PLANT

STRUCTURE
& FUNCTION
Prepared by:
Ramon Philip Lagrimas Bacayo
THE PLANT BODY
Plants are one of the most essential living organisms on earth. They are immensely
beneficial to both animals and human beings. They produce oxygen which is crucial for
the survival of living organisms. Trees provide shelter to animals and are also known for
their medicinal benefits. Overall, different parts of plants have different roles to perform.
They act as a source of food and oxygen and maintain the ecological balance.
A plant has many parts. Different parts perform different functionalities. The part of the
plant that appears above the ground level is called the shoot system while the part of
the plant which lies underneath the soil is called the root system.

The main parts of a plant include:

• Roots
• Stem
• Leaves
• Flowers
• Fruits
Plant Parts
Roots
Roots are the most important and underground part of a plant, which are collectively
called the root system. They are the major part that anchors the plant firmly in the soil.
They absorb water and minerals from the soil, synthesize plant growth regulators, and
store reserve food material. The apical part of the root is covered by the root cap that
protects the root apex.
The direct elongation of radicle leads to the formation of primary roots that grow inside
the soil in dicots. It bears lateral roots that are known as secondary and tertiary roots.
In monocots, the primary root is replaced by a large number of roots because it is short-
lived. In some plants such as Banyan tree, the roots arise from the parts of the plant
and not from the radical. Such roots are known as adventitious roots.
A few plants that grow in swampy areas have roots growing vertically upwards to get
oxygen for respiration. Such roots are known as pneumatophores.

Stem
The stem is the part of the plant which is found above the ground. The bark of trees is
brown in color and younger stems are green in color. It forms the basis of the shoot
system and bears leaves, fruits and flowers. The region where the leaves arise is known
as the node and the region between the nodes is known as the internode.
Stems arise from the plumule, vertically upwards to the ground. Initially, stems are
usually weak and cannot stand straight. It eventually grows to become the toughest part
of the plant called the trunk. The trunk is covered by a thick outer covering known as the
bark. Overall stem provides a definite framework and structure to a plant, which later
develops into a tree.
The stem provides support to the plant. They also protect the plant and help in
vegetative propagation. A few underground stems such as potato and ginger are
modified to store food.

The important functions of a stem include:

• A stem carries out a number of functions essential for various processes such as
photosynthesis.
• Provides a definite framework and structure to a plant which later develops into a
tree.
• Support: Primary function of the stem is to hold up buds, flowers, leaves, and
fruits to the plant. Along with the roots, a stem anchors the plants and helps them
to stand upright and perpendicular to the ground.
• Transportation: It is the part which transports water and minerals from the root
and prepared food from leaves to other parts of the plant.
• Storage: Stems are one of the storerooms of plants where the prepared food is
stored in the form of starch. The stems of a few plants in the desert areas, such
as Opuntia, get modified into thick, fleshy structures that store food and prevent
excessive water loss due to transpiration.
• Reproduction: A few stems help in reproduction through vegetative propagation
and also help to bear flowers and to produce fruits.
• Guards: Protects Xylem and phloem allowing them to perform their functions.
The stem tendrils are spirally coiled and help the plant to climb support. The
axillary buds also get modified into thorns that protect the plant from grazing
animals.
• The stems of a few plants in the desert areas, such as Opuntia, get modified into
thick, fleshy structures that store food and prevent excessive water loss due to
transpiration.
Leaves
Leaves are the most important part of a plant. They contain chlorophyll that helps the
plants to prepare their food using sunlight, carbon dioxide and water. A leaf consists of
three main parts- petiole, leaf base and lamina.
The petiole keeps the leaf blade exposed to wind and cools the leaf.
The leaf base is a protruding part of a leaf.
The lamina of the leaf contains veins and veinlets that provide rigidity to the leaf blade
and help in the transport of mineral nutrients.

Primarily, leaves have three main functions:

• Photosynthesis: Green leaves prepare food for plants by using water and carbon
dioxide in the presence of sunlight. This process is called photosynthesis.
• Transpiration: Other than photosynthesis, leaves play a crucial role in the
removal of excess of water from plants through tiny pores called stomata. This is
the process of transpiration.
• Reproduction: Leaves of some plants helps in reproduction also. For e.g. leaves
of Bryophyllum give rise to a new Bryophyllum plant.
STEM
An Overview of Plant Stem
Stems are part of the shoot system of
a plant that is usually cylindrical in form and
are usually green when they are herbaceous
and brown when they are woody. They may
range in length from a few millimeters to
hundreds of meters, and also vary in
diameter, depending on the plant type. A
stem may be unbranched, like that of a palm
tree, or it may be highly branched, like that of
a magnolia tree. Plant stems, whether above
or below ground, are characterized by the
presence of nodes and internode; have buds
which may be terminal or axillary.

Figure 1. Stem of Plant

❑ A stem with its leaves is termed a shoot.
❑ All the stems, branches and leaves of a plant constitute its shoot system
❑ Stems that grow above the ground are called epiterranean/aerial stems
❑ Stems that grow below the ground are called underground stems or
subterranean stems.

Course Objectives:
1. List the functions of stems.
2. Recognize features of stem morphology: node, internode, terminal bud, axillary bud,
bud scales, bud scale scars, leaf scar, bundle scars, and lenticels.
3. Describe and cite examples of stem modifications.
4. Compare the cross sections of monocot and eudicot stems.

Importance of Stems
❑ Stems move leaves toward the light
❑ To support the weight of leaves and to withstand the force of wind.
❑ Conduct water, mineral and organic molecules between the roots and the leaves.
❑ Hold leaves, flowers and buds.
❑ Store food for the plant.
❑ Helps to transport the products of photosynthesis, namely sugars, from the
leaves to the rest of the plant.
Origin of Stems
The first stem of the plants develops from
a part of the seed embryo known as the
epicotyl, which is a continuation of the
hypocotyl.
Epicotyl- cylindrical structure with a
mass of meristematic cells and often a
pair of small leaves at its apex.
Figure 2. Epicotyl, together with a part of the hypocotyl

The tip of the epicotyl is called the

plumule, where the main stems grow.
As it grows, the primary stems produce
branches called secondary stems.

The primary stems grow upward, while the

secondary stems grow in horizontal
directions.
Figure 3. Plumule at the tip of the epicotyl

Aerial stems are commonly classified into two types:

1. Herbaceous Stems
❑ Soft and green
❑ Little growth in diameter
❑ Tissues are chiefly primary
❑ Chiefly annual
❑ Covered by an epidermis
❑ Buds mostly naked

2. Woody Stems
❑ Tough and brown
❑ Considerable growth in diameter
❑ Tissues chiefly secondary
❑ Chiefly perennial
❑ Covered by corky bark
❑ Buds covered by scales

Figure 4. Herbaceous stems and Woody stems

 ❑ Stem growth is more complex than root growth because the stem not only grows
in length but also produces leaves and axillary branches with the shoot apical
meristem giving rise to leaf primordia and axillary buds.
❑ Since the leaf primordia are close together, the internodes are very short at first.
❑ Stem typically lengthens simultaneously in several internodes below the shoot
apical meristems.
❑ Some plants including grasses such as wheat, have a region of dividing cells at
each internode, known as intercalary meristems.
❑ Intercalary meristems allow the stem to grow rapidly all along its length.

EXTERNAL STRUCTURES OF STEMS:

1.Nodes: swollen portion of the stem
where leaves, branches, buds,
flowers or fruits may arise.
2. Internodes: distances between
two successive nodes in the stem.
3. Leaf Scar: mark on the stem by a
fallen leaf. It is the place at which the
leaf stalk grows from the stem.
4. Lenticels: tiny pores found
especially on dicot stems used for
gaseous exchange.
5. Bundle Scar: mark on the stem
left after cutting branch or a leaf.
These are the broken ends of
vascular bundles.
6. Fruit Scar: mark on the stem after
picking a fruit. It is the place at which
the fruit stalk is attached to the stem.
7. Buds: underdeveloped structures
which may grow into a leaf, flower, or
another stem.
8. Bud Scar: ring of small narrow
scar left by the falling away of the
bud scales and forming a complete
thin circle around a twig.
Figure 5. External morphology of stem
I. Strong Stems
1. Excurrent: the main axis shows
continues growth and the latera branches
develop regularly which gives a conical
appearance to the trees. Example is the
Casuarina equisetifolia commonly known
as Agoho tree.

Agoho

2. Deliquescent: the growth of lateral

branch is more vigorous than that of main
axis. The tree has a rounded or spreading
appearance. Example is the Mangifera
indica commonly known as Mango.

Mango
3. Caudex: it is an unbranched, stout,
cylindrical stem, marked with scars or fallen
leaves. Example is the Cocos nucifera,
commonly known as coconut or coconut
palm.

Coconut/ Coconut palm

4. Culm: erect stems with distinct nodes
and internodes. Stems showed jointed
appearance. Example is the Bambusa
species, commonly known as bamboo.

Bamboo
II. Weak Stems
1. Trailing: a weak stem that spreads over the surface of the ground without rooting at
the nodes. It falls into three categories:

1.1 Prostrate: a stem that lie flat on the

ground. Example is Portulaca oleracea,
commonly known as Moss rose.

1.2 Decumbent: a stem that lies flat

but its apex is raised. Example is
Tridax procumbens, commonly known
as Daisy.

1.3 Creepers-When the plant grows horizontally

over the surface of the soil and produces
branches profusely and spreads out in all
directions and gets rooted at each nodes.
Example is the Ipomoea batatas, commonly
known as sweet potato.
2. Upright weak stems are of two kinds: climbers and twiners

2.1 Climbers: The stem is weak and flexible but is unable to coil around an upright
support by itself. It requires the help of certain clasping or clinging structures.

Accordingly, climbers are of several types:

a. Root Climbers: Several weak plants

like Pothos scandens (Araceae), Piper
betle (Piperaceae), Scindapsus officinalis
(Araceae), Ficus pumila (Moraceae),
climb up suitable objects with the help of
adventitious roots which develop from the
nodes of the stem.

b. Tendril Climbers: There are some

plants which develop special type of
climbing organs called tendrils. These are
slender, spirally coiled, thread-like
structures which may be modifications of
either branches or leaves or
inflorescence stalk and are very sensitive
to contact. They help the plant to climb
up any other plant or object. Different
plant organs are modified to form tendrils.

c. Hook climbers: In Artabotrys uncinatus

(Anonaceae) curved hooks are developed
from the flower stalks (pedicels) which
help the plant to climb up any other plant
or support. Hook-like structures are also
formed due to the modification of terminal
leaflets which also serve the climbing
purpose such as Bignonia unguis-cati
(Bignoniaceae); sometimes hooks are
modified inflorescence axes such as Anabotrys sp. (Anonaceae).
d. Leaf Climbers: Leaf climbers are the climbing plants
in which the leaf or pan of the leaf is modified into tendril
and which act as climbing organ. The leaf-stalk i.e.
petiole of Clematis sp. (Ranunculaceae) is sensitive to
contact and coil round any neighbouring object helping
the plant to climb. In Nepenthes Sp. (Nepenthaceae) the
modified petiole often twists round the support like tendril
holding the pitcher in vertical position.
e. Rambler or Scramblers: Several
plants have been found to climb
neighbouring plants in forests with the
help of prickles and thorns. Example are
Bougainvillea spectabilis (Nyctaginaceae),
climbing rose, Calamus rotang (Palmae)
etc. In Calamus rotang, Iong slender whip-
like stalk covered by prickles (hook-like) is
produced from the leaf sheath this helps
the plant to climb neighbouring plants or
objects.
f. Adhesive climber: These climbers are
provided with adhesive discs to some of their
organs, by means of such adhesive discs they
may adhere to flat surfaces, Ampelopsis veitchii
(Vitaceae). In this plant the adhesive discs are
developed from the tendrils.

g. Lianas or Lianes: Long and woody perennial

stem climbers which climb up tall forest trees.
are called lianes. They climb up owing to the
different growth and curvature of the stem
Hiptage benghalensis (Malpighiaceae),
Beaumontia grandiflora (Apocynaceae),
Bauhinia vahlii (Legnmihosae Caesalpinaceae).

2.2 Twiners: The stem is long, flexible and sensitive. It can coil around an upright
support like a rope. Depending upon the direction of coiling, twiners can be indifferent,
sinistrorse (anticlockwise, upper coil disappears at observer’s right; dextrorse
(clockwise, upper coil disappears at observer’s left).

Twiners are classified into two groups according to the direction of twining.

(i) Dextrorse- When the climbers twine

clockwise or to the right then it is
called dextrorse.

(ii) Sinistrorse- When the climbers

twine in anticlockwise or to the left
then it is called sinistrorse.
MODIFIED STEMS: stems which have specialized or unusual functions and thus have
an unusual structure.
Sub-aerial modification of stem
Runner – a type of stolon, horizontally growing on top of the ground and rooting
at the nodes, aids in reproduction. Ex. Garden strawberry
Stolon – a horizontal stem that produces rooted plantlets at its nodes and ends,
forming near the surface of the ground.
Sucker- underground runner which soon goes up and form a daughter plant after
striking root. Ex. Chrysanthemum
Offset- found in aquatic plants and is just like the runner, only it is shorter and
thicker. Ex. Water hyacinth

Underground modification of stem

Rhizome – a horizontal underground stem that functions mainly in reproduction
but also in storage. Ex. Most ferns
Corm – a short-enlarged underground, storage stem. Ex. Taro
Bulb- a short vertical underground stem with fleshy storage leaves attached. Ex.
Onion, tulip
Tuber – a swollen, underground storage stem adapted for storage and
reproduction. Ex. Potato
Aerial modification of stem
Cladode (including phylloclade) – a flattened stem that appears more-or-less leaf
like and is specialized for photosynthesis. Ex. Cactus
Thorn – a modified stem with a sharpened point.
Tendrils- slender whip-like or threadlike strand, produced usually from the node
of a stem by which vine or other plant may climb. Ex. Grape, squash or melon
family
BUD:
* Underdeveloped structures which may grow into a leaf, flower or a stem
* Located in the axils of leaves, that is, in the upper angles between the points of
juncture of leaves with stems.
Ways of classifying buds:
In terms of position or location in the stem:
1. Terminal or apical bud- located at the tip of the stem or twig
2. Lateral or axillary bud- located along the side of the stem/ buds in the axils of the
leaves
3. Adventitious bud- located at the internodes or other parts of the plant body

In terms of morphology:
1. Naked. Bud is not covered by bud scales
2. Covered. Bud is covered by bud scales
3. Scaly. bud is covered with scale which protect the embryonic parts it contains
4. Hairy. Bud is covered by hairs

Naked Covered Scaly Hairy

In terms of development:
1. leaf- develop into a leaf
2. Flower- develop into a flower
3. Mixed- develop either a flower or a leaf
4. Stem- develop into a stem
5. Vegetative- develop either a stem or a leaf
The Monocot Stem
The monocot stem also has the single
layered epidermis along with the thick cuticle,
although the epidermal hairs are absent in the
case. Due to the presence of the lateral
branches, circular stems are absent in
monocots. As we know that the main
difference between dicot and monocot stem is
due to the arrangement of the vascular
bundles.
In monocot stems, the vascular bundles
are scattered and are arranged indefinitely in
such a way that they are spread throughout
the steam area. Although the bundle sheath is
present in this case, which surrounds these scattered bundles. The hypodermis in the
monocot stem is made up of the sclerenchyma. As the vascular bundles are scattered,
they also lack the distinct cortex and stele.

The Dicot Stem

The dicot stem has single layered
epidermis along with the thick cuticle. Mainly
the difference in arrangement of the vascular
bundles make the difference between them and
the monocot stem. As the dicots are more
complex as compared to the monocots, they
may or may not have the epidermal hairs,
which are essential for the insulation, warmth
and absorption in plants. Their vascular
bundles are arranged in the form of one or two
broken rings. These bundles are definite in
shape and size and are smaller in size as
compared to the bundles in the monocots.
The hypodermis in the dicot stem is made up of the collenchyma. The main
function of the hypodermis is to secrete the chitinous cuticle; it is present in the
epidermal layer of cells in plants. In dicot stem, the epidermis is the outermost layer
along with the multicellular epidermal stem hairs. The other important regions of the
dicot stem are cortex, medullary rays, pericycle and pith. The vascular system in dicots
comprises of the two distinct regions cortex and stele, which are absent in the monocot
stems. The bundle sheaths, which have the main function to regulate the substances
between the parenchyma and vascular tissues, are absent in the dicots.
SUMMARY
1. Most stems are found above ground, but some of them grow underground. They can
be either unbranched or highly branched; they maybe herbaceous or woody.
2. Stems connect the roots to the leaves, helping to transport water, minerals and
sugars to the different parts of the plant.
3. Plant stems always have nodes and internodes. The petiole is the stalk that extends
from the stems to the base of leaf. While an axillary bud give rise to a branch or a
flower; it is usually found in the axil; the junction of the stem and petiole.
4. The stem is composed of three tissue systems that include the epidermis, vascular,
and ground tissues, all of which are made from the simple cell types.
5. The epidermis is a single layer of cells that makes up the dermal tissue covering the
stem and protecting the underlying tissue.
6. The vascular tissue of the stem consists of the complex tissues xylem and phloem
which carry water and nutrients up and down the length of the stem and are arranged in
distinct strands called vascular bundles.
7. Ground tissues helps support the stem and is called pith when it is located towards
the middle of the stem and called the cortex when it is between the vascular tissue and
the epidermis.
8. Modified stems that grow horizontally underground are either rhizomes, from which
vertical shoots grow, or flesher, food storing corms; Potatoes are examples of tubers,
the swollen end of stolons that may store starch; Bulb is the stem modification that has
enlarged fleshy leaves emerging from the stem; Aerial modifications of stems include
tendrils, thorns, bulbs and cladodes.
ROOTS
INTRODUCTION
The root system is the descending (growing downwards) portion of the plant axis. When
a seed germinates, radicle is the first organ to come out. It elongates to form primary or
the tap root. It gives off lateral branches (secondary and tertiary roots) and thus forms
the root system. It branches through large and deep areas in the soil and anchors the
plant very firmly. It also plays another vital role of absorbing water and mineral salts
from the soil and transporting them upwards.

Module Objectives:

1. Define and identify root.

2. Distinguish between different types of root systems.
3. Describe and illustrate different regions of a root apex.
4. Describe various modifications and functions of roots.
5. Describe and distinguish between primary structure of dicot and monocot root.
6. Illustrate and explain the mode of secondary growth in a dicot root.
7. Describe the deep-seated (endogenous) origin of lateral roots.

CHARACTERISTICS OF ROOTS
1. Non green due to absence of chlorophyll
2. Not divided into nodes and internodes
3. Absence of leaves and buds
4. Normally under the ground
4. Positively geotropic (grow towards gravity)
5. Positively hydrotropic (grow towards water)
6. Negatively phototropic (grow away from light)
FUNCTIONS OF ROOTS
1. Anchorage – Roots anchor the plant firmly to the soil (mechanical function).
2. Absorption – Roots absorb water and mineral salts and conduct them upwards
(physiological function).
3. Special functions – By undergoing modifications in their structure, roots perform
special physiological functions like food storage, assimilation, absorption of atmospheric
moisture, sucking food from host, better gaseous exchange and mechanical functions
like floating (buoyancy), stronger anchorage and climbing.
TYPES OF ROOT SYSTEMS
1. Tap root system — It is the root system that
develops from the radicle and continues as the
primary root (tap root) which gives off lateral
roots. They provide very strong anchorage as
they are able to reach very deep into the soil. It
is the main root system of dicots e.g. gram,
china rose, neem (fig.1).
Figure 1. Taproot

2. Fibrous root system — In this root system,

the primary root is short lived. A cluster of
slender, fiber-like roots arises from the base of
the radicle and plumule which constitute the
fibrous root system. They do not branch
profusely, are shallow and spread horizontally,
hence cannot provide strong anchorage. Fibrous
root system is the main root system of
monocots, e.g. maize, grasses, wheat. Figure 2. Fibrous roots

3. Adventitious root – These are roots that

develop from any part of the plant except the
radicle. They may be aerial or underground (Fig.
6.1b). They may grow from node (money plant,
bamboo), stem cutting (rose), tree branch
(banyan)or stem base (fibrous roots in
monocots).

Figure 3. Adventitious roots

REGIONS OF ROOTS

1. Root cap region — It is a thimble- like structure produced by meristematic (rapidly

dividing) zone and protects the tender apex (apical meristem) from harsh soil particles.
As the root grows further down in soil, root cap wears out but it is constantly renewed. In
aquatic plants (Pistia and water hyacinth) root cap is like a loose thimble called root
pocket (figure 4c).

2. Region of meristematic cells — is a small region of actively dividing cells called the
apical meristem (figure 4c). It consists of: (i) Dermatogen (outermost layer whose cells
mature into epiblema and root cap); (ii) Periblem (inner to dermatogen whose cells
mature into cortex) and (iii) Plerome (central region whose cells mature into stele)
(figure 4b). In monocots, cap is formed by independent group of cells known as
Calyptrogen.

3. Region of elongation — Lies next to the meristematic region, the cells elongate and
enlarge to make the root grow in length.

4. Region of maturation — Lies next to the region of elongation. The cells mature and
differentiate into various tissues constituting (i) Root hair or piliferous region having
unicellular hairs which absorb water and mineral salts from soil and (ii) Permanent
region which lies behind the root hair zone and is without hairs. It produces lateral
roots, anchors the plant in soil and conducts water and minerals upwards.

Figure 4. (a) Apical region of root showing four different regions; (b) L.S. through
root apex; (c) longitudinal section of roots apex.
MODIFICATIONS OF ROOTS
Tap roots and adventitious roots can get modified into a variety of forms to
perform various functions. Let us have a detailed look at the modification of roots.

Tap roots modifications:

1. Conical roots: base is broad and tapers gradually towards apex. Ex. Carrots

2. Fusiform roots: swollen in middle tapering towards both ends. Ex. Radish

3. Napiform roots: spherical at base tapering sharply towards the tip. Ex. Turnip

4. Tuberous roots: thick and fleshy with no definite shape. Ex. 4 O’clock plant

(a) (b) (c) (d)

Figure 5. Taproots modifications: (a) conical root of carrot;

(b) fusiform root of radish (c) napiform root of turnip;
(d) tuberous root of 4 O’clock plant

Adventitious roots modifications:

I. Modification for food storage

1. Tuberous roots: Swollen roots developing from nodes of prostrate stem.

Ex. Sweet potato
2. Fasciculated roots: Swollen roots developing in a cluster from the stem.
Ex. Dahlia
3. Nodulose roots: Only apices of roots become swollen like single beads.
Ex. Mango-ginger
4. Moniliform: Roots alternately swollen and constricted presenting a beaded or
moniliform appearance. Ex. Grasses, Sedges
 5. Annulated: Look as if formed by a number of discs placed one above the
other. Ex. Ipecac

(a) (b) (c) (d) (e)

Figure 6. Modification for food storage. (a) tuberous roots of sweet potato;
(b) fasciculated roots of dahlia; (c) nodulose roots of mango-ginger;
(d) moniliform roots of grass; (d) annulated roots of Ipecac

II. Modification for photosynthesis

1. Assimilatory roots: Root which when exposed to sun develop chlorophyll, turn
green (aerial root), and manufacture food orchid.
Ex. Tinospora (aerial root), orchid.

III. Modification for absorbing atmospheric moisture

1. Epiphytic roots: Aerial roots of epiphytes are greenish and covered with
spongy tissue (Velamen) with which they absorb atmospheric
moisture. Ex. Orchids (Vanda).

Figure 7. Assimilatory and epiphytic roots for orchids

 IV. Modification for better gaseous exchange

1. Pneumatophores: Some roots grow vertically up or respiratory roots

(negatively geotropic) into air. Exposed root tips possess
minute pores through which roots respire, appear like conical
spikes coming out of water. Ex. Mangroves (marshy plants)
Rhizophora.

V. Modification for sucking nutrition from host

1. Sucking roots or haustoria: Parasitic plants give out sucking roots or haustoria
which penetrate living host plant and suck food.
Ex. Cuscuta

(a) (b) (c)

Figure 8. Modification for gaseous exchange. (a) Pneumatophores of mangrove plants

Modification for sucking nutrients from host. (b) Cuscuta (parasite) on host
(c) section showing the sucking roots or haustoria penetrating the host plant

VI. Modification for strong support

1. Prop roots: Roots develop from tree branches, hang downwards and
ultimately penetrate the ground, thus support heavy branches.
Ex. Banyan

2. Stilt roots: Extra roots developing from nodes near the base of stem, grow
obliquely and penetrate the soil giving strong anchorage.
Ex. Sugarcane, Screwpine

3. Climbing roots: Weak climbers twine around and clasp the support with the
help of climbing roots arising from their nodes. Ex. Money
plant, betel
 4. Clinging roots: Special clinging roots arise, enter the crevices of support and
fix the epiphyte. Ex. Epiphytes, orchids

(a) (b) (c) (d)

Figure 7. Modification for strong support. (a) prop roots as in banyan; (b) stilt roots as in
sugarcane; (c) climbing roots as in betel; (d) clinging roots as in orchids

(vii) Modification for buoyancy & respiration

1. Floating roots: Spongy, floating roots filled with air, arise from nodes of some
aquatic plants, and help in floating and respiration. Ex. Jussiaea

Figure 8. Floating roots as in Jussiaea

DICOT ROOT

A thin transverse section of dicot root (fig.9) shows the following structures:

1. Epiblema: Single, outermost layer

of thin-walled cells. Some cells are
prolonged to form unicellular root hairs. It
protects and absorbs water.

2. Cortex: Large zone, many layered,

cells thin-walled parenchymatous with
intercellular spaces, stores food and water.

3. Endodermis: Innermost layer of

cortex, cells barrel-shaped, closely packed,
show band like thickenings on their radial
walls called casparian strips. Some cells
(opposite the protoxylem) which lack these
strips are called passage cells. They help in
the movement of water and dissolved salts
from cortex directly into xylem. Stele: All
tissues inner to endodermis comprise stele.

4. Pericycle: Inner to endodermis lies

a single layer of pericycle. It is the seat of
origin of lateral roots and vascular cambium
and cork cambium during secondary growth. Figure 9. A portion of dicot root in
transverse section

5. Vascular Bundle: It consists of xylem and phloem patches lying on alternate

radii i.e., it is radial. Xylem is exarch where protoxylem (first formed, having narrow
vessels and tracheids) lies towards the periphery and metaxylem (differentiates later,
has wider vessels and tracheids) lies towards the center. Depending upon the number
of xylem patches a root may be diarch (di-2 patches) to hexarch (hexa- 6 patches).

6. Pith: Sometimes the metaxylem of all xylem patches meet in the centre, in that
case pith is absent or is small and parenchymatous.

7. Conjunctive parenchyma: Parenchyma which separates xylem and phloem.

MONOCOT ROOT

A thin transverse section of monocot root (fig.10) shows the following structures.

1. Epiblema: Outermost, single layer of

thin-walled, closely packed cells. Some
cells are prolonged into unicellular root
hairs.

2. Cortex: Large zone, multilayered, of

parenchymatous cells with intercellular
spaces, stores water and food material.

3. Endodermis: Innermost layer of cortex

with characteristic casparian strips and
passage cells.

4. Stele: All the tissue inner to endodermis

is called stele

5. Pericycle: Single layered, of thin walled

cells. The lateral roots originate from this
layer.

6. Vascular Bundle: it consists of many

patches of xylem and phloem arranged Figure 10. A portion of monocot root
radially. The xylem is exarch and polyarch. in transverse section

7. Pith: Lies in the center, large, well developed, parenchymatous or

sclerenchyamatous, stores food.

8. Conjunctive Parenchyma: Lies in between strands of xylem and phloem.

ORIGIN OF LATERAL ROOTS

➢ The origin of lateral roots is Endogenous i.e. from deeper layer.

➢ The seat of its origin is pericycle where cells opposite protoxylem divide and form
a hump in the endodermis (fig. 10 a-b)

➢ The hump penetrates into the cortex (fig. 10 c-d)

Figure 11. a-d Formation of lateral root (Endogenous origin)- Stages as seen in
longitudinal sections of root.

➢ Later, the hump differentiates into 3 regions of the root apex i.e. dermatogen,
periblem and plerome.

➢ Finally, the lateral root comes out.

➢ The number of lateral roots correspond to the number of xylem bundles.

SECONDARY GROWTH IN DICOT ROOTS

The roots grow in length with the help of apical meristem. It is called primary growth.
Apart from primary growth, roots grow in width i.e., they increase in girth. This increase
is called secondary growth. It is found only in dicot roots.

The tissues involved in secondary growth are lateral meristem i.e., vascular cambium
and cork cambium.

It is important to remember that the vascular cambium and cork cambium are secondary
in origin and arise from pericycle.

Secondary growth is as follows:

Pericycle cells outside the protoxylem divide to form a strip of cambium (Fig 12b).

Another strip of vascular cambium appears in the conjunctive tissue on the inner side of
phloem bundle (Fig. 12 a, b).
Figure 12. T. S. Dicot Root – (a) and (c) (diagrammatic) – Early stages in secondary
growth (b) Stele enlarged (cellular)

These two vascular cambium strips join laterally to form a ring which may initially be
wavy (Fig. 12 c) but later becomes circular due to over production of secondary xylem
tissue inner to primary phloem (Fig. 12 a).

Cambium cells consist of brick shaped cells which divide and add cells on its either side
i.e. towards periphery and towards center. Those added towards the periphery
differentiate into secondary phloem and the ones formed towards the center differentiate
into secondary xylem.

Figure 13. T. S. Dicot root (Diagrammatic) a-b Later stages in secondary growth.
Secondary tissue formed outer to the protoxylem bundle differentiates into prominent
primary medullary ray thus, protoxylem does not get crushed (Fig. 13a).

Later, cork cambium (Phellogen) also differentiates in the pericycle (Fig. 13b).

The cork cambium divides and gives rise to cork (Phellem) towards outside and
secondary cortex (Phelloderm) towards inside.

All the three i.e. Phellogen, Phellem and Phelloderm together form the Periderm of the
root and has protective function.

Finally, all the primary tissue outside the developing cork (i.e. endodermis, cortex and
epiblema) are sloughed off.
LEAVES
An Overview of Plant Leaf
The leaf is the lateral, expanded, flattened outgrowth of a stem, arising at a nod,
and possessing a bud in its axil. Most leaves are flattened and expanded but modified
kinds of leaves do not necessarily have this flattened structures. They usually grow
above the ground, supported and elevated to an aerial position by the stem, thereby
permitting access to light energy from the sun, water from the raindrops, and carbon
dioxide from the air.
Leaves are the major sites of photosynthesis. They take in carbon dioxide from
the air and produce oxygen through stomata. Leaves come in many sizes and shapes,
they are often used to help identify plants.
In the tropical regions, the leaves live only for single growing season, then they
fall off. Such plants are called deciduous. Plants which retain their leaves for more than
one growing season, and thus have leaves at all times of the year are called
evergreens.
Objectives:
1. Describe the main parts of a leaf.
2. Describe some major types of leaves
3. Discuss common vein patterns found in leaves.
4. Explain how leaf is organized.
5. Recognize the economic importance of leaves.

External Parts of a Leaf

Figure 1. Leaf of Ficus religiosa (pipal) showing various parts of the leaf
1. Petiole – a cylindrical structure that grows out
from the node of the stem. It holds and supports
the leaf blade; provide the leaf maximum exposure
to sunlight; serve for conduction of food and water.
1.1 Petiolate – a leaf with petiole
1.2 Sessile – a leaf without petiole

2. Leaf blade – the expanded, thin, flattened,

usually green portion continuous to the leaf stalk or
petiolate. In some monocot plants, the blade is
supported by a flattened structure called the leaf
sheath which clasps the stem. Outgrowths found at
the junction on the leaf sheath and the leaf blades
are collectively known as the ligules. The ligule
prevents water entering inside the leaf-sheath where
it might be retained and cause rotting.

3. Stipule- a pair of small leaves, or small flaps of

tissues that grow out from the base of the petiole.
3.1 Stipulate- leaves with stipules
3.2 Exstipulate- leaves without stipules

4. Leaf margins- edges of the leaves

5. Leaf base- lowermost part of the petiole of the leaf sheath which joins the entire leaf
to the stem.
6. Midrib- line that divides the leaf in half which serve to carry the weight of the leaf,
and also serve for the conduction of food and water to the cells of the leaf.
There are two types of leaf venation
1. Parallel venation or striate venation - the
veins run alongside each other at a similar angle
to the midrib of the blade; characteristic of
monocotyledonous plants.
2. Net venation or reticulate venation - the veins
branch out repeatedly and form a network over the
blade; characteristic of dicotyledonous plants.
INTERNAL ANATOMY OF LEAVES
A cross section of a leaf under the microscope shows three regions.
Epidermis - single layer of cells found on
the upper and lower portions of the leaf;
cells are tightly together and are of two
types: the ordinary epidermal cells and the
crescent-shaped guard cells, which
contain chloroplasts and occurs in pairs
with an opening called the stoma enclosed
by each pair.
Epidermal cell- protect the inner
tissues from desiccation, from mechanical
injury, and from the entrance of parasites
and other microorganisms that cause
disease; it also regulates the exchange of
gases such as carbon dioxide, oxygen and
water vapor that are needed by or
produced by the leaf; and it secrete a waxy
cutin on their outer surface.

Cutin – a fatty substance that is impervious to water; some plants produce a

layer of wax outside the cuticle, which gives extra protection against water loss. Cuticle
and its external waxy layer also provide a slick surface that discourages the attachment
and germination of fungal spores and creates an unsure footing for many insects. Some
of this trichomes, which make leaves feel furry or fuzzy, protect against water loss and
excessive buildup of heat. Others contain toxic substances that repel insects or other
animals that might eat the leaf.
Stoma/Stomata - each consisting of a pore regulated by two epidermal cells
called guard cells. Typically located in greater numbers on the underside of the leaf,
where they are protected from the highest temperatures and accumulate less dust and
fewer fungal spore. Through the stomata, carbon dioxide enters the leaf for
incorporation into carbohydrates by photosynthesis, and water vapor and oxygen exit
the leaf in large quantities. When guard cells take up water, the bands of cellulose in
their cell walls cause them to swell and curve so that the stoma opens; when guard cells
loss water, the stoma closes. In most plants, the amount of water in the leaf is an
important factor controlling whether stomata are open or closed. Stomata serve for the
respiration of the leaf.
Transpiration - evaporation of water
through the stomata and serves as a suction
to pull water and minerals up the stem from
the roots. Advantage of transpiration is it
creates a cooling effect within the leaves.
Disadvantage is it can cause wilting and
desiccation of the leaves and the whole plant
body. If too much, it can cause the plants to
die.
Evaporation - also cools the leaves. It is the process by which water evaporates
to the atmosphere. Water becomes vapor. Evaporation is important in the water cycle.
When the molecules of water are already condensed in the clouds, the water will return
back to the earth in the form of precipitation. Water precipitates include rain, snow or
ice, fog, dew
Guttation - exudation of
water in liquid form, usually through
a special pores called hydathodes.
Guttation usually occurs from well-
watered plants on cool, moist
nights following warm days.

Mesophyll- central portion of the leaf (from the Greek mesos “middle” and phyllon,
“leaf”). It is located between the upper and lower layer of epidermis; composed of two
tissues: palisade tissue which consist of vertically elongated, cylindrical cells and lined-
up underneath the epidermis. Usually one cell thick, although multiple layers may occur
where sunlight is intense. While spongy tissue consists of loosely-packed
photosynthetic cells that have sufficient space between them to allow diffusion of carbon
dioxide from the stomata to other parts of the leaf.
Veins- branched continuations in the mesophyll found in the vascular bundles of the
petiole. It consists of xylem cell such as vessel and tracheid; and phloem cells such as
sieve tubes, companion cells which conduct water and food upward and downward into
the petiole. In most leaves, xylem cells are in the upper part of the veins while the
phloem cells are on the lower part of the veins.
MODIFIED LEAVES:
Leaves have been modified to perform a number of specific functions by plants.
1. For support – like tendrils, floats, hooks, supporting leaf bases to form false
trunk.
 2. For absorption – like thin, uncutinized epidermis, insectivorous leaves.
3. For attraction – like bright coloration of entire blade or portion of the blade,
petalloid, bracts.
4. For reproduction – like buds, plantlets develop at certain parts of the leaf.
5. For protection – like bud scales, motile leaves, spiny leaves, trichomes.
6. For storage – like fleshy or thickened blades, bulb or bulblets, pocket leaves
7. Additional photosynthesis – development of leaf-like petioles and stipules.

FACTORS NECESSARY FOR PHOTOSYTHESIS

1. Carbon dioxide- gaseous compound which makes up 0.03-0.04% of the air. It
reaches the stomata in the leaves of plants through the mesophyll.
2. Water – absorbed by the roots and passed through the mesophyll of the leaves.
3. temperature- favorable temperature for photosynthesis is usually between 50C to
450C. There is a higher rate of photosynthetic activity during warmer temperatures than
colder temperatures.
4. Light- any wavelength of visible light may be photosynthetically effective. About 3% of
the total energy of sunlight reaching a leaf is actually used in photosynthesis. The
unused 97% is reflected, passes through the leaf or absorbed as heat.
5. chlorophyll- green coloring pigment found in plant cells.
chlorophyll a- bluish green material found in higher plants and algae
chlorophyll b- yellowish-green material found in other green plants
chlorophyll c, d, e- light green found in other green plants
carotenoids- yellow or yellow-orange pigments found in carrots, squash,
tomatoes
and other plants.
phycoerythrin- red pigment found in red algae and other plants
phycocyanin- blue-green found in blue-green algae and other plants
cytochromes- iron-containing compounds that act in the energy-transfer system
by
accepting and releasing electrons.

Phyllotaxy- system or pattern of leaf arrangement of a stem.

Types of Phyllotaxy

1. Alternate or spiral- only one leaf is developed each node.

2. Opposite- two leaves develop opposite each other at a node
3. Whorled or verticillate- three or more leaves develop equidistantly around the node
4. Fasciculate- two or more leaves develop at only one side of the node.

Alternate Opposite Whorled Fasciculate

VARIATION OF LEAVES: Leaves vary widely in size and form and may be classified
according to blade pattern including shape, margin, base modification and apices.

Leaf Shapes- any of the various shape that leaves of plants can assume leaf form

1. Aciculate- very slender, long and pointed, needle-like

2. Acuminate- tapering to a long point
3. Subulate- small, short, sharp-pointed, broadened at the base, scale-like
4. Linear- narrow and long, with approximately parallel sides
5. Oblong- longer than broad, with nearly parallel sides and with a rounded base and
apex
6. Lanceolate- Long, wider in the middle, shaped like a lance tip
7. Oblanceolate - Much longer than wide and with the widest portion near the tip,
reversed lanceolate.
8. Ovate- broadest part belong to the middle, more or less narrow, narrowed toward the
tip, egg-shaped
9. Obovate- broader part above the middle, the reverse of ovate
10. Elliptical- broads at the middle tapering, more or less equally to the apex
11. Oval- broadly elliptical, with the width greater than one-half the length
12. Orbicular- more or less circular, in outline, flat
13. Cordate- heart-shaped
14. Deltoid- triangular
15. Reniform- kidney-shaped
16. Rhomboid- diamond-shaped, with equal sides but unequal angle
17. Falcate- more or less curved
18. Spatulate- broadly rounded above and long and narrow belong
19. flabellate- fan-shaped.
20. rosette- leaflets in tight circular rings.
21. Tripinnate- leaflets also bipinnate.
22. Bipinnate- leaflets also pinnat
23. Pinnatisect- deep opposite lobing
24. Palmate- like a hands with fingers
25. Odd pinnate- leaflets in row, one at a tip
26. Even pinnate- leaflets in row, two at a tip
27. Whorled- ring of three or more leaflets
Leaf Base - The part of the leaf which is attached to the stem or a branch.

1. Oblique-base with unequal sides

2. Acute- forming an acute angle of less than 90 degrees
3. Cuneate- wedge-shaped, tapering evenly to a narrow point
4. Obtuse- the sides forming an angle of more than 90 degrees
5. Truncate- abruptly cut-off transversely at the base
6. Cordate- heart-shaped
7. Rounded- forming an arc
8. Sheathing- wrapped around the stem
9. Perfoliate- the lobes meet around the stem
10. Connate- united or grown together from the first formation
11. Auriculate- with ear-like appendages formed by two projecting sides of the base
12. Decurrent- leaves prolonged on the stem beneath their insertion
13. Sagittate- arrow-shaped, the auricles turned inwards
14. Peltate- a shield-shaped leaf, petiole is attached to the lower side
15. Hastate- spear-shaped, more or less triangular, with two lateral lobes diverging
laterally.
Leaf Margins- the boundary area extending along the edge of the leaf.

1. Entire- without teeth or lobes

2. Serrate- with sharp teeth pointing toward the apex, like a saw
3. Double serrate- coarsely serrated, the teeth margin again serrated
4. Crenate- with regular blunt or rounded teeth
5. Doubly crenate- coarsely crenated, the teeth margin again crenated
6. Sinuate- deeply or strongly wavy but not lobed
7. Repand- slightly wavy
8. Dentate- with sharp teeth pointed outward
9. Lobed- deeply cut, but the incisions do not reach much more than halfway the
midrib
10. Cleft- nearly the same as lobed, but incisions extend more than halfway to the
midrib
11. Parted- division extend nearly to the midrib
12. Crenulate- crenated with minute teeth
13. Serrulate- serrated with minute teeth
14. Ciliate- with line hairs
15. Incised - margins are sharply and deeply indented.

cleft
LEAF APICES - The outer end or apex of a leaf lamina that is opposite the petiole.
1. Rounded- broad and semi-circular in outline
2. Acuminate- pointed but the tapering line is curved
3. Acute- ending in an acute angle with straight sides
4. Emarginate- with shallow notch at the tip
5. Mucronate- abruptly tipped by a small short point
6. Aristate- the mucronate point is extended into a larger and more or less bristle-
like
7. Cuspidate- tipped with sharp point
8. Obtuse- blunt-pointed or rounded
9. Caudate- abruptly ending in a long tail
10. Retuse- with a rounded sinus at the tip
11. Truncate- cut-off square or nearly so
Types of Leaves

The two different types of leaves found in a plant are:

Simple Leaf

When a single lamina is connected to the

main stem by a petiole, the leaf is said to
be simple. A simple leaf may be incised to
any depth but not down to the midrib or
petiole; leaf blade is a single continuous
unit. Example, Guava leaves.

Compound Leaf

A compound leaf is a leaf made up of two

or more leaflets. In a compound leaf, the
midrib of the leaf is branched into different
leaflets and is connected by a single
petiole. Example, Pea, palm leaves.

The compound leaves are further sub-divided into the following types of leaves:

Palmately Compound Leaf. In a palmately compound leaf, the leaflets are attached at
the tip of the petiole. Example, Silk cotton. These can be differentiated into:

1. Unifoliate: This type of leaves has only one leaflet. Ex., Citrus
2. Bifoliate: These leaves have two leaflets. Ex., Balanites
3. Trifoliate: These leaves have three leaflets emerging from the same point. Ex.,
Oxalis
4. Quadrifoliate: These leaves have four leaflets arising from the same point. Ex.,
Marsilea
5. Multifoliate: This type of leaf has many leaflets arising at a common point. Ex.,
Bombax
Pinnately Compound Leaf. In a pinnately compound leaf, the midrib of the leaf is
divided into numerous leaflets and all connected by a common axis. Example, Neem.
These can be further differentiated into:

1. Pinnate: A compound leaf that has an axis on each side of the midrib is known
as a pinnate leaf.
2. Unipinnate: The leaf with leaflets on each side of the axis. Ex., cassia
3. Bipinnate: Here, a secondary axis bearing the leaflet is produced by the central
axis. Ex., Acacia
4. Tripinnate: Here, a tertiary axis bearing leaflets emerges from the secondary
axis. Ex., Moringa
5. Decompound: Leaf with more than three pinnate. Eg., old leaves of coriander
6. Parapinnate: A leaf without a terminal leaflet. Ex., Cassia
7. Imparipinnate: Leaf with an odd terminal leaflet. Ex., Pea

Pinnate Unipinnate Unipinnate

(Parapinnate) (Imparipinnate)

Bipinnate Tripinnate Decompound

REFERENCES

Vicencio, M. C. (2024). Textbook in General Botany. College of Science. University of

Eastern Philippines. Catarman, Northern Samar. Philippines.

BYJU’S. (2024). Parts of a Plant. https://byjus.com/biology/parts-of-plants/