Anatomy of Flowering Plants - Magadh University · 2020. 8. 12. · Anatomy of Flowering Plants Anatomy definition: is the branch of biology concerned with the study of the structure
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Anatomy of Flowering Plants
Anatomy definition: is the branch of biology concerned with the study of the structure of
organisms and their parts.
Difference between Morphology and Anatomy:
o Plant morphology or phytomorphology is the study of the physical form and
external structure of plants, whereas plant anatomy is the study of the
internal plant structure, mostly at the cellular/microscopic level.
Cell – is the smallest structural and functional unit of an organism, which is
typically microscopic and consist of cytoplasm and nucleus enclosed in a
membrane.
Difference between Protoplasm and cytoplasm:
o Protoplasm is the colourless material consisting of the living part of a cell,
including the cytoplasm, nucleus, and other organelles. Cytoplasm is the fluid
that consist of all of the content outside the nucleus and enclosed within the
cell membrane of a cell.
The tissue
A group of cells having a common origin and usually performing common function are
called tissues.
Plant Tissue
Meristematic Tissue Permanent Tissue
1. Apical meristem Simple permanent tissue Complex permanent tissue
2. Intercalary 1. Parenchyma 1. Xylem
3. Lateral 2. Collenchyma 2. Phloem
3. Sclerenchyma
Meristematic Tissues or Meristems
(1) They contain immature and young cells and are capable of repeated divisions.
(2) Intercellular spaces are not present in meristematic tissue.
(3) They contain a homogeneous thin wall.
(4) They contain large nuclei associated with abundant cytoplasm.
(5) They are metabolically very active but they do not store food material.
(6) Only proto-plastids are present instead of plastids, chloroplast absent.
(7) Dense cytoplasm is present which contains several prematuremitochondria.
(8) Vacuoles are absent.
(9) Meristematic cells are isodiametric (roughly spherical inshape, not a proper sphere but almost spherical) in shape.
Isodiametric shape ofparenchymatic cell
Types of meristems
The meristems may be classified on the basis of their mode of origin, position or function:
(i) According to origin and development: On the basis of origin, meristematic tissues are of
three types :
(a) Promeristem or Primordial meristem: The promeristem originates from
embryo and, therefore, called primordial or embryonic meristem. It is present in
the regions where an organ or a part of plant body is initiated.
(b) Primary meristem: A primary meristem originates from promeristem and
retains its meristematic activity. It is located in the apices of roots, stems and the
leaf primordia.
(c) Secondary Meristem: They always arise in permanent tissues and have no
typical promeristem. Some living permanent cells may regain the meristematic nature.
(ii) According to position: On the basis of their position in the plant body meristems are
classified into three categories:
(a) Apical meristem: This meristem is located at the growing apices of main and
lateral shoots and roots. These cells are responsible for linear growth of an organ.
(b) Intercalary meristem: These are the portions of apical meristems which are
separated from the apex during the growth of axis and formation of permanent
tissues. It is present mostly at the base of node (e.g., Mentha viridisMint), base of
internode (e.g., stem of many monocots viz., Wheat, Grasses, Pteridophyts like
Equisetum) or at the base of the leaf (e.g., Pinus).
(c) Lateral meristem: These meristems occur laterally in the axis, parallel to the
sides of stems and roots. This meristem consists of initials which divide mainly in
one plane (periclinal) and result increase in the diameter of an organ.
(iii) According to function: Haberlandt in 1890 classified the primary meristem at the apex
of stem under the following three types:
(a) Protoderm: It is the outermost layer of the apical meristem which develops into
the epidermis or epidermal tissue system.
(b) Procambium: It occurs inside the protoderm. Some of the cells of young
growing region which by their elongation and differentiation give rise to primary
vascular tissue constitute the procambium.
(c) Ground meristem: It constitutes the major part of the apical meristem develops
ground tissues like hypodermis, cortex, endodermis, pericycle, pith and medullary
rays.
(iv) According to plane of cell division: On the basis of their plane of cell division meristem
are classified into three categories:
(a) Mass meristem: The cells divide anticlinally in all planes, so mass of cells is
formed. e.g., formation of spores, cortex, pith, endosperm.
(b) Plate meristem: The cells divide anticlinally in two planes, so plate like area
increased. e.g., formation of epidermis and lamina of leaves.
(c) Rib or File meristem: The cells divide anticlinally in one plane, so row or
column of cells is formed. e.g., formation of lateral root.
Permanent Tissues
Permanent tissues are made up of mature cells which have lost the capacity to divide and
have attained a permanent shape, size and function due to division and differentiation in
meristematic tissues. The cells of these tissues are either living or dead, thin-walled or
thick-walled. Permanent tissues are of three types:
(1) Simple tissues: Simple tissues are a group of cells which are all alike in origin, form
and function. They are further grouped under three categories:
(i) Parenchyma: Parenchyma is most simple and unspecialized tissue which is
concerned mainly with the vegetative activities of the plant.
(ii) Collenchyma: The term collenchyma was coined by Schleiden (1839). It is the
tissue of primary body.
o The cells of this tissue contain protoplasm and are living.
o The cell walls are thickened at the corners and are made up of cellulose,
hemicellulose and pectin.
(iii) Sclerenchyma: It was discovered and coined by Mettenius (1805).The main
feature of sclerenchyma are:
o It consist of thick-walled dead cells.
o The cells vary in shape, size and origin.
(2) Complex Permanent Tissue
Unlike simple permanent cells which look the same and are made up of one type of cells,
complex permanent tissues are made up of more than one type of cells. These different
types of cells coordinate to perform a function. Xylem and Phloem are complex
permanent tissues and are found in the vascular bundles in the plants.
Xylem- It consists of tracheids, vessels, xylem parenchyma and xylem fibres.
Tracheids and vessels are hollow tube-like structures that help in conducting water
and minerals. The xylem conducts only in one direction i.e vertically. The xylem
parenchyma is responsible for storing the prepared food and assists in the
conduction of water. Xylem fibres are supportive in function.
Phloem- It consists of four of elements: sieve tubes, companion cells, phloem
fibres and the phloem parenchyma. Unlike the xylem, phloem conducts in both
directions. It is responsible for transporting food from the leaves to the other
parts of the plant. Phloem contains living tissues except for fibres that are dead
tissues.
Primary xylem is of two types- protoxylem and metaxylem. In stem,
protoxylem lies in centre and metaxylem towards periphery. This type of
primary xylem is called endarch.
In roots, protoxylem lies in periphery and metaxylem lies towards the centre.
This type of primary xylem is called exarch.
In gymnosperms, albuminous cells and sieve cells lack sieve tube and
companion cells.
Secondary growth
Unlike most animals, who grow to a specific body size and shape and then stop growing
(determinate growth), plants exhibit indeterminate growth where the plant will continue
adding new organs (leaves, stems, roots) as long as it has access to the necessary resources.
Plants are able to continue growing indefinitely like this due to specialized tissues called
meristems, which are regions of continuous cell division and growth. Meristematic tissue
cells are either undifferentiated or incompletely differentiated, and they continue to
produce cells that quickly differentiate, or specialize, and become permanent tissues
(dermal, ground, and vascular).
Meristems contribute to both primary (taller/longer) and secondary (wider) growth.
Primary growth is controlled by root apical meristems or shoot apical meristems, while
secondary growth is controlled by the two lateral meristems, called the vascular cambium
and the cork cambium. Not all plants exhibit secondary growth.
Key Points
Indeterminate growth continues throughout a plant’s life, while determinate growth
stops when a plant element (such as a leaf) reaches a particular size.
Primary growth of stems is a result of rapidly-dividing cells in the apical meristems
at the shoot tips.
Apical dominance reduces the growth along the sides of branches and stems, giving
the tree a conical shape.
The growth of the lateral meristems, which includes the vascular cambium and the
cork cambium (in woody plants), increases the thickness of the stem during
secondary growth.
Cork cells (bark) protect the plant against physical damage and water loss; they
contain a waxy substance known as suberin that prevents water from penetrating the
tissue.
The secondary xylem develops dense wood during the fall and thin wood during the
spring, which produces a characteristic ring for each year of growth.
Key Terms
Lenticel: small, oval, rounded spots upon the stem or branch of a plant that allow the
exchange of gases with the surrounding atmosphere
Periderm: the outer layer of plant tissue; the outer bark
Suberin: a waxy material found in bark that can repel water
Growth in Stems
Growth in plants occurs as the stems and roots lengthen. Some plants, especially those that
are woody, also increase in thickness during their life span. The increase in length of the
shoot and the root is referred to as primary growth. It is the result of cell division in the
shoot apical meristem. Secondary growth is characterized by an increase in thickness or
girth of the plant. It is caused by cell division in the lateral meristem. Herbaceous plants
mostly undergo primary growth, with little secondary growth or increase in thickness.
Secondary growth, or “wood”, is noticeable in woody plants; it occurs in some dicots, but
occurs very rarely in monocots.
Figure: Primary and secondary growth: In woody plants, primary growth is followed by
secondary growth, which allows the plant stem to increase in thickness or girth. Secondary
vascular tissue is added as the plant grows, as well as a cork layer. The bark of a tree extends
from the vascular cambium to the epidermis.
Some plant parts, such as stems and roots, continue to grow throughout a plant’s life: a
phenomenon called indeterminate growth. Other plant parts, such as leaves and flowers,
exhibit determinate growth, which ceases when a plant part reaches a particular size.
Primary Growth
Most primary growth occurs at the apices, or tips, of stems and roots. Primary growth is a
result of rapidly-dividing cells in the apical meristems at the shoot tip and root tip.
Subsequent cell elongation also contributes to primary growth. The growth of shoots and
roots during primary growth enables plants to continuously seek water (roots) or sunlight
(shoots).
The influence of the apical bud on overall plant growth is known as apical dominance,
which diminishes the growth of axillary buds that form along the sides of branches and
stems. Most coniferous trees exhibit strong apical dominance, thus producing the typical
conical Christmas tree shape. If the apical bud is removed, then the axillary buds will
start forming lateral branches. Gardeners make use of this fact when they prune plants by
cutting off the tops of branches, thus encouraging the axillary buds to grow out, giving the
plant a bushy shape.
Conifers Angiosperm Tree
Secondary Growth
The increase in stem thickness that results from secondary growth is due to the activity of
the lateral meristems, which are lacking in herbaceous plants. Lateral meristems include
the vascular cambium and, in woody plants, the cork cambium. The vascular cambium is
located just outside the primary xylem and to the interior of the primary phloem. The cells of
the vascular cambium divide and form secondary xylem (tracheids and vessel elements) to
the inside and secondary phloem (sieve elements and companion cells) to the outside. The
thickening of the stem that occurs in secondary growth is due to the formation of
secondary phloem and secondary xylem by the vascular cambium, plus the action of cork
cambium, which forms the tough outermost layer of the stem. The cells of the secondary
xylem contain lignin, which provides hardiness and strength.
In woody plants, cork cambium is the outermost lateral meristem. It produces cork cells
(bark) containing a waxy substance known as suberin that can repel water. The bark
protects the plant against physical damage and helps reduce water loss. The cork
cambium also produces a layer of cells known as phelloderm, which grows inward from the
cambium. The cork cambium, cork cells, and phelloderm are collectively termed the
periderm. The periderm substitutes for the epidermis in mature plants. In some plants,
the periderm has many openings, known as lenticels, which allow the interior cells to
exchange gases with the outside atmosphere. This supplies oxygen to the living- and
metabolically-active cells of the cortex, xylem, and phloem.
Annual Rings
The activity of the vascular cambium gives rise to annual growth rings. During the spring
growing season, cells of the secondary xylem have a large internal diameter; their primary
cell walls are not extensively thickened. This is known as early wood, or spring wood.
During the fall season, the secondary xylem develops thickened cell walls, forming late
wood, or autumn wood, which is denser than early wood. This alternation of early and
late wood is due largely to a seasonal decrease in the number of vessel elements and a
seasonal increase in the number of tracheids. It results in the formation of an annual ring,
which can be seen as a circular ring in the cross section of the stem. An examination of the
number of annual rings and their nature (such as their size and cell wall thickness) can
reveal the age of the tree and the prevailing climatic conditions during each season.
Figure: Annual growth rings: The rate of wood growth increases in summer and
decreases in winter, producing a characteristic ring for each year of growth. Seasonal
changes in weather patterns can also affect the growth rate. Note how the rings vary in
thickness.
Secondary Meristem in Monocot
Secondary growth is increase in the circumference / girth of the plant organs due to the
formation of secondary tissues in stelar and extra stelar regions. Normally secondary growth
takes place in roots and stem of dicotyledons and gymnosperms. Due to lack of cambium
in monocotyledons, secondary growth is absent. But exceptionally, secondary growth takes
place in some monocotyledons, such as palm, Yucca, Dracaena etc.
Difference between Dicot and Monocot Root
Dicot Root Monocot Root
Pericycle
Gives rise to cork cambium, parts of the vascular cambium, andlateral roots
Gives rise to lateral roots only
Vascular Tissues
Has a limited number of Xylem and Phloem Has a higher number of Xylem andPhloem
Shape of Xylem
Angular or Polygonal Round or Oval
Number of Xylem and Phloem
2 to 8 8 to many
Pith
Absent or very small and undeveloped Larger and well developed
Conjunctive tissue
Parenchymatous Sclerenchymatous
Secondary growth
Secondary growth in Monocot
Most monocotyledons consist entirely of primary tissues. The usual vascular cambium is
absent from this group and so there is no normal secondary growth. However, in some
monocots, the thickening and elongation of stem occurs through primary thickening
meristem, diffuse secondary thickening and secondary thickening meristem.
Primary thickening meristem:
Secondary growth occurs Secondary growth does not occur
Cambium
Present and formed by the Conjunctive parenchyma Absent
Xylem
Usually tetrarch Polyarch
Cortex
Comparatively Narrow Very wide
Covering
Older roots are covered by a Cork Older roots are covered by anExodermis
Examples
Pea, beans, peanuts, etc. Maize, banana, palm, etc.
This meristem is observed in palms, in the rhizomes of Musa (Banana) and in the bulbs of
Allium cepa (Onion) etc. In these plants, the shoot apex is not large and produces only a
small part of the primary body. A considerable thickening occurs below the shoot apical
meristem. This is due to the intensive cell division of primary thickening meristem.
This meristem lies below the young leaf bases and originates by periclinal division of the
cells situated below the region of attachment of young leaf primordia. The meristem appears
as a flat zone in longitudinal section of the developing embryo. The zone gradually assumes a
concave form in the mature young plant.
The meristem consists of several layer of cells which are rectangular, elongated and
oriented parallel to the surface of shoot apex. The derivatives of meristem are the ground
parenchyma cells. Sometimes localized mitotic activity within this meristem forms
procambial strands, which run horizontal to the surface. These procambial strands gradually
mature into vascular bundles.
The term meristematic cap has been coined by Zimmermann and Tomlinson (1976) to
designate the zone of procambium formation. Primary thickening meristem contributes
mainly to the increase in width of stem and later it causes the elongation of young stem. In
palms, Musa and a number of other monocotyledons with a similar growth habit the
procambium originates from two sources —the shoot apical meristem and meristematic cap.
Diffuse secondary thickening:
In palm stem the ground parenchyma cells, close to and distant from the shoot apex (ex.
Roystonea, Actinophloeus etc.), expand along with proportional increase of the
intercellular spaces, thus causing diffuse secondary growth.
Sometimes these ground parenchyma and the procambium cells, destined to produce the
outer fibres of bundle sheath, divide, expand and thus contribute to the diameter of
stem. The diffuse secondary thickening occurs after the completion of expansion and
elongation caused by primary thickening meristem.
Secondary thickening meristem:
Secondary thickening with this meristem occurs in a number of monocotyledonous species
such as, Xanthorrhoea, Dracaena, (Figs. 21.1 & 31.23) Cordyline, Aloe, Yucca, Kingia,
Dioscorea etc. This meristem is a type of vascular cambium, which originates in the
parenchyma cells present on the peripheral sides of the entire mass of primary vascular
bundles.
This region may be the inner layer of cortex or pericycle. Though this lateral meristem is
distant from the stem apex, in seedling stages it develops in continuity to the primary
thickening meristem. In mature plants these two meristems are found to be discontinuous.
The secondary thickening meristem consists of several tiers of cells that may be of different
shapes, such as fusiform, rectangular, polygonal or only with one end tapered. This cambium
divides tangentially, some of the peripheral derivatives divide repeatedly and the others form
secondary cortex. These parenchyma cells usually remain thin walled and sometimes may
contain crystals.
In Xanthorrhoea these cells secrete resin, which form a sheath around the stem. A large
number of tissues are formed to the inner side. The inner tissues form the ground parenchyma
and vascular bundles. The ground parenchymatous tissues are termed as conjunctive tissues,
which may sometimes become lignified. The secondary vascular bundles remain embedded
in the conjunctive tissue.
The bundles are either collateral (e.g. Kingia) or amphivasal (e.g. Dracaena, Aloe etc.). In
Kingia, the phloem is surrounded by xylem only on three sides and so in cross section the
vascular bundles appear as U-shaped. The xylem consists of small amount xylem
parenchyma that may be lignified and tracheids only.
Some localized region of the tiers of cambium is involved in the formation of a single
bundle. The conjunctive tissues with embedded secondary vascular bundles show radial
arrangement in contrast to primary strands, which are irregularly arranged.
Protective tissues are developed in the perennial monocotyledons by periderm and storied
cork formation.
Periderm is formed with the origin of cork cambium or phellogen and the subsequent
formation of phellem and phelloderm from it like dicotyledons. The periderm formation of
Dracaena, Aloe and Yucca has no difference than those of dicotyledons.
Storied cork:
It is a special type of protective tissue composed of cells, which are radially arranged in
files and have suberized walls. The storied corks are observed in Curcuma, Cordyline and in
many palms. These corks are formed by the activity of storied meristem. This meristem
originates at the outer cortex. In contrast to phellogen, the storied meristem does not form a
regular uninterrupted cylinder.
The cortical parenchyma cells, which are destined to be the initials of this meristem, become
three to eight layered by periclinal division. The derivatives of this meristem become
suberized and are termed as storied cork. The cork cells are arranged in irregular tangential
bands and radial files.
Sometimes these bands may coalesce tangentially and radially when they enclose some
undividing cortical cells, which also become suberized. Thus the storied cork along with
enclosed suberized cells form a hard protective tissue (Fig. 21.2).
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