DEPARTMENT OF BIOLOGICAL SCIENCES

REV. FR. MOSES ORSHIO ADASU UNIVERSITY MAKURDI

COURSE TITLE: INTRODUCTION TO PLANT DIVERSITY (2 UNITS)
COURSE CODE: PSB 101
Lecture Note by Akintade Ojo

Diversity in Plant Structures and Functions

Plants differ from each other in size and morphological features. The reason for this vast diversity
is the adaptations of plants to survive in different environments. Plant diversity encompasses two
key aspects; structural diversity and functional diversity. The structural diversity refers to the
variations in physical characteristics of plants such as their size, shape and arrangement while
functional diversity on the other hand refers to the various ecological roles plants play; such as
nutrient cycling, interacting with other organisms and contributing to ecosystem processes.
Structural and functional diversity are closely related. The variety of plant structures affects the
range of ecological functions they can perform. For example, a forest with a mix of different plant
sizes and shapes will likely support a wider range of animals and other organisms, contributing to
the overall health and resilience of the ecosystem.

Structural Diversity

This aspect of plant biodiversity focuses on the physical characteristics of plants. It includes the
variation in:

• Size and shape: From tiny herbs to giant trees, plants display a wide range of sizes and
forms.

• Canopy structure: The way plants are arranged vertically, influencing light availability and
habitat structure.

• Root systems: Differences in root structure can impact nutrient uptake and soil stability.

• Example: A forest with a mix of tall trees, shrubs, and low-lying plants exhibits high
structural diversity.

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Functional Diversity:
This aspect emphasizes the roles that plants play in their ecosystems. It includes:

• Nutrient cycling: Plants absorb nutrients from the soil and release them back
through decomposition, influencing soil health.

• Carbon sequestration: Plants store atmospheric carbon, playing a crucial role in

climate regulation.

• Water cycling: Plants influence water movement through transpiration, affecting

local water resources.

• Pollination and seed dispersal: Some plants rely on animals for pollination and
seed dispersal, contributing to the flow of genetic material. Example: A community
with diverse plant species, each with unique roles in pollination or nitrogen fixation,
exhibits high functional diversity.

The basic parts of most land plants are roots, stems, leaves, flowers, fruits and seeds. The structure
and function of each plant parts will be discussed extensively.

A. THE LEAVES
Leaves are considered as the most significant part of a plant. Most of the leaves are green in colour.
The main function of a leaf is photosynthesis. Food is produced in plants having chlorophyll by
using carbon dioxide, water and solar energy. This process is known as photosynthesis. Leaves are
the photosynthetic organs of the plant. Their morphology and anatomy have been adapted over
evolutionary time to optimize light absorption and carbon dioxide uptake. The green substance,
chlorophyll, captures light energy and uses it to convert water and carbon dioxide into plant food
and oxygen. The leaf also loses water to the atmosphere (transpiration) and grows at the node of a
stem. A typical green leaf consists of three principal parts; the leaf base, the petiole and the leaf
blade (or lamina). The leaf base is the part of the leaf attached to the stem while the petiole is the
stalk between the leaf base and the leaf blade (although absent in leaves in some plants). The leaf
blade or lamina is the green flattened part of the leaf. At the centre is a strong vein known as midrib
which gives rise to the secondary veins at its sides. Leaves of dicotyledons are net veined while
those of monocotyledons are parallel veined.

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Leaf Types
Simple: Leaf blade is one continuous unit (cherry, maple, Guava, hibiscus and elm).

Compound: Several leaflets arise from the same petiole (Neem leaf)

Palmately Compound: Leaflets radiate from one central point, like fingers from a palm. (Ohio
buckeye, Cassava and horse chestnut).

Pinnately Compound: Leaflets arranged on both sides of a common rachis (leaf petiole), like a
feather (mountain ash, Neem leaf).

Bi-pinnately (Doubly) Compound: Leaflets are themselves compound, with smaller leaflets on
a secondary rachis.

Leaf Arrangement on Stems

Alternate: Arranged in staggered fashion along stem (willow).

Opposite: Pair of leaves arranged across from each other on stem (maple).

Whorled: Arranged in a ring (catalpa).

Rosette: Leaves arranged tightly at the plant crown (dandelion)

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Shapes of leaves
Leaf shape is a primary tool in plant identification. Descriptions often go into minute detail about
leaf shapes and margins.

Leaf Shape Descriptions

Cordate: Heart-shaped.

Cuneate: Leaves with small width at base, widening near the top (think wedge).

Elliptical: Leaves widest in the middle, tapering on both ends.

Hastate: Arrowhead shaped leaves.

Lanceolate: Leaf is three times or longer than width and broadest below the middle.

Linear: Leaves narrow, four times longer than width and have the same width.

Obcordate: Reverse appearance of cordate leaves. (The heart shape is upside down).

Oblanceolate: Leaf is three times longer than wide and broadest above the middle.

Oblong: Leaf is two to three times as long as it is wide and has parallel sides.

Obovate: Leaf is broadest above the middle and about two times as long as the width. Ovate: Leaf
is broadest below the middle and about two times as long as the width, also called oval (egg
shaped).

Peltate: Leaves rounded with petiole attached under the leaf base.

Reniform: Leaves wider than they are high. Spatulate – Generally narrow leaves widening to a
rounded shape at the tip.

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 Figure 1. Leaf Shapes
Leaf Margins
The leaf margin is another tool in plant identification
Crenate: Leaf edge has blunt, rounded teeth.
Dentate: Leaf has triangular or tooth-like edges.
Doubly Serrate: Edges with saw like teeth that have even smaller teeth within the larger ones.

Entire: Leaf edge is smooth.

Incised: Leaf margins have deep, irregular teeth.
Lobed: Leaf edges are deep and rounded. Serrate – Leaf edges are sharp and saw-like (think
serrated knife).
Serrulate: Leaf edges with smaller, more evenly spaced serrations than a serrated leaf.
Sinuate: Margins are slightly wavy.

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Undulate: Very wavy margins.

Figure 2. Leaf Margins

Leaf Venation

Monocots Parallel Venation: Veins run in parallel lines (common in monocots, e.g., grasses, lilies,
tulips)

Figure 3. Parallel veined monocot leaf

Dicots
Pinnate Venation: Veins extend from a midrib toward the edge, resembling a feather. (elm, peach,
apple, cherry).

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Palmate Venation: Veins radiate from a central point in a fan-shape from the petiole, like fingers
on a palm (maple, grapes).

Figure 4. Venation of Dicot Leaves

Internal Structural Features
The leaf blade is composed of several layers.

Epidermis: Outer layer of cells

Cuticle: Waxy protective outer layer of epidermis that prevents water loss from leaves, green
stems, and fruits. The amount of cutin or wax increases with light intensity.

Leaf Hairs/Trichomes: Uni- or multicellular projections that can provide physical defense or
excrete chemical compounds.

Stomates (Stomata): Natural openings in leaves and herbaceous stems that allow for gas exchange
(water vapor, carbon dioxide and oxygen) and plant cooling. Most stomates are found on the
underside of leaves.

Guard Cells: Specialized kidney-shaped cells that open and close the stomata.

Vascular bundle: Xylem and phloem tissues comprising the leaf veins.

Mesophyll: Cells within the leaf directly involved with photosynthesis, storage, and other
metabolic processes. Mesophyll organization is variable within the leaves of different plant
species.

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 Palisade Layer: Closely ranked cells directly beneath the epidermis, very photosynthetically
active. Not all plants have a well differentiated palisade layer.

Spongy Mesophyll: Loosely organized ground tissue (mostly parenchyma cells) that are involved
with photosynthesis, water and nutrient exchange and metabolism.

Figure 5. Cross sectional view of a leaf

Relationship between structure and function in dicotyledon leaf

Tissue Structure Function

Upper and One cell thick. Flattened cells lacking Protective
Lower chloroplasts. External walls covered with a Cutin is waterproof and protects from
Epidermis cuticle of cutin (waxy substance). Contains desiccation and infection
stomata (pores) which are normally confined to Stomata are sites of gaseous exchange
or more numerous in the lower epidermis. Each with the environment. Their size is
stoma is surrounded by a pair of guard cells regulated by guard cells , special
epidermal cells containing chloroplasts.
Palisade Column shaped (palisade) cells with numerous Main photosynthetic tissue.
Mesophyll chloroplasts in a thin layer of cytoplasm Chloroplasts may move towards light.
Spongy Irregularly shaped cells fitting together loosely Photosynthetic, but few chloroplasts
Mesophyll to leave large air spaces than palisade cells.
Gaseous exchange can occur through
the large air spaces via stomata.
Stores starch
Vascular tissue Extensive finely branching network through the Conducts water and mineral salts to the
leaf leaf in xylem.
Removes products of photosynthesis
(mainly sucrose) in phloem.
Provides a supporting skeleton to the
lamina, aided by collenchyma of the
midrib, turgidity of the mesophyll cells
and sometimes sclerenchyma.

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Leaf fall process
As the leaves grow old, they change colour and eventually drop off. Before a leaf falls, a thin layer
of tissue called the absciss layer grows across the base of the petiole and forms a line of weakness;
the leaf, cut of from its supplies of water and mineral salts, is soon blown off. As the leaf drops it
leaves a mark on the stem known as leaf scar. This is made of cork tissue which is formed beneath
the absciss layer.

Modified Leaves
Modified leaves are leaves that have adapted to perform functions other than photosynthesis, such
as climbing, protection, or nutrient capture. These modifications can involve changes to the leaf's
structure, size, or shape. For example, tendrils (thread-like structures) help plants climb, spines
protect them from herbivores, and traps capture insects.

Adhesive Disc
Modified leaf used as an attachment mechanism. Sometimes referred to as a holdfast (Boston
ivy).

Bract

Specialized, often highly colored leaf below flower that often serves to lure pollinators
(poinsettia, dogwood).

Tendril
Modified leaf, stipule or other plant part used for climbing or as an attachment mechanism
(Virginia creeper, peas, grapes). Distinguished from twining stems by the absence of leaves along
their length (since they are themselves leaves). E.g Gloriosa lily

Hairs and spines

In a few plants, the leaves are either absent or modified into hairs or spines whicah are unable to
carry out photosynthesis; in these plants the work of the leaf in manufacturing food is taken on by
the stem as in cactus or Euphorbia or by the roots as in some orchids.

Finally, it should be pointed out that some plants lack leaves entirely. Parasitic plants such as
dodder (Cuscuta sp.) and love vine (Cassytha filiformis) derive all their nutrition from their host

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plant and have no need for photosynthesis, not even in their stems, which lack chlorophyll and are
yellow.

B. STEMS
Stems are typically aboveground organs that grow toward light (positive phototropism) and away
from the ground (negative gravitropism). They provide support for the aerial portions of plants and
serve multiple other functions in a plant. Stems play an important role as the conduits of the
vascular tissues (xylem and phloem) needed for long-distance transport of water, minerals,
photosynthetically derived sugars, and hormones. Accordingly, the evolution and design of the
stem vascular system (a.k.a. stele) is of great interest to plant anatomists. We will take an in-depth
look at stem diversity, structure, function and introduce the basics of the plant vascular system.

One of the primary functions of a stem is the production of leaves and the proper arrangement of
those leaves in space. Stems are also the organ from which flowers are generated and are held in
the proper position for pollinators. Stems have tissues that provide a means to increase length
(apical meristems), girth (lateral meristems), and additional branching (axillary buds). Roots can
develop directly from a stem; such roots are called adventitious. By generating xylem and phloem,
stems are a conduit for the long-distance transport of water, nutrients, and photosynthate and allow
the leaves and the roots to engage in whole-plant communication

The stem is typically thought of as a long, thin projection from which leaves or flowers emerge.
Indeed, that form is found on many extant plants and is the form to have evolved first. However,
over time, natural selection produced several other forms and functions. Flattened stems that lack
leaves and perform photosynthesis are called cladodes; cactus “pads” are a common form.
Cladodes that resemble leaves, such as those found on Acacia (Dong and He 2017), are also
referred to as cladophylls phyllodes and phylloclades. Rhizomes are underground stems that can
play the role of a root (anchorage and water uptake), allow for asexual reproduction, or serve as a
storage organ as seen in the common potato tuber. Stolons differ from rhizomes in that rhizomes
grow beneath the soil surface, while stolons grow across the soil surface. Stolons allow for plant
dispersal and asexual reproduction. Stems may also serve as perennating organs. Tendrils may be
modified leaves or stems that aid in attachment of viney plants.

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Not all plants possess a stem. For instance, the much-reduced members of the duckweed family
(Lemnaceae) are monocots that have adapted an aquatic habitat. The true duckweeds (Lemna sp.)
are composed of small leaf-like fronds and one to a few roots. Watermeal plants (Wolffia sp.) are
even more reduced and consist of two connected individuals—a mother frond that asexually
produces multiple daughter fronds from a pouch at one end.

Several basic stem types are apparent based on (a) the presence or absence of secondary growth
and (b) on the plant group. Stems with only primary growth are said to be herbaceous, a
characteristic found in most annuals (one-year life cycle) and biennials (two-year life cycle). Stems
with true secondary growth are commonly called woody stems. The secondary growth arises from
a vascular cambium which, in some plants, may remain active for decades, centuries, or even
millennia leading to accumulations of xylem and a very large stem diameter.

Horticultural uses of stems include; Aesthetic (winter interest in the landscape, appealing bark,
etc.); Feed and food, Plant identification, Propagation (cuttings and layering), Wildlife habitat,
Wood industry and construction.
Internal Features
Shoot Apical Meristem: “Immortal” cells at the tips of stems that generate new cells for
differentiation and growth in stem length.

Epidermis: Outer layer of wax-coated cells that provides protection and covering.

Cortex: Primary structural and storage tissues of a stem.

Vascular Tissues

Vascular Bundle: grouped phloem, xylem and associated cells in primary stems. Vascular bundles
give rise to the Vascular Cambium in plants that are capable of secondary growth (stem
thickening).

Vascular Cambium: the layer of meristematic (dividing) tissues that forms in some plants to
generate secondary growth (growth in girth). The cambium divides to form phloem tissues toward
the outside of the stem and xylem tissues toward the inside. Cell division of the cambium tissues
adds width to the stem.

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 Figure 6. Cross section of stem in secondary growth

Secondary Phloem (inner bark): In plants with secondary growth (woody plants), the phloem is
located to the outside of the vascular cambium and just beneath the bark. If the stem is damaged
or girdled so as to disrupt or block the phloem, it can enlarge just above the blockage due to the
sugars moving down from the leaves for distribution throughout the plant. Tissues below the
blockage slowly starve. Roots die back, eventually leading to death of the plant.

Secondary Xylem (wood): distributes water and minerals from the roots up through the plant.
Typically only the xylem tissue nearest the vascular cambium (the youngest xylem) functions for
water transmission; older xylem provides structural support.

Pith: the soft center of dicot plant stems, consisting of parenchyma cells. In some plants the pith
breaks down forming a hollow stem.

External Features

Bud: A stem's primary growing point. Buds can be either leaf buds (vegetative) or flower buds
(reproductive). These buds can be remarkably similar in appearance, but flower buds tend to be
plumper than leaf buds.

Terminal bud: Bud at the tip of a stem. In many plants, auxin (a plant hormone) released from
the terminal bud suppresses development of lateral buds, thereby focusing the growth of the plant
upward rather than outward. If the terminal bud is removed during pruning (or natural events) the
lateral buds will develop and the stem becomes bushy.

Lateral Buds: They grow from the leaf axils on the side of a stem.

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Leaf Scar: Mark left on stem where leaf was attached. Often used in woody plant identification.

Bundle Scar: Marks left in the leaf scar from the vascular tissue attachment. Used in woody plant
identification.

Lenticel: Pores that allow for gas exchange.

Node: Segment of stem where leaves and lateral buds are attached. [Figure 6] Note: Roots do not
have nodes.
Internode: Section of a stem between two nodes.

Bark: Protective outer tissue that develops with age. Used in woody plant identification.

External Features of a stem Node and Internode

Modified Stems
Corm: Short, thickened, underground monocot stem.

Crown: Compressed stem having leaves and flowers growing above and roots beneath (strawberry
plant, dandelion, African violet).

Rhizome: Horizontal, underground stem, typically forms roots and plantlets at tips or nodes (iris,
bentgrass, cannas).

Spur: Very compressed (shortened), fruiting twig found on some apples, pears, cherries, and
ginkgo.

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Stolon (or runner): Horizontal, above-ground stems often forming roots and/or plantlets at their
tips or nodes (strawberry runners, spider plants).

Thorn: A stem modified for plant defense. Thorns maintain cell types and morphology of stems,
whereas prickles are superficial outgrowths of the epidermis. Hawthorns have thorns, roses have
prickles.

Twining stems: Modified stems used for climbing. Some twist clockwise (hops, honeysuckle);
others twist counterclockwise (pole beans, Dutchman’s pipe).

Tuber: A solid thickened portion or outgrowth of an underground stem containing stored food
(e.g., potato, the eyes of the potato are axillary buds).

Corm Crown Rhizome Spur

Stolon Tuber

C. ROOTS
Roots have crucial functions: anchoring the plant, absorbing water and nutrients, and conducting
these materials throughout the plant. They also store food and can be involved in reproduction. The
root's structure, including root hairs, epidermis, cortex, and vascular tissue, is adapted to facilitate
these functions. The structure and growth habits of roots have a pronounced effect on: Size and
vigor of the plants and Adaptation to certain soils. Because they are out of sight, roots are often

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out of mind. They are widely overlooked as to their significance in plant health. The majority of
all plant problems start with soil/root problems.

The delicate tips of the roots are protected by cup layer of tissue known as the root cap. Behind
the root cap, the root has numerous delicate hairs known as root hairs which grow between the
particles of the soil and thus help the branch roots to hold the plant firmly. Roots of any type can
be distinguished from stems because they do not bear leaves or buds and thus lack internodes
instead they have root caps and root hairs at their tips.

Epidermis: The outer layer of cells.

Root hairs: Absorptive unicellular extensions of epidermal cells of a root. These tiny, hair-like
structures function as the major site of water and mineral uptake. Root hairs are very delicate and
subject to desiccation. Root hairs are easily destroyed in transplanting.

Cortex: Primary tissues of a root bordered on the outside by the epidermis and on the inside by
the endodermis. When roots begin thickening (secondary growth), the cortex and epidermis are
gradually shed and replaced by the periderm.

Endodermis: A single layer of cells in a root that separates the cortex tissues from the pericycle.
The endodermis includes the Casparian Strip, an impermeable layer that allows plants to control
which substances can move from the cortex into the vascular system for transport to the rest of the
plant.

Pericycle: A layer of parenchyma cells immediately inside the endodermis. Branch roots arise
from the pericycle.

Vascular system
Phloem tissue conducts products of photosynthesis from leaves throughout the plant including
down to the roots.

Xylem tissue conducts water and minerals from the roots up through the plant.

Root Meristems

Root Tip Meristem: Region of cell division that supports root elongation, found at the root tips
just behind the root cap.

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Root Cap: A thimble-shaped group of thick-walled cells at the root tip serves as a “hard hat” to
push through soil. The root cap protects the tender meristem tissues

Vascular Cambium – The site of secondary root growth (root thickening). Vascular cambium
develops in association with primary xylem and phloem and annually generates new vascular
tissue in a ring shape, increasing the root girth and gradually crushing and sloughing off the
pericycle, endodermis, cortex, and epidermis, replacing it with periderm.

Fibrous: Profusely branched roots that occupy a large volume of shallow soil around a plant's base
(petunias, beans, peas)

Taproot: Main, downward-growing root with limited branching, where soils permit (carrots, beets,
radishes).

Adventitious Roots: Generated in the ground system and arise at an “unexpected” place. For
example, the buttress roots on corn and the short whitish bumps along a tomato stem are
adventitious roots.

Aerial Roots: Arise from above-ground stem tissues. Aerial roots are common on ficus,
philodendrons, pothos, and Christmas cactus.

Lateral Roots: The building blocks of the root system; branching roots that grow horizontally
from the pericycle of the primary root.

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Sinker Roots: Make a sharp dive into deeper soils, wherever oxygen is available. Sinker roots are
common on some tree species.

Storage or Tuberous Root – Enlarged roots that serve as storage organs (Canada thistle, morning
glory, sweet potato, dahlia).

D. FRUITS
In botanical terms, a fruit is the mature ovary of a flowering plant, containing seeds. In culinary
terms, it's often defined as the sweet or sour, edible part of a plant containing seeds, like apples,
oranges, and bananas. A fruit develops from the ovary of a flower after fertilization and contains
seeds. Fruits play vital role in seed dispersal and protection. They consist of the pericarp (outer
layer) and the seed(s). The main function of a fruit is to facilitate seed dispersal and protect the
seeds. Structurally, a fruit includes the pericarp (outer layers, including exocarp, mesocarp, and
endocarp) and the seeds. The ovary wall typically develops into an expanded structure termed the
pericarp. Three zones are often seen in the pericarp, an outer exocarp, a central mesocarp, and an
inner endocarp. However, in many plants, the endocarp, the mesocarp, or both may be missing or
fused with each other. In some fruit types like berries and in many dry fruits, these zones are
difficult to distinguish in the mature fruit and can only be identified by carefully studying fruit
development. Fruits occur only in angiosperms, not in gymnosperms; “angio” means vessel and
“sperm” means seed. Therefore, angiosperms produce seeds inside a vessel (fruit). Gymnosperms
(literally “naked seed”) do not produce a fruit

Figure 7. Parts of a Fruit

Fruit taxonomy is based on morphological features such as the consistency of the pericarp (dry
and hard or soft and fleshy). Fruits may also be classified on the basis of whether they dehisce (i.e.,
release their seeds) or not when ripe and whether they contain a single carpel or multiple carpels.

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 • Epicarp: Outermost layer, forms the peel.

• Mesocarp: Middle layer, fleshy, edible portion of the fruits

• Endocarp: Innermost layer, the inner rough portion where the seed is accommodated

Fruit Types

Dehiscent Fruit: Fruit splits open at maturity, releasing (usually multiple) seeds (beans, flax,
penstemon).

Indehiscent Fruit: Fruit formed from an ovary in which usually only one seed develops, and
within which the seed remains during distribution (sunflowers, grasses).

Fleshy Fruit: Fruit developed from unicarpellate (one-seeded) or multicarpellate (many-seeded)

ovaries. The ovary wall develops rapidly proliferating cells that take on diverse roles in the
resulting fruit.

Pome: A fleshy enlarged fruit derived from non-ovarian “accessory tissue” (like the receptacle)
surrounding a leathery or papery core (the true “fruit”). Typical of the rose family, including apples,
pears, and rosehips.

Berry: A pulpy fruit with many seeds scattered throughout (tomato, blueberry).

Drupe: A fleshy fruit with a single seed originating from a single carpel, the pit (peaches, plums).

Note: A flower is a reproductive unit and the fruits are the outcome of reproduction.

Based on the number of ovaries and the number of flowers involved in the fruit formation, fruits
are classified into three major groups namely:

Simple Fruits

These fruits develop from a single matured ovary in a single flower. Apple, banana, cherry pear,
plum, tomato are few examples of simple fruits. The simple fruits are classified into the following
categories:

• Drupes: These are also known as stone fruits since it contains a very hard seed inside the
simple fruits. For eg., plum, cherry, peach.

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 • Berries: These types of fruits have a single seed in the center and are very juicy. For eg.,
grapes, blueberries, tomatoes

• Pomes: Such fruits bloom from trees. For eg., apple, papaya

• Hesperidium: Citrus fruit, with a rind and easily divisible segments.

• Pepo: A type of berry usually with a hard outer rind, specifically characteristic of the family
Cucurbitaceae (zucchini, pumpkin, cucumber).

Aggregate Fruits

These fruits develop from a number of matured ovaries formed in a single flower. Individual
ovaries are called “fruitlets.” Blackberry, raspberry, strawberry are few examples of aggregate
fruits.

• Drupelet: A single-seeded fruit making up part of a larger composite fruit, as in

blackberries and raspberries.

Composite Fruits
These fruits develop from a complete inflorescence. these are also known as multiple fruits.
Composite fruits are of two types:

• Sorosis: These are found in mulberry, jackfruits and pineapple. They develop from catkin,
spikes and spadix type of inflorescence.

• Syconus: This develops from hypanthodium type of inflorescence.

E. SEEDS

The seed in a plant is the part that develops from the ovules after fertilization. They are enclosed
in the fruit which develops from the fertilized ovary. Seeds, the reproductive structures of
flowering plants, consist of a protective seed coat, a nutritive endosperm, and an embryo. The seed
coat protects the embryo and aids in dispersal, while the endosperm provides stored nutrients for
the developing embryo. The embryo itself contains the radicle (embryonic root) and plumule
(embryonic shoot), which will develop into the new plant. Seeds are the means of plant
dissemination (dispersal) through space. Seeds help the tender embryo survive under cold, dry, and
other unfavorable conditions and supply the embryo with nutritive materials for germination and

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prior to autotrophic growth. Plants that possess seeds are called spermatophytes (also known as
phanerogams).

Structure and Function

• Seed Coat
The outer protective layer, often referred to as the testa, protects the delicate embryo from damage
and desiccation. It also plays a role in seed dormancy and germination.

• Endosperm

The nutritive tissue, primarily composed of starch and other nutrients, provides nourishment to the
developing embryo during germination and early growth. In some seeds, the cotyledons
(embryonic leaves) also store food reserves.

• Embryo
The young plant, containing the following key structures:
• Radicle: The embryonic root that develops into the primary root of the new plant.

• Plumule: The embryonic shoot, which will develop into the stem and leaves of the
new plant.

• Cotyledons: Embryonic leaves, which may or may not be involved in food storage
depending on the seed type. In some cases, they emerge from the seed during
germination and become the first leaves.

Figure 7. Structure of Seed

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Functions of Seeds
• Reproduction: Seeds allow for the continuation of plant species through sexual
reproduction.

• Dispersal: Seeds are dispersed from the parent plant, enabling the plant to colonize new
areas.

• Dormancy: Seeds can remain dormant under unfavorable conditions, waiting for suitable
environmental conditions for germination.

• Protection: The seed coat protects the embryo from physical damage and environmental
stress.

• Nourishment: The endosperm and/or cotyledons provide essential nutrients for the
embryo's growth and development.

F. FLOWERS
A flower is the reproductive structure of angiosperms (flowering plants). It typically consists of
four main parts: sepals, petals, stamens, and pistils. It is a characteristic system of reproductive
organs in which two basic processes of sexual reproduction, meiosis and the fusion of male and
female gametes, occur resulting in the production of a new generation, the embryo. Floral parts
are typically borne in whorls (circular patterns) on the axis of the flower stalk known as the
receptacle—a modified stem. From the outside, moving to the center, the three main whorls of
floral organs are the perianth (“surrounding flower,” sum of sepals and petals), the androecium
(“male household,” sum of all the stamens), and the gynoecium (“female household,” sum of all
the carpels). Flowers may or may not have all three whorls. Perfect flowers have both an
androecium and a gynoecium. Imperfect flowers have one or the other and are termed pistillate if
they contain female reproductive structures or staminate if only male reproductive structures.
Monecious (meaning “one household”) plants have both male and female reproductive structures
on the same plant.

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 Figure 8. A flower
Sepal
Sepals are the exterior parts of a flower that protect the interior flower while it emerges. Sepals are
typically green and leaf-like, as they are in fact modified leaves, but it is possible for them to be
almost any color depending on the type of plant. The sepal is the first part of the flower to grow,
forming at the uppermost end of a stem. The sepal creates a bud around the emerging flower, and
its key responsibilities are to protect the flower as it grows and prevent it from drying out. Not all
flowers have sepals, and in some cases, the sepals are modified into bracts that surround the flower.
They are often brightly colored, and in many cases, the bract draws more attention than the flower
itself.

Petals
Petals exist to draw pollinators to the flower. It is for this reason that they are often brightly colored,
showy, and of interesting patterns and sizes. The petals together form what is known as the corolla
of the plant. Petals are probably the part of the flower that has most variation from plant to plant.

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Not only do they differ in color, size, and shape, but some petals form in several layers to create
very full-looking flowers, while others appear to not have separate petals, but instead are one solid
petal.

Stamens
The stamen is the male reproductive organ of a flower. Each stamen contains two main parts. The
filament is the long cylindrical tendril part of the stamen, while the anther is a sac that sits at the
top of the filament. The function of the filament is simply to hold up the anther, extending it up to
an accessible part of the flower for pollinators reach, or for the wind to disperse the pollen.

The anther is where the pollen is produced, and each anther contains many grains of pollen that
each have the male reproductive cells present in them. Each flower can have just a few stamens,
or hundreds of them. The function of the stamen is to produce pollen and make it available for
pollinators to allow reproduction. When a pollinator, such as a bee or a bird, touches the anther the
pollen will stick to them, and then get transported to other flowers they visit.

Carpel
The carpel, which is also sometimes called the pistil, is the female reproductive organ of a flower.
Each carpel is usually bowling pin-shaped, and features a sac at its base in the center of a flower,
and this sac is the ovary that produces and contains developing seeds, or ovules. Moving upward,
the ovary extends to support a style, that is a tube-like structure leading up to the stigma at the very
top.

The stigma features a flat surface with a sticky texture, that is ideal for capturing pollen that has
been transported to the stigma of the flower by wind or pollinating insects and birds. Upon arriving
on the stigma, pollen will germinate to produce a pollen tube down the style. When it reaches the
ovary sac, the pollen tube fertilizes the ovules. At this point, pollination is complete. A fertilized
ovary swells to protect the developing seeds and transforms the flower into a fruit. Inside the fruit,
a fertilized ovule becomes a seed, from that the plant can be sown and an entirely new plant created.

Other Parts of a Flower

Corolla
The corolla presents differently in different types of flowers, but it always makes up the inner
perianth that immediately surrounds the reproductive part of the plant. Typically, the corolla is
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made up of a circle of distinct petals, but it can also be formed from one solid petal in the case of
petunia. It may also be lobed or layered in the case of some roses, which can be referred to as
double or even triple blooms with many layers of petals.

The corolla attracts pollinators to the plant with its vivid coloring and interesting scents, except for
in the case of flowers that are pollinated by the wind. Wind-pollinated flowers have not needed to
evolve to attract birds or insects for the survival of the species and, therefore, their corollas are
often plain or dull.

Filament

The filament is the thin tubular part of the stamen that extends and supports the pollen sac at the
top.

Ovary
The ovary produces and contains unfertilized seeds. It sits centrally inside the flower at the base
of the carpel. Once fertilized, it is the ovary that develops into the fruit of the plant.

Ovule

Ovules are contained within the ovary, and in the event of successful pollination, they will become
the seed of the fruit.

Anther
The anther sits at the top of the filament of a stamen and produces and contains the pollen.
Bract
A bract works similarly to a sepal on plants that do not have sepals. It is a modified leaf that looks
more like a petal than a leaf, as it is usually brightly colored and shaped differently to other leaves
on the plant.

Style
The style is the elongated part of a carpel that joins the ovary to the stigma. It is the tube through
which pollen is delivered to the ovary.

Stigma

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The stigma sits at the top of the carpel, and its job is to capture pollen. It is often sticky in texture
or contains tiny hair-like structures to help pollen adhere to its surface.

Receptacle
A receptacle sits at the top of a stalk underneath the main portion of the flower. It is often enlarged
to support the weight of the flower, or the fruit when it develops. Its main function is to both
connect the stalk to the flower and to support the flower.

Peduncle
A peduncle is the stalk of a flower, or the stem from which a cluster of flowers bloom.
Pedicel
A pedicel is the secondary stalk from which flowers grow off the main stem. Only plants that have
inflorescence in the form of clusters or similar will have pedicels.

Perianth
A perianth is the scientific term for the parts of the flower that surround the reproductive organs.
The perianth can be divided into two segments, the inner perianth and the outer perianth. The inner
perianth is usually composed of the corolla, which is made up of a series of petals. The outer
perianth is the calyx, which is typically made up of sepals. The purpose of the perianth as a whole
is to protect the flower as it develops, protect the fully grown reproductive organs, and to lure
pollinators to the flowers for the purpose of pollination and reproduction, ensuring the continuation
of the species.

Calyx
The calyx is the technical term for a group of sepals, leaf-like structures that surround and protect
the bud as it forms into a flower.

NUTRITION IN PLANTS
Just like other organisms, plants also require food that can supply energy for their various
metabolic activities. Though animals can move from one place to another in search of food, plants
just stand still in one place and make their own food. As we already know that all plants are
autotrophs and synthesize their own food through the process of photosynthesis. Where ‘Photo’
means ‘light’ and ‘synthesis’ means ‘to build’, thus ‘photosynthesis’ means ‘building up by light’.

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Nutrition in plants is the process by which plants acquire the necessary substances for their growth
and sustenance. Through photosynthesis, plants convert sunlight, water, and carbon dioxide into
glucose and oxygen, using chlorophyll to capture sunlight energy. Additionally, roots absorb water
and minerals from the soil, while leaves release excess water through transpiration. Plants respire
to generate energy from glucose and absorb various nutrients from the soil for their metabolic
needs. Some plants even form partnerships with nitrogen-fixing bacteria to access the nitrogen
cycle.

Mode of Nutrition in Plants

Mode of nutrition refers to the way organisms obtain the nutrients they need for growth, energy,
and maintenance. Mode of nutrition in plants can be broadly categorized into two modes:
Autotrophic Nutrition and Heterotrophic Nutrition. Autotrophic organisms, like plants, produce
their own food through photosynthesis. Heterotrophic organisms, like animals and many species
of plants, rely on consuming other organisms or organic matter for nutrients.

Modes of Nutrition
Mode Description
Autotrophic Nutrition Plants make their own food using sunlight,
water, and carbon dioxide through
photosynthesis. They do not depend on other
organisms for food and nutrients.
Heterotrophic Nutrition Heterotrophic nutrition in plants involves
obtaining nutrients by consuming organic
matter from other sources. Heterotrophic
nutrition is mainly of four different types-
Insectivorous, Parasitic, Symbiotic and
Saprophytic.

Autotrophic Nutrition in Plants

Autotrophic nutrition is a mode of nutrition in which organisms produce their own food using
simple inorganic compounds, like glucose, from carbon dioxide and water, usually through the
process of photosynthesis. Plants are a common example of autotrophic organisms. They use
sunlight to convert carbon dioxide and water into glucose and oxygen, with the help of chlorophyll
in their cells. This process allows them to create their own energy-rich molecules for growth,
development, and other metabolic activities.

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Functions of Autotrophic Nutrition in Plants
Autotrophic nutrition in plants allows them to produce their own energy-rich molecules (glucose)
using sunlight, carbon dioxide, and water. This process supports their growth, development, and
the sustenance of the entire ecosystem.

Photosynthesis: The primary function of autotrophic nutrition in plants is photosynthesis. Plants

use light energy from the sun to convert carbon dioxide and water into glucose (a type of sugar)
and oxygen. This process takes place in the chlorophyll-containing structures called Thylakoids.

Energy Production: The glucose produced during photosynthesis serves as a source of energy for
plants. Through cellular respiration, plants break down glucose and other organic molecules to
release energy that fuels various metabolic processes within the plant.

Storage: Excess glucose produced during photosynthesis is often stored as starch in various parts
of the plant, such as roots, stems, and seeds. Starch serves as a reserve energy source that can be
used actively occurring, such as during nighttime.

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Growth and Development: The glucose produced by photosynthesis is used to synthesize
complex organic molecules like cellulose, proteins, and lipids. These molecules are essential for
the growth, repair, and development of plant cells, tissues, and organs.

Ecosystem Support: Plants with autotrophic nutrition form the foundation of the terrestrial
ecosystem. They provide food and habitat for other organisms, contribute to nutrient cycling, and
play a crucial role in maintaining ecological balance.

Photosynthesis

Photosynthesis is the process by which plants, some bacteria and some protistans use the energy
from sunlight to produce glucose from carbon dioxide and water. This glucose can be converted
into pyruvate which releases adenosine triphosphate (ATP) by cellular respiration. Oxygen is also
formed.

Sunlight
6CO2 + 6 H2O C6H12O6 + O2
Chlorophyll

Photosynthesis occurs in the chloroplast of plant cells. Chloroplasts contain chlorophyll which is
the green pigment which transfers light energy into chemical energy to make carbohydrates.
Chlorophyll is a complex molecule. Several modifications of chlorophyll occur among plants and
other photosynthetic organisms. All photosynthetic organisms have chlorophyll a. Accessory
pigments absorb energy that chlorophyll a does not absorb. Accessory pigments include
chlorophyll b (also c, d, and e in algae and protistans), xanthophylls, and carotenoids (such as beta
carotene). Chlorophylls absorb mainly red and blue violet light, reflecting green light and
therefore giving plants their characteristics green colour.

Process of Photosynthesis

a. Light Absorption
Plants contain chlorophyll pigments in their chloroplast, which absorb light energy from the sun.
Other pigments, like carotenoids, also contribute to light absorption.

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 b. Light Reaction (Light-Dependent Phase)
The light reaction takes place in the thylakoid membrane of the chloroplasts. Light energy splits
water molecules into oxygen, electrons, and protons. Oxygen is released as a byproduct, while
electrons are transferred along the electron transport chain

c. Electron Transport Chain

High-energy electrons move through protein complexes in the thylakoid membrane. Energy from
electrons is used to pump protons (H+ions) into the thylakoid space, creating a proton gradient.

d. ATP and NADPH Formation

Protons flow back into the chloroplast stroma through ATP synthase, producing ATP. Electrons
reduce NAPH+ to form NADPH, a high-energy electron carrier.

e. Carbon Fixation (Calvin Cycle or Dark Reaction)

Carbon fixation takes place in the stroma of the chloroplast. Carbon dioxide is captured and
combined with a five-carbon compound (RuBP) in a reaction catalyzed by the enzyme rubisco.
The resulting six-carbon compound immediately splits into two three-carbon molecules.

f. Reduction and Formation of G3P

ATP and NADPH produced in the light reaction are used to convert the three-carbon molecule into
Glyceraldehyde-3-Phosphate (G3P). Some G3P molecules are used to regenerate Rubp, while
others proceed to produce glucose and other carbohydrates.

g. Regeneration of RuBP
G3P molecules are rearranged and combined to regenerate the initial five-carbon molecule,
RuBP. ATP from the light reaction provides energy for this regeneration process.

h. Glucose Formation and Oxygen Release

Some G3P molecules are used to produce glucose and other sugars. Glucose is transported to other
parts of the plant for energy storage and growth. Overall, the process releases oxygen into the
atmosphere.

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Heterotrophic Nutrition in Plants
Heterotrophic nutrition in plants refers to the process by which certain plants obtain nutrients from
organic matter produced by other organisms. Unlike autotrophic plants that can produce their own
food through photosynthesis, heterotrophic plants rely on external sources for nutrients. They
usually obtain these nutrients by decomposing dead organic material or forming symbiotic
relationships with fungi that help them absorb nutrients from their surroundings. This adaptation
allows them to survive in environments with limited sunlight or nutrients. Heterotrophic nutrition
is mainly of four different types: Insectivorous, Parasitic, Symbiotic, and Saprophytic.

Types of Heterotrophic Nutrition in Plants

Types Description
Insectivorous Nutrition Insectivorous nutrition is a survival strategy where certain
plants capture and digest insects for nutrients.
Saprophytic Nutrition Saprophytic nutrition involves obtaining nutrients by
decomposing dead organic matter in the environment.
Parasitic Nutrition Parasitic nutrition is when an organism or plant feeds on
another organism or plant, harming the host for its
sustenance.
Symbiotic Nutrition Symbiotic nutrition refers to a mutually beneficial
relationship between different organisms, sharing nutrients
and resources.

1. Insectivorous Nutrition in Plants

Insectivorous nutrition is also known as carnivorous nutrition, as in this type of nutrition
carnivorous plants capture and digest insects or other small organisms to obtain nutrients like
nitrogen, which may be deficient in their habitat. Examples of insectivorous plants include Venus
flytraps, Pitcher plants, and Sundew.

2. Saprophytic Nutrition in Plants

Saprophytic plants obtain nutrients from dead and decaying organic matter. They play a crucial
role in breaking down and recycling nutrients in the ecosystem. The saprophytic plants are
fascinating organisms that play a vital role in ecosystems by breaking down dead and decaying
organic matter. One example of a saprophytic plant is the Indian Pipe (Monotropa iniflora), and
another example is the Ghost Plant (or Pine drops, Pterospora andromeda).

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3. Parasitic Nutrition in Plants
Parasitic plants attach to the host plants and derive nutrients from them. They can either be their
total parasite (obtaining all their nutrients from the host) or partial parasites (these plants still
conduct some amount of photosynthesis). Or we can say that parasitic plants are unique botanical
entities that have evolved to draw nutrients from other living plants. One of the well-known
examples is the Dodder (Cuscutta species), and another example is the Broomrape (Orobanche
species). Loranthus, Viscum (mistletoe), Orobanche etc. Plants like Cuscuta and Loranthus are
called stem parasites as they grow on the stems of other plants. Mistletoe grows on the branches
of trees.

4. Symbiotic Nutrition in Plants

Symbiotic nutrition is also known as Mycoheterotrophic Nutrition. In this type of nutrition, plants
form symbiotic relationships with fungi. These plants lack chlorophyll and cannot perform
photosynthesis, so they rely on fungi to extract nutrients from the soil and deliver them to the
plants. Examples include Pines, Ferns, and Orchids; as the pine tree and other conifers have
mycorrhizal relationships.

Functions of Heterotrophic Nutrition in Plants

Nutrient Absorption: Plants obtain essential nutrients such as minerals and organic compounds
from external sources.
Energy acquisition: Heterotrophic nutrition supplies the energy needed for various metabolic
processes in the absence of photosynthesis.
Respiration: Organic compounds acquired are used for cellular respiration, generating ATP, the
energy currency of the cell.
Adaptation to Varied Environment: Heterotrophic Nutrition allows plants to thrive in diverse
habitats, including those with limited sunlight.
Supplementing Photosynthesis: In some situations, heterotrophic nutrition complements
photosynthesis, aiding the plant’s overall nutrient and energy intake.

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REPRODUCTION IN PLANTS
All the living organisms including plants and animals have the capability to produce new
individuals during their lifespan. This process of producing a new organism from the existing
organism (or the parent) of the same species is called reproduction. The new individuals produced,
are the copies of their parents. The process of reproduction is one of the important life processes
and is essential for the continuity of the species. Thus, reproduction makes the life continuous
which is not only essential for the survival of an organism but it is also very necessary for the
perpetuation and preservation of the species because it increases the number of members of a
species. The various parts of a plant such as roots, stem and leaves each with a specific function is
called vegetative parts. After a certain period of growth, plants bear flowers. These flowers develop
into fruits and seeds. The parts of a plant that participate in the process of sexual reproduction are
called reproductive parts or organs. In plants, the reproductive parts are a flower which may have
the male or female part or both the parts on the same flower. Different organisms reproduce in a
different way. In plants, there are two different methods of reproduction: Asexual reproduction and
Sexual reproduction. The term ‘sexual’ means involving the fusion of sex cells or gametes while
‘asexual’ means without involving the fusion of gametes.

Asexual Reproduction
The process in which only one parent is involved in the production of new individuals of the same
kind is called asexual reproduction. In plants, asexual reproduction results in the formation of
offsprings or new plants without seeds spores. Asexual reproduction in plants, also known as
vegetative reproduction, involves producing new plants from non-reproductive parts like stems,
roots, or leaves, without the need for seeds, flowers, or fertilization. This process results in
genetically identical offspring, or "clones" of the parent plant, as there's no mixing of genetic
material from different sources. Asexual reproduction in plants occurs through the following
methods: Budding, Fission: Binary and Multiple, Fragmentation and Spore Formation.

Budding
Budding is an asexual mode of producing new organisms. In this process, a new organism is
developed from a small part of the parent’s body. A bud which is formed detaches to develop into
a new organism. The newly developed organism remains attached as it grows further. It is separated
from the parent organism when it gets matured by leaving scar tissues behind. As this is asexual

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reproduction, the newly developed organism is a replica of the parent and is genetically identical.
For reproduction, Hydra uses regenerative cells where a bud expands as an outgrowth because of
repeated cell division at one specific location. These buds then developed into new small
individuals which when completely matured, detach from the parent body.

Budding in Hydra
Binary Fission

Binary fission is a form of asexual reproduction in which an organism divides into two, each part
carrying one copy of genetic material.”

Multiple Fission
Multiple fission is the process of asexual reproduction in which instead of 2 daughter cells, many
daughter cells are produced from the parent cell. In this, the nucleus undergoes repeated division
to produce a large number of nuclei. Each nucleus along with little bit of cytoplasm forms a
membrane around it. All the daughter cells are equal sized and are similar.

Spore formation

In non-flowering plants (the plants which do not produce seeds- ferns, mosses), fungi (dung
mould), formation of spores is a common method of reproduction. Spores are very small in size.
They have thick walls. The thick walls help the spores to survive in adverse conditions like high
temperature, low humidity scarcity of water and lack of food. When favourable conditions arrive
the spores burst and germinate to develop into new plants. pores are very light asexually

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reproducing bodies which can be carried over a long distance by air or wind. The spores give rise
to new organisms under favourable conditions.

Fragmentation
Some algae (Spirogyra) that are present in waterbodies reproduce by fragmentation. In this-
method, the body of the parent breaks into small pieces called fragments and each fragment grows
up to become a new plant. Fragmentation of parent body occurs when they are matured.

Vegetative Propagation
This is an asexual method of reproduction in plants where vegetative parts namely the root,
stem, leaf or buds give rise to new plants. No reproductive organs take part in this method
of reproduction and therefore, no seeds are produced.

Natural Vegetative Propagation

a) By Roots
In this process, new plants grow out of the modified roots called tubers. In fact, in some plant
species, roots develop adventitious buds. These buds grow and form new plants/sprouts under the
right conditions. For example, Sweet potato
b) By Stems
Stems are the most common part of vegetative reproduction. Many plants propagate and form new
plants through their stems. Following are the different types of vegetative reproduction by stems:

i) Tuber – A swollen stem that stores food is known as tuber. A tuber has some
depressions called eyes. Each eye has one or more buds from which new plants arise.
Potato reproduces by forming tubers. If a potato is planted in soil, the roots arise from
its ‘eyes’ and new plants grow.
ii) Rhizome – Rhizome, also called creeping rootstalk, horizontal underground plant stem
capable of producing the shoot and root systems of a new plant. Rhizomes are used to
store starches and proteins and enable plants to perennate (survive an annual
unfavourable season) underground. In addition, those modified stems allow the parent
plant to propagate vegetatively (asexually), and some plants, such as poplars and
various bamboos, rely heavily on rhizomes for that purpose. In plants such as water
lilies and many ferns, the rhizome is the only stem of the plant. Notably, the rhizomes

34
 of some species including ginger, turmeric and lotus are edible and valued for their
culinary applications.
iii) Bulbs – Bulbs are also underground stems encased in thick fleshy bulb scales. These
scales are modified leaves and store food. The new plant arising from the bulb takes
food from the fleshy scales. Bulbs are found in onion plants, shallot, garlic.
iv) Runner – Some plants grow horizontal stems parallel to the ground. They are called
runners. These stems contain nodes from which new plants grow. Oxalis and grass
propagate by this method.
v) Sucker – In mint and Chrysanthemum, underground stems are divided into nodes and
internodes. New shoots develop from the nodes. When the internodal regions decay,
each shoot separates and forms a new plant. These plants are called suckers.
vi) Corm – Corm is an underground stem with scale leaves and buds. In favourable
conditions, the buds give rise to new plants. Colocasia and Gladiolus reproduce by this
method.

c) By Leaves
Begonia and Bryophyllum are examples of vegetative propagation by leaves. This is a form of
asexual reproduction in which new plants grow from the buds growing on the margin of the leaves.
These buds are reproductive in nature and when they fall on the ground they germinate and form
a new plant.

2. Artificial Vegetative Propagation

This is a type of vegetative reproduction carried out by humans in the fields and laboratories. The
most common types of vegetative reproduction occurring artificially include:

Cutting

In this, a part of a plant, specifically a stem or leaf is cut and planted in the soil. These cuttings are
sometimes treated with hormones to induce root development. The new plant is formed from the
adventitious roots developing from the cutting.

Grafting

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In this, the cutting from some other plant is attached to the stem of a plant rooted in the ground.
The tissues of the graft become integrated with the tissues of the rooted plant and develop as a
single plant over time.

Layering
In this, the stem of the plant is bent to the ground and covered with soil. Adventitious roots emerge
from the plant parts covered with the soil. This attached stem with developing roots is known as a
layer.

Tissue Culture
In this, the plant cells from different parts of a plant are cultured in the laboratory to develop a new
plant. This technique is helpful in increasing the number of rare and endangered plant species that
are unable to grow under natural conditions.

Sexual Reproduction
Sexual reproduction in flowering plants, or angiosperms, involves the fusion of male and female
gametes, resulting in the formation of seeds and fruits. This process begins with pollination, where
pollen is transferred from the male anther to the female stigma. Following pollination, the pollen
germinates, forming a pollen tube that delivers the male gametes to the ovule for fertilization. After
fertilization, the ovules develop into seeds, and the ovary develops into a fruit, which aids in seed
dispersal. Two types of reproductive cells called gametes are produced from the reproductive
organs of two parents. Male parent produces the male gamete and the female parent produces the
female gamete. The fusion of the two gametes is called fertilization. The product of fusion of the
two gametes is called zygote. The male gamete in a flowering plant is formed by the pollen grain
whereas in animals, it is the sperm. The female gamete in plants is a large egg-cell in the ovule,
while in animals, it is the ovum. After fertilization, the zygote undergoes cell division and growth.
Ultimately, it forms the new individual.

Features of Sexual Reproduction

• Two parents are involved (both male and a female).

• Gamete formation and fertilization take place.

• The whole process is slow and lengthy.

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• Variation occurs; offspring are different from parents, genetically and physically

Sexual reproduction in plants takes place in flowers. The complete flower typically consists
of four parts:

• Petals

• Sepals

• Stamen (male reproductive part)

• Pistil/Carpel (female reproductive part)

Stamen (male reproductive part) consists of anther and filament.

• The anther is a sac-like structure that produces and stores pollen.

• The filament supports the anther.

The pistil (female reproductive part) comprises three parts- stigma, style, and ovary.

• Stigma is the topmost part of a flower.

• The style is the long tube which connects the stigma to the ovary.

• The ovary contains a lot of ovules. It is the part of the plant where the seed formation takes place.

A flower may consist of either stamen or pistil or both. Based on this, a flower can be either
unisexual or bisexual. A bisexual flower is composed of all the four parts mentioned above, e.g.
Rose, China rose. Whereas, plants like papaya and cucumber produce only unisexual flowers.

Pollination
In order to form a zygote, male gametes in pollen grains have to fuse with the egg in the ovule.
This is achieved by a process called pollination. Pollination is the process of transferring pollen
grains from the anther – male part of a flower, to the stigma – female part of a flower. Depending
on the pollen landing, pollination can be classified into two types-

• Self-Pollination: A pollination where the pollen transfer takes place between the anther and
stigma of the same flower.

• Cross-Pollination: A pollination where the pollen transfer takes place between the anther and
the stigma of different flowers of the same plant or different plants of the same species.

37
Cross pollination often involves various external agencies to carry pollen grains from one flower
to another one. These agencies may be air, water, insects or animals. Most flowers are pollinated
by insects. When an insect visits a flower, the pollen grains get deposited on the body of the insect.
When this insect visits another flower, the deposited pollen grains now get dusted on the stigma of
the second flower, thus bringing about the transfer of pollen grains from the anther to the stigma
(pollination). Pollen grains of all flowers are not carried by insects. In some cases, they are carried
by wind (wind pollination). In case of water plants, pollen grains are carried by water (water
pollination).

Fertilization in flowering plants

Fertilization is a step between pollination and seed formation. The fusion of the male gamete with
the female gamete is called fertilization.

During fertilization, the following events take place

1. The pollen grains germinate on the stigma and pollen tubes develop. The pollen tubes move
downwards into the style. These tubes are the carriers of male gametes.
2. One pollen tube finally enters the ovule where female gamete is located. Female gamete or
egg cell is present inside the ovule.
3. Finally the male gamete fuses with the female gamete. This completes the process of
fertilization. The fusion product or the cell formed as a result of fusion of the two gametes
is called zygote. The zygote soon develops into an embryo (baby plant).

After fertilization, the ovule develops into a seed, which contains the embryo and endosperm. The
ovary develops into a fruit, which helps protect the seeds and aids in their dispersal.

THE ROLE OF PLANTS IN SUSTAINING THE ECOSYSTEM

Plants are essential for sustaining ecosystems, serving as primary producers, oxygen providers,
and habitat creators. They also play a crucial role in regulating climate, water cycles, and soil
stability. Through photosynthesis, plants convert carbon dioxide into oxygen, supporting all life
on Earth. They form the base of food chains, providing sustenance for numerous organisms,
including humans. Plants are fundamental to life on Earth, playing a crucial role in maintaining
ecological balance and supporting diverse ecosystems. Their contributions go beyond providing
oxygen and food; they offer a range of ecosystem services that benefit both the environment and

38
human societies. These services can be broadly categorized into four types: provisioning,
regulating, supporting and cultural services.

A. PROVISIONING SERVICES
Provisioning services refer to the tangible benefits that ecosystems provide to humans, including
food, fuel, fiber, medicine, and raw materials. Various plant species have been utilized for centuries
for their economic and ecological contributions.

Food Production: Plants constitute the primary base of the food chain, supplying essential
nutrients to humans and animals. Crops such as rice, wheat, maize, and various fruits and
vegetables provide necessary sustenance. Staple crops like wheat, rice, and maize serve as primary
food sources globally, offering carbohydrates, proteins, and other vital nutrients. Fruits and
vegetables, including apples, bananas, citrus fruits, carrots, spinach, and potatoes, are crucial
sources of vitamins and minerals. Additionally, plants serve as primary fodder sources for
livestock, ensuring sustainability in the meat, dairy, and poultry industries.

Medicinal Resources: Plants have long been valued for their medicinal properties, forming the
foundation of traditional and modern healthcare systems. Traditional medicine systems such as
Ayurveda, Traditional Chinese Medicine (TCM), and herbal remedies rely heavily on plant-based
ingredients. Many pharmaceutical drugs originate from plant compounds. For instance, aspirin is
derived from willow bark, and quinine is used for treating malaria. Herbal remedies continue to be
widely used for treating various ailments. Aloe vera is well known for treating burns, wounds, and
skin conditions due to its healing properties.

Timber and Fiber: Forests provide a significant source of timber and fiber, supporting
construction, furniture, textile, and paper industries. Timber from trees such as oak, teak, and pine
is used for constructing buildings, furniture, and flooring. Pulpwood, derived from species like
eucalyptus and pine, is essential for the paper industry, producing books, newspapers, and
packaging materials. Fibers from plants such as cotton, jute, hemp are widely used in textile
production. Cotton remains the most popular natural fiber globally, used in the production of
clothing, bed linens, and industrial fabrics.

Fuel and Bioenergy: Plants have been a primary energy source for centuries, whether in the form
of firewood, charcoal, or biofuels. Traditional biomass fuels such as firewood and charcoal are still

39
widely used in many developing regions for cooking and heating. Biofuels derived from crops like
sugarcane, maize, and oil palm contribute to renewable energy solutions. Brazil is a leading
producer of ethanol, primarily derived from sugarcane, used as an alternative fuel for automobiles.
Algae-based biofuels are an emerging technology with the potential to provide sustainable energy
solutions. The United States has been researching and investing in algae-based biofuels as an
alternative to fossil fuels.

B. REGULATING SERVICES
Plants play a crucial role in maintaining ecosystem stability through regulating services that
influence climate, water cycles, air quality, and pest populations. These services not only benefit
biodiversity but also enhance human well-being by ensuring a sustainable environment.

Carbon Sequestration and Climate Regulation: One of the most significant regulating services
provided by plants is carbon sequestration, which directly impacts climate regulation. Through
photosynthesis, plants absorb atmospheric carbon dioxide (CO2) and convert it into organic matter,
storing carbon in their biomass and soil. This process helps mitigate the effects of climate change
by reducing greenhouse gas levels. Forests, especially tropical rainforests and boreal forests, act
as major carbon sinks, storing vast amounts of carbon in their trunks, leaves, and soil. For instance,
the Amazon rainforest alone holds approximately 150-200 billion metric tons of carbon, playing a
vital role in stabilizing global temperatures. Moreover, afforestation (planting trees in barren lands)
and reforestation (restoring forests in degraded areas) are recognized as effective strategies to
combat climate change by enhancing carbon sequestration. A notable example of carbon
sequestration is the afforestation program in China, known as the "Great Green Wall," which aims
to combat desertification by planting billions of trees across arid regions. This initiative has
contributed to increased carbon absorption and improved local climatic conditions.

Air Purification: Air pollution is a major environmental challenge, and plants play a crucial role
in filtering and improving air quality. They remove pollutants such as sulfur dioxide (SO2),
nitrogen oxides (NOx), carbon monoxide (CO), and particulate matter (PM) from the air. Trees
and shrubs, particularly those with dense foliage, trap airborne pollutants on their leaves and bark,
reducing their concentration in the atmosphere. Urban green spaces, such as parks, gardens, and
green roofs, contribute significantly to air purification. Studies have shown that trees in city
environments can reduce particulate matter by up to 60%, thereby decreasing respiratory illnesses

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among urban populations. The "Million Trees NYC" initiative in New York City focused on
planting one million trees across urban areas, significantly improving air quality by absorbing
pollutants and reducing the urban heat island effect.

Water Cycle Regulation: Vegetation plays an essential role in the hydrological cycle by
regulating water flow, facilitating groundwater recharge, and preventing soil erosion. Plants absorb
and store rainwater, reducing surface runoff and preventing floods. The roots of trees and shrubs
help bind soil, minimizing erosion and protecting water bodies from sedimentation. Wetlands and
mangroves are particularly effective in water regulation. They act as natural water filters, trapping
pollutants and excess nutrients before they enter rivers, lakes, and oceans. Moreover, mangroves
serve as barriers against storm surges and coastal erosion, providing protection to coastal
communities. In Bangladesh, the Sundarbans mangrove forest acts as a natural flood defense,
protecting millions of people from the impacts of cyclones and storm surges. The dense root
systems of mangroves stabilize coastlines, reducing the risk of land loss due to erosion.

Pest and Disease Control: Plants contribute to natural pest control by attracting beneficial insects
and other natural predators that feed on harmful pests. Many plant species release biochemical
compounds that deter pests and diseases, reducing the need for chemical pesticides. This function
is particularly important in agriculture, where intercropping (planting different species together)
and agroforestry (integrating trees into farming systems) enhance pest management and promote
sustainable farming practices. For instance, marigolds are known to produce compounds that repel
nematodes, reducing root infections in vegetable crops. Similarly, neem trees contain azadirachtin,
a natural pesticide that disrupts the growth and reproduction of insect pests.

Regulating services provided by plants are essential for maintaining ecological balance and
ensuring environmental sustainability. From carbon sequestration and climate regulation to air
purification, water cycle management, and natural pest control, plants play an indispensable role
in supporting life on Earth.

C. SUPPORTING SERVICES
Supporting services are fundamental ecological processes that maintain the functioning of
ecosystems and enable other services to be sustained. These services form the backbone of all
ecological interactions, ensuring the stability and health of natural environments.

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Soil Formation and Fertility: Soil formation is a slow yet fundamental process that supports plant
growth and maintains the productivity of ecosystems. Plants contribute significantly to soil
formation by breaking down organic matter and promoting microbial activity. The decomposition
of fallen leaves, twigs, and roots releases essential nutrients into the soil, making them available
for uptake by other plants. Moreover, plant roots play a critical role in stabilizing soil structure,
preventing erosion caused by wind and water. In areas with abundant vegetation, root systems bind
soil particles together, reducing the likelihood of landslides and desertification. For instance,
mangrove forests along coastlines serve as buffers against erosion and storm surges by securing
the soil with their extensive root networks. In addition to preventing soil erosion, plants facilitate
nutrient cycling. Their roots take up nutrients from the soil, and when they shed leaves or die, these
nutrients are returned to the earth, where microorganisms break them down into simpler
compounds. This process enhances soil fertility and ensures the continuous availability of essential
elements such as nitrogen, phosphorus, and potassium. A practical example of plant contributions
to soil fertility can be seen in leguminous plants such as peas and beans, which have nitrogen-
fixing bacteria in their root nodules. These bacteria convert atmospheric nitrogen into forms that
plants can absorb, enriching the soil and reducing the need for chemical fertilizers.

Pollination: Pollination is a crucial ecological process that ensures the reproduction of flowering
plants. Many plant species rely on pollinators such as bees, butterflies, birds, and bats to transfer
pollen from one flower to another, enabling fertilization and seed production. Without pollinators,
many plant species would struggle to reproduce, leading to a decline in biodiversity and disruptions
in food supply chains. The presence of diverse plant species ensures the stability of pollinator
populations. When a variety of flowering plants are available, pollinators can find consistent food
sources throughout the year. This is particularly important for species such as honeybees, which
require a steady supply of nectar and pollen to sustain their colonies. For example, apple orchards
heavily depend on honeybees for pollination. Without bee activity, apple trees would produce
significantly fewer fruits, leading to lower agricultural yields. Similarly, crops such as almonds,
blueberries, and coffee rely on specific pollinators to ensure successful reproduction and fruit
development. Human activities such as deforestation, pesticide use, and habitat destruction have
led to a decline in pollinator populations. Conservation efforts, including the establishment of
pollinator-friendly gardens and the reduction of harmful pesticide use, are essential to maintaining
these supporting services.

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Habitat and Biodiversity Conservation: Forests, grasslands, wetlands, and other natural
ecosystems provide essential habitats for numerous animal species. These habitats support
biodiversity by offering food, shelter, and breeding grounds for various organisms, thereby
maintaining ecological equilibrium. Plants play a pivotal role in habitat conservation by forming
the foundation of ecosystems. Trees provide nesting sites for birds, while fallen logs and leaf litter
create microhabitats for insects and fungi. Wetlands serve as another example of how plants
contribute to biodiversity conservation. Marshes and swamps support a wide array of aquatic
plants that offer refuge for fish, amphibians, and migratory birds. These ecosystems also act as
natural water filters, improving water quality by trapping sediments and absorbing pollutants.

D. CULTURAL SERVICES
Plants offer a vast array of cultural services that enhance human well-being through aesthetic,
recreational, spiritual, and educational benefits. These services, often intangible, significantly
influence human culture, traditions, and everyday life. Native plants, in particular, hold great value
for recreational and spiritual purposes. Cultural services provided by plants fall under three key
categories: aesthetic and recreational value, spiritual and religious significance, and educational
and scientific importance.

Aesthetic and Recreational Value: Green spaces, parks, and gardens significantly contribute to
human wellbeing by enhancing mental health, reducing stress, and encouraging outdoor activities.
The beauty of nature and scenic landscapes creates a serene environment that fosters relaxation
and recreation. Research indicates that exposure to natural environments, such as forests and
botanical gardens, helps reduce stress, anxiety, and depression. The presence of trees, flowers, and
greenery provides a calming effect, which is beneficial for individuals experiencing mental fatigue.
Studies suggest that walking in green spaces can lower cortisol levels, a hormone associated with
stress. Parks and gardens encourage physical activities such as walking, jogging, and cycling,
which promote overall health and fitness. Families and communities utilize green spaces for
gatherings, picnics, and social events, reinforcing social bonds and community engagement.

Spiritual and Religious Significance: Plants hold deep spiritual and religious significance across
various cultures and traditions. Many sacred groves, trees, and flowers are revered for their divine
associations and ritualistic importance. In India, sacred groves—protected forest patches—are
associated with religious beliefs and biodiversity conservation. Flowers and plant materials play a

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vital role in religious ceremonies, festivals, and rituals. For instance, marigolds are widely used in
Hindu festivals, weddings, and temple offerings. In Christianity, palm fronds are used during Palm
Sunday to commemorate Jesus’ entry into Jerusalem.

Educational and Scientific Importance: Plants serve as essential resources for scientific research
and environmental education. They contribute to advancements in genetics, biotechnology, and
climate change studies. Plants have been at the forefront of scientific discoveries. Gregor Mendel’s
experiments with pea plants laid the foundation for modern genetics. Today, plants such as
Arabidopsis thaliana serve as model organisms for genetic research. Furthermore, biotechnology
utilizes plants for developing medicines, biofuels, and genetically modified crops that enhance
food security. Botanical gardens, arboretums, and nature reserves educate people about plant
biodiversity, conservation, and sustainable practices. Schools and universities incorporate plant
studies into their curriculum to raise awareness about ecological preservation and climate change
adaptation strategies. Forest act as carbon sinks, absorbing carbon dioxide and mitigating climate
change effects. Studies on afforestation and reforestation projects help scientists develop strategies
to combat global warming. Additionally, mangrove forests protect coastal areas from erosion and
storms, highlighting their ecological and scientific importance.

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