OVERVIEW OF PLANT STRUCTURE
BASIC TYPES OF PLANT CELLS
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Parenchyma
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Collenchyma
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Sclerenchyma
Parenchyma Cells
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The most common type of living differentiated cell.
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Greek parenchein, meaning “to pour in beside”
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Usually spherical, cubical or elongated.
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Parenchyma are alive at maturity with thin primary walls and typically
no secondary wall.
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Parenchyma cells serve for photosynthesis and storage.
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Parenchyma tissue
(a). Starch storage
parenchyma cells occur in
leaves, stems and roots
(b). Parenchyma can also
serve as water storage
cells in roots and stems
(c). Chlorenchyma
are chloroplast-containing
parenchyma cells that are
primarily found in leaves
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Composed of living parenchyma cells with thin primary cell
walls
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Functions include photosynthesis, storage, and secretion
Collenchyma Cells
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Greek kolla, meaning “glue”
Alive at maturity and lacking secondary walls
Collenchyma cells can attain a variety of shapes and have thicker
primary walls than parenchyma cells.
Parenchyma and collenchyma cells must be filled with water to
become turgid enough for support.
Sclerenchyma Cells
Collenchyma cells provide
flexible support.
Collenchyma cells have
cell walls that are
thickened in some places
with additional cellulose,
enabling them to provide
more support than
parenchyma while still
allowing living stems to be
flexible.
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Provide rigid support
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Greek scleros, meaning “hard”
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Sclerenchyma cells are usually dead at maturity
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Have secondary walls, often hardened with lignin (a rigid molecule
that strengthens & stiffens cell walls in vascular plants)
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They can provide support without being filled with water.
Two Main Types of Sclerenchyma
1. Fibers
-elongated cells with thick secondary walls reinforced with lignin, making
them flexible as well as strong.
-enable stems to move in the wind without snapping
2. Sclereids
-vary more in shape than fibers but are often cubical or spherical.
-make structures such as nutshells and fruit pits typically rock-hard and
inflexible.
-the gritty sandlike pieces in fleshy part of a pear are also sclereids,
commonly called stone cells.
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Collenchyma tissue
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Composed of collenchyma cells with unevenly thickened
primary cell walls
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Provides flexible structural support
Sclerenchyma cells provide
rigid support.
(a) This stem has long,
tapering sclerenchyma cells
called fibers, which provide
sturdy support for a plant.
(b) Thick, hard cell walls of
sclereid cells, also known as
stone cells provide the
structural support in fruit pits
and nutshells, and also give a
rough texture to the flesh of a
pear.
KEY TERMS
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PARENCHYMA CELL
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COLLENCHYMA CELL
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Sclerenchyma
tissue
Composed of
sclerenchyma cells with
both primary and
secondary cell walls
Sclerenchyma cells are
often dead at maturity, but
provide structural support
Living plant cell with moderately but unevenly
thickened primary walls
SCLERENCHYMA CELL
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Relatively unspecialized plant cell; thin walled, may
contain chlorophyll, loosely packed
Plant cell with extremely thick walls; provides strength
and support to plant body
Tissues of Vascular Plants
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A tissue- a group of cells that performs a function.
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May be simple (one cell type) or complex (multiple cell types).
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Simple tissue- parenchyma, collenchyma and sclerenchyma
cells.
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Complex tissue- mixture of parenchyma, sclerenchyma and
water-conducting cells
THE THREE TISSUE SYSTEMS
1. The dermal tissue system
2.
The vascular tissue system
3.
The ground tissue system
The Dermal Tissue Systems
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Forms the plant’s outer protective covering
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Greek derma, meaning “skin”
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A single layer of epidermis is replaced during second year of growth
by periderm, (mainly composed of nonliving cells).
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Dermal cells can be modified into hairlike trichomes.
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The aboveground dermal tissue often forms a waxy protective layer
called a cuticle.
KEY TERMS
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EPIDERMIS
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Outermost tissue layer, usually one cell thick
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Covers the primary plant body (leaves, young stems and
roots)
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Epidermis covering aerial parts secretes a wax layer (cuticle) that
reduces water loss
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Gas is exchanged between interior of shoot system and surrounding
atmosphere through stomata
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PERIDERM
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Outermost layer of cells covering a woody stem or
root (the outer bark that replaces epidermis when it is
destroyed during secondary growth)
KEY TERMS
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The Vascular Tissue Systems
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Conducts water, minerals and food
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Two complex tissues of vascular system:
1. xylem- for transporting water and minerals
2. phloem- for transporting food
XYLEM
A complex vascular tissue that conducts water and
dissolved minerals throughout the plant body
Actual conducting cells of xylem are tracheids and
vessel elements
Pits are gaps
in the
secondary cell
wall.
Tracheids
These are stacked with pits allowing for
movement of water and minerals from
one tracheid to the other.
Vessel elements.
A large, water-conducting cell in the xylem of most flowering plants;
transports water and minerals more rapidly than tracheid cells.
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PHLOEM
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A complex vascular tissue that conducts food
(carbohydrate) throughout the plant body
Conducting cells of phloem are sieve-tube
elements assisted by companion cells
Sieve-tube Members.
The phloem of flowering plants consists of cells called sieve-tube
members (sieve-tube elements).
Sieve-tubes.
They conduct organic nutrients from the leaves to other parts of the
plants.
The Ground Tissue System
An Overview of Vascular Plant Organs
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Also called the fundamental tissue system.
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Simple and complex organ tissues form structures known as organs.
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Ground tissue fill up space not occupied by vascular or dermal
tissue.
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An organ consists of several types of tissues adapted as a group to
perform particular functions.
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Carries out photosynthesis and stores nutrients.
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Vascular plants have three types of organs:
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Cortex- ground tissue forms between the dermal and vascular
tissues.
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(1). Stems (2). Leaves (3). Roots
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Pith- it also appears to the inside of the vascular tissues, where it is
called pith.
3 Tissue Systems in Plant Body
STEMS
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Position leaves for maximum photosynthesis
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A stem is any part of a plant that supports leaves or reproductive
structures.
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Stems can vary greatly in size, such as slender stalk supporting a
small flower or a huge tree trunk dozens of feet in diameter and
hundreds of feet high.
2. Transpiration
-the loss of water through the pores in leaves, which pulls water and
mineral nutrients up from the roots to the leaves.
Functions of Stems
1.
Stems supports leaves and reproductive structures,
2.
Transport water and nutrients and
3.
Protect the plant.
LEAVES
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The earliest land plants were leafless photosynthetic stem systems.
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Eventually their flattened stems grew close together, evolving into
continuous structures known as leaves.
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A leaf is the main photosynthetic organ of modern plants.
Cactus stem and leaf adaptations. The stem and leaves of cacti are well
adapted for hot, dry climate of the desert. The thick stems serve as water
storage, while the small surface area of the thin spines (leaves) loses less
water to evaporation.
Functions of Leaves
1. Photosynthesis
-the process by which the plants use solar energy to make their own
food, transforming carbon dioxide and water into sugars that
store chemical energy.
ROOTS
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A root has two main functions:
1. Anchoring the plant in the soil
2. Absorbing water and minerals.
Root hairs. Root hairs are found within a centimeter of the tips of
roots just above the region of cell elongation. They specialized to
take up both water and ions needed by the plant. (a) Here are root
hairs on a radish seedling. (b) This micrograph provides a close-up
view of root hairs.
An Overview of Plant Growth and
Development
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A typical plant has a:
1.
Root system (usually below ground) and a
2.
Shoot system (usually above ground) consisting of all
stems, leaves and reproductive structures.
SHOOT
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A shoot is any individual stem and its leaves, as well as any
reproductive structures that extend from the stem such as flowers.
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A shoot that has leaves but no reproductive structures is called a
vegetative shoot.
EMBRYOS
SEED COAT
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Give rise to stems, leaves and roots of adult seed plants.
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A seed plant begins as a fertilized egg or zygote that grows into an
embryo within a seed.
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The typical seed plant embryo includes the following embryonic
“organs” that develop into the roots and shoots.
PARTS OF A SEED (EMBRYO)
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A seed is enclosed with a covering called the seed coat.
COTYLEDONS
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One or more “seed leaves” called cotyledons, which are usually the
largest, most visible parts of the embryo.
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Monocot- one cotyledon (corn, rice, palms)
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Dicot- two cotyledon (beans, peas, mango)
1. Seed coats
4. Plumule
2. Cotyledons
5. Hypocotyl
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Latin radix, meaning “root”
3. Radicle
6. Endosperm
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An embryonic “root” which is always prominent.
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It is the first root that grows from the seed.
RADICLE
PLUMULE
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Latin plumula, meaning “soft leather”
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An embryonic “”shoot” which is usually scarcely developed.
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It gives rise to the epicotyl, a portion of the embryonic “stem” located
over the cotyledon.
HYPOCOTYL
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A portion of the embryonic “stem” which is located under the
cotyledon and above the radicle and may be either short or long.
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It is the first stem that develops from the seed.
ENDOSPERM
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The embryo is surrounded by a storage tissue, called endosperm in
flowering plants, that breaks down to provide nourishment to the
developing embryo.
Root Tip
Sometimes endosperm also supplies nutrition to the sprouting seed.
KEY TERMS
PRIMARY GROWTH
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An increase in stem and root length due to the activity of apical
meristems at the tips of roots and at the buds of stems
APICAL MERISTEM
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An area of cell division at the tip of a stem or root in a plant;
produces primary tissues
BUD
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A dormant embryonic shoot that eventually develops into an apical
meristem.
Apical Meristems
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Primary growth (growth in length) comes from apical meristems at
tips of roots and shoots.
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Leaves originate on shoot apical meristems as leaf primordia.
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Each mature leaf is attached to a stem by petiole at a point called a
node.
Stem Tip
Stem Tip Development
SECONDARY GROWTH
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An increase in a plant’s stem and root girth due to the activity
of lateral meristems (the vascular cambium and cork
cambium) Secondary growth is localized, typically as long
cylinders of active growth throughout the lengths of older
stems and roots
LATERAL MERISTEM
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An area of cell division on the side of a vascular plant; the two
lateral meristems (vascular cambium and cork cambium)
give rise to secondary tissues
Lateral Meristems and
Secondary Growth