.
• Pericycle: Next to endodermis lie a few layers of thickwalled
parenchymatous cells known as pericycle.
• Pericycle is responsible for initiation of lateral roots and vascular
cambium during the secondary growth.
• Pith: The pith is small or inconspicuous.

35. • Vascular bundles: Vascular bundles is
single (Monoarch). There are usually two to
four xylem and phloem patches. Later, a
cambium ring develops between the xylem
and phloem.
• Conjunctive tissue: The
parenchymatousm cells which lie between
the xylem and the phloem are called
conjuctive tissue.
• Stele: All tissues on the innerside of the
endodermis such as pericycle, vascular
bundles and pith constitute the stele.

36. T.S. of Monocot Root
• Epidermis, cortex, and endodermis: Same as that of dicot root.
• Epidermis: It has a single layer of epidermal cells, many of which
protrude in the form of unicellular root hairs.
• Cortex: The cortex consists of several layers of thin-walled
parenchyma cells with intercellular spaces.
• Endodermis:
• The innermost layer of the cortex is called endodermis.
• It comprises a single layer of barrel-shaped cells without any
intercellular spaces.
• The tangential as well as radial walls of the endodermal cells have a
deposition of water impermeable, waxy material suberin in the form
of casparian strips.
• Pericycle: Monocotyledonous roots do not undergo any secondary
growth.
• Pith: Pith is large and well developed.
• Vascular bundle: There are usually more than six (polyarch) xylem
bundles in the monocot root.

37. T.S. of Dicot Stem
• Epidermis: It is the outermost protective layer of the stem,
covered with a thin layer of cuticle. It may bear trichomes and
a few stomata.
• Cortex: The cells arranged in multiple layers between
epidermis and pericycle constitutes the cortex. It consists of
three sub-zones: Hypodermis, cortical layers and endodermis.
• Hypodermis: The outer hypodermis consists of a few layers of
collenchymatous cells just below the epidermis. It provides mechanical
strength to the young stem.
• Cortical layers: It is found below hypodermis and consist of rounded
thin walled parenchymatous cells with conspicuous intercellular
spaces.
• Endodermis: The innermost layer of the cortex is called the
endodermis. The cells of the endodermis are rich in starch grains and
the layer is also referred to as the starch sheath.

38. • Pericycle: It is present on the inner side of the
endodermis and above the phloem in the form of
semi-lunar patches of sclerenchyma.
• Vascular bundles: A large number of vascular
bundles are arranged in a ring. This arrangement is
characteristic of dicot stem. Each vascular bundle is
conjoint, open, and with endarch protoxylem.
• Medullary rays: In between the vascular bundles there
are a few layers of radially placed parenchymatous cells,
which constitute medullary rays.
• Pith: A large number of rounded, parenchymatous
cells with large intercellular spaces which occupy the
central portion of the stem constitute the pith.
Trichome

39. DICOT STEM MONOCOT STEM

40. T.S. of Monocot Stem
• Epidermis: It is the outermost protective layer of the stem,
covered with a thin layer of cuticle.
• Cortex: Cortex contains few layers of sclerenchymatous cells.
• Pericycle: It is present on the inner side of the endodermis
and above the phloem in the form of semi-lunar patches of
sclerenchyma.
• Vascular bundles: A large number of scattered vascular
bundles are present. Vascular bundles are conjoint, closed with
peripheral vascular bundles generally smaller than the centrally
located ones. Each vascular bundle is surrounded by
sclerenchymatous bundle-sheath cells. Phloem parenchyma is
absent, and water-containing cavities are present.
• Pith: Pith is absent in monocot stem.

41. T.S. of Dicot Leaf• Dorsiventral leaves are found in dicots. The vertical section
of a dorsiventral leaf contains three distinct parts.
• Epidermis: Epidermis is present on both the upper surface
(adaxial epidermis) and the lower surface (abaxial epidermis).
The epidermis on the outside is covered with a thick cuticle.
Abaxial epidermis bears more stomata than the adaxial
epidermis.
• Mesophyll: Mesophyll is a tissue of the leaf present
between the adaxial and abaxial epidermises. It is
differentiated into the palisade parenchyma (composed of
tall, compactlyplaced cells) and the spongy parenchyma
(comprising oval or round, loosely-arranged cells with inter
cellular spaces). Mesophyll contains the chloroplasts which
perform the function of photosynthesis.
• Vascular system: The vascular bundles present in leaves are
conjoint and closed. They are surrounded by thick layers of
bundle-sheath cells.

43. T.S. of Monocot Leaf
• Isobilateral leaf is found in monocot leaf.
• The anatomy of isobilateral leaf is similar to that of
the dorsiventral leaf in many ways.
• It shows the following characteristic differences.
• In an isobilateral leaf, the stomata are present on both
the surfaces of the epidermis.
• The mesophyll is not differentiated into palisade and
spongy parenchyma.
• Bulliform cells, which are modified adaxial epidermal
cells, are present along the veins, which absorb water
and make the cells turgid.
• When they are flaccid due to water stress, they make
the leaves curl inwards to minimize water loss.
• The parallel venation in monocot leaves is reflected in
the near similar sizes of vascular bundles (except in
main veins) as seen in vertical sections of the leaves.

44. SECONDARY GROWTH
• The growth of the roots and stems in length with the help of apical meristem is called the
primary growth.
• Apart from primary growth most dicotyledonous plants exhibit an increase in girth. This
increase is called the secondary growth.
• The tissues involved in secondary growth are together known as lateral meristem.
• Lateral meristem consists of
(A) Vascular cambium and
(B) Cork cambium.

45. (i) Vascular Cambium
• It is the meristematic tissue that is responsible for cutting
off vascular tissues – xylem and phloem.
• In the young stem, it is present in patches as a single layer
between the xylem and phloem. Later it forms a complete
ring.
• Formation of cambial ring
• In dicot stems, the cells of cambium present between
primary xylem and primary phloem is the intrafascicular
cambium.
• The interfascicular cambium is formed from the cells of
the medullary rays adjoining the interfascicular cambium,
as they become meristematic.
• This results in the formation of a continuous ring of
cambium.

46. • Activity of the cambial ring:
• The cambial ring becomes active and cuts off new cells,
towards its either sides.
• The cells present toward the outside differentiate into the
secondary phloem, while the cells cut off toward the pith
give rise to the secondary xylem.
• The amount of the secondary xylem produced is more
than that of the 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.

47. • The primary and secondary phloem gets
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.
• The secondary growth in plants increases the
girth of plants, increases the amount of water
and nutrients to support the growing number of
leaves, and also provides support to plants.

49. Secondary Growth in Dicot Root
• In the dicot root, the vascular cambium is completely secondary in origin.
• It originates from a portion of pericycle tissue, above the protoxylem and below phloem
bundles.
• They form a complete and continuous wavy ring, which later becomes circular.
• Further events are similar to those already described above for a dicotyledonous stem.
• Secondary growth also occurs in stems and roots of gymnosperms. However, secondary growth
does not occur in monocotyledons.

50. Spring Wood and Autumn Wood
• The activity of cambium is under the control of many physiological and environmental factors.
• Spring wood (Early wood): In the spring season, cambium is very active and produces a large
number of xylary elements having vessels with wider cavities. This wood is called spring wood
or early wood. The spring wood is lighter in colour and has a lower density
• Autumn wood (Late wood): In winter, the cambium is less active and forms fewer xylary
elements that have narrow vessels. This wood is called autumn wood or late wood. The
autumn wood is darker and has a higher density.
• Annual ring: When two kinds of wood appear as alternate concentric rings, it is known as
annual ring. Annual rings seen in a cut stem give an estimate of the age of the tree.

51. Heartwood and Sapwood• Heartwood
• It is composed of dead elements with highly lignified walls.
• It imparts dark brown colour to the older trees, due to the deposition of organic compounds
like tannins, resins, oils, gums, aromatic substances and essential oils in the central or innermost
layers of the stem.
• It makes the plant hard, durable and resistant to the attacks of microorganisms and insects.
The heartwood does not conduct water but it gives mechanical support to the stem.
• Sapwood
• It constitutes the the peripheral region of the secondary xylem.
• It is lighter in colour.
• It is involved in the conduction of water and minerals from root to
leaf.

52. Cork Cambium (Or Phellogen):
• As secondary growth starts due to the activity of vascular cambium, the outer cortical and
epidermis layers break down.
• These are replaced down to provide new protective cell layers. Hence, sooner or later,
another meristematic tissue called cork cambium or phellogen develops, usually in the
cortex region.
• Phellogen is composed of thin-walled, narrow and rectangular cells.
• Phellogen cuts off cells on its either side.
• The cells cut off toward the outside give rise to the phellem or cork. The suberin deposits in
its cell wall make it impervious to water.
• The inner cells give rise to the secondary cortex or phelloderm. The secondary cortex is
parenchymatous.
• Phellogen, phellem, and phelloderm are collectively known as periderm.
• Due to activity of the cork cambium, pressure builds up on the remaining layers peripheral
to phellogen and ultimately these layers die and slough off.

53. OR
phellogen
Phallum
OR

54. Bark
• It is a non-technical term that refers to all tissues
exterior to the vascular cambium, therefore
including secondary phloem.
• Bark is divided into two types: Early (or soft bark) and late
(or hard bark).
• Early or soft bark: Bark that is formed early in
the season is called early or soft bark.
• Late or hard bark: Bark that is formed towards
the end of the season late or hard bark is formed.

55. Lenticles
• At certain regions, the phellogen cuts off
closely arranged parenchymatous cells on
the outer side instead of cork cells.
• These parenchymatous cells soon rupture
the epidermis, forming lens-shaped
openings called lenticels.
• Lenticels permit the exchange of gases
between the outer atmosphere and the
internal tissue of the stem.
• It is found in most woody trees.

56. Secondary Growth in Roots
• In the dicot root, the vascular cambium is
completely secondary in origin.
• It originates from the tissue located just below the
phloem bundles, a portion of pericycle tissue,
above the protoxylem forming a complete and
continuous wavy ring, which later becomes circular
• Further events are similar to those already
described above for a dicotyledon stem.
• Secondary growth also occurs in stems and roots
of gymnosperms.
• However, secondary growth does not occur in
monocotyledons.

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