Chapter 23
Roots, Stems, and Leaves
Roots
Roots are plant organs
that anchor a plant,
usually absorb water and
dissolved minerals, and
contain vascular tissues
that transport materials to
and from the stem.
Types of Roots


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The surface area of a plant’s roots can be as much
as 50 times greater than the surface area of its
leaves
Most roots grow in soil but some do not
There are two main types of root systems1. taproots
2. fibrous roots
Taproots


Carrots and beets are taproots, which are single,
thick structures with smaller branching roots
Taproots accumulate and store food
Fibrous root


Fibrous roots systems have
many, small branching roots
that grow from a central
point.
Some plants, such as corn
have a type of root called
prop roots, which originate
above ground and help
support a plant
Other types of Roots

Many climbing plants have aerial roots that cling
to objects such as walls and provide support for
climbing stems
Other types of roots
When bald cypress trees grow in swampy soils, they
produce modified roots called pneumatophores,
which are referred to as “knees.”
The knees grow upward
from the mud, and
eventually, out of the
water.
Knees help supply
oxygen to the roots

The structure of Roots
Root hairs
Xylem and phloem are located in the
center of the root.
Xylem
The arrangement of xylem
and phloem tissues accounts
Phloem
for one of the major
Pericycle
differences between
Endodermis
monocots and dicots.
The tip of each root is
covered by a protective layer
of parenchyma cells called
Apical meristem
the root cap.
Root cap
Root Growth

There are two areas of rapidly dividing cells in
roots where the production of new cells initiates
growth.

The root apical meristem produces cells that cause
a root to increase in length.

In dicots, the vascular cambium develops
between the xylem and phloem and contributes
to a root’s growth by adding cells that increase
its diameter.
Root Growth

As the root grows through the soil, the cells of the
root cap wear away.

Replacement cells are produced by the root apical
meristem so the root tip is never without its
protective coverings.
Stems
Stems


Stems usually are the aboveground parts of plants
that support leaves and flowers. They have
vascular tissues that transport water, dissolved
minerals, and sugars to and from roots and leaves.
Green, herbaceous
stems are soft and
flexible and usually
carry out some
photosynthesis.
Stems

Trees, shrubs, and some other perennials have
woody stems.

Woody stems are
hard and rigid and
have cork and
vascular
cambriums.
Stem Adaptations

Some stems are adapted to storing food.

Stems that act as food-storage organs include:
1. Corms
2. Tubers
3. rhizomes.
Corm

A corm is a short, thickened, underground stem
surrounded by leaf scales.
Tuber & Rhizome


A tuber is a swollen, underground stem that has buds
from which new plants can grow.
Rhizomes also are
underground stems
that store food.
Tuber
Internal Structure

The vascular tissues in stems are arranged differently
from that of roots.

Stems have a bundled arrangement or circular
arrangement of vascular tissues within a surrounding
mass of parenchyma tissue.
Vascular Bundle

In most dicots, xylem and phloem are in a circle of
vascular bundles that form a ring in the cortex.

The vascular bundles of most monocots are
scattered throughout the stem.
Vascular
bundles
Vascular
bundle
Woody Stem




As the stems of woody plants grow in height, they
also grow in thickness
This added thickness, called secondary growth,
results from cell divisions in the vascular
cambium of the stem
The xylem tissue produced by secondary growth is
also called wood
In temperate regions, a tree’s annual growth rings
are the layers of vascular tissue produced each
year by secondary growth
How old is my tree?
Annual growth rings

These annual
growth rings can
be used to
estimate the age of
the plant.

Woody stems are
composed
primarily of dead
xylem cells.
Cork
Phloem
Vascular
cambium
Xylem
Woody Stems

As secondary growth continues, the outer portion
of a wood stem develops bark.

Bark is composed of phloem cells and the cork
cambium.

Bark is a tough, corky tissue that protects the stem
from damage by burrowing insects and browsing
herbivores.
Stems transport water
Water lost
through
leaves


Water, sugars, and
other compounds are
transported within the
stem
As water moves up
through the xylem, it
also carries dissolved
minerals to all living
plant cells
Xylem

Water
Stems transport sugar


The contents of phloem are primarily dissolved
sugars but phloem also can transport hormones,
viruses, and other substances.
The sugars originate in photosynthetic tissues that
are usually in leaves.
Stem Transport


Any portion of the plant
that stores these sugars
is called a sink, such as
the parenchyma cells
that make up the cortex
in the root.
The movement of
sugars in the phloem is
called translocation
Source of
sugars
Sugar

Phloem
Sink
Sieve plate
Companion cell
Leaves
Function of leaves



The primary function of the leaves is
photosynthesis
Most leaves have a relatively large surface area
that receives sunlight
Sunlight passes through the transparent cuticle
into the photosynthetic tissues just beneath the leaf
surface
Parts of the Leaf

When you think of a leaf, you probably think only of a flat,
broad, green structure

Sizes, shapes, and types of leaves vary enormously.
– Blade: entire leaf unit
– Margin: edge of the leaf
– Base: Part of the leaf closest to the stem
– Apex: tip of the leaf
– Petiole: connects the leaf blade to the stem
– Stipule: tiny leaf like structures that may or may not be present
close to the stem
Simple and Compound Leaf

A simple leaf is one with a
blade that is not divided

When the blade is divided
into leaflets, it is called a
compound leaf
Leaf arrangement on a stem

The arrangement of leaves on a stem can vary.
1. Opposite: Directly
opposite each other on
a stem
2. Alternate: on both
sides but not opposite
each other
Leaf arrangement on a stem
3. Whorled: Three or more
leaves growing around a
stem at the same position
The lines that appear on the surface of a leaf and
look like blood vessels are called Veins
1. Pinnate
2. Palmate
Leaf structure
Cuticle
Upper
epidermis
Palisade
mesophyll
Vascular
bundle
Xylem
Phloem
Lower epidermis
Spongy mesophyll
Stomata
Guard cell
Leaf Structure




Most photosynthesis takes place in the palisade
mesophyll
Below the palisade mesophyll is the spongy
mesophyll, which is composed of loosely packed,
irregularly shaped cells
These cells usually are surrounded by many air
spaces that allow carbon dioxide, oxygen, and
water vapor to freely flow around the cells
Gases can also move in and out of a leaf through
the stomata
Stomata
Stomata (STOH mah tuh) (singular, stoma) are
openings in leaf tissue that control the exchange of
gases.
Stomata are found on
green stems and on the
surfaces of leaves.
Guard cells
Cells called guard cells control the opening and closing
of stomata.
The opening and
closing of stomata
regulates the flow of
water vapor from leaf
tissues.
The loss of water
through the stomata
is called
transpiration
Transpiration
Water



The guard cells have
flexible cell walls
When water enters the
guard cells, the
pressure causes them
to bow out, opening
the stoma
As water leaves the
guard cells, the
pressure is released
and the cells come
together, closing the
stoma
Thickened
walls
Guard
cell
Epidermal
cells
Leaf Modifications
Many plants have leaves with structural
adaptations for functions besides photosynthesis
1. Some plant leave have epidermal growths that
release irritants when broken or crushed

Leaf Modifications

Cactus spines are
modified leaves that help
reduce water loss from the
plant and provide
protection from predators
Leaf Modifications

Carnivorous plants have
leaves with adaptations
that can trap insects or
other small animals
Pitcher Plant
Chapter 24
Plant Reproduction
Reproduction in plants




The process of sexual
reproduction in flowering
plants takes place in a
flower
A flower’s structure is
genetically determined
and usually made up of
four kinds of organs:
1. sepals 2. petals
3. Stamens 4. pistils
Stigma
Petals
Style
Ovary
Anther
Stamen
Filament
Sepal
Peduncle
Pistil
Complete Flower

A flower that has all four
organs—sepals, petals,
stamens, and pistils—is
called a complete flower
Incomplete Flower


A flower that lacks one or
more organs is called an
incomplete flower
For example the flowers
of plants such as sweet
corn, and grasses, have no
petals and are adapted for
pollination by wind rather
than by animals.
Photoperiodism


The relative lengths of daylight and darkness each
day have a significant effect on the rate of growth
and the timing of flower production in many
species of flowering plants
The response of flowering plants to daily daylightdarkness conditions is called photoperiodism
– Plant biologists originally thought that the length of
daylight controlled flowering, but they now know that it
is the length of darkness that controls flowering, and
that the darkness must be uninterrupted
Types of Photoperiodism

Plants are
1. short-day plants
2. long-day plants
3.day-neutral plants
4. intermediate day plants
Short Day Plants

A short-day plant
flowers when the
number of daylight
hours is shorter than
that of its critical period

Short-day plants usually
flower sometime during late
summer, fall, winter, or
spring
Long Day Plants

A long-day plant
flowers when the
number of daylight
hours is longer than
that of its critical
period

Long-day plants usually
flower in summer, but also
will flower if lighted
continually
Day Neutral Plants

Some plants will
flower over a range in
the number of daylight hours. These
plants are called dayneutral plants

Includes many plants
Flowering in cucumbers,
tomatoes, and corn are not
influenced by dark period

Intermediate Day Plants

An intermediate-day
plant will not flower if
days are shorter or
longer than its critical
period

Several grasses and
sugarcane are in this
category
Pollination

The process of transferring pollen grains
from the anther to the stigma
1. Wind



Wind is random
It scatters pollen
randomly
Pollen can land places
besides the stigma
2. Animals



Ensure pollen gets in
the right place
Use beetles,
butterflies, moths,
bees, flies,
hummingbirds, and
bats
These are the most
successful plant
groups on Earth
3. Nectar




Produced by flowers
Attracts & serves as
food for animal
pollinators
Liquid is made up of
proteins & sugars
Usually collects in a
cup like area that the
base of the petals
Animals as pollinators



The animals position
on the petals & brush
against the anthers
Pollen grains stick to
the body & brush off
onto the stigma of
another plant
Results in pollination
Nectar feeding pollinators are attracted
by color and scent

Butterflies: attracted to bright colors, with
platforms or cluster petals in the daytime
– Daisies, phlox & rhododendrons
Nectar feeding pollinators are attracted
by color and scent
Moths: attract to plants that
stay open all night, with pale
colors but has a strong sweet
scent
 Do not need a landing pad,
moths hover

– Tobacco, night-blooming cereus, &
honeysuckle
Nectar feeding pollinators are attracted
by color and scent
Bees: collect pollen & nectar
 Attracted to yellow/blue flowers with a
sweet scent

– Peas, mint, primrose, irises…
Seed Dispersal

Dispersal of seeds is important because it
reduces competition
1. Animals such as raccoons, deer, bears, and birds
help distribute many seeds by eating fruits.
Seed Dispersal
2. Seeds that are eaten usually pass through the
digestive system undamaged and are deposited in
the animal’s wastes
3. The ripened fruits of many plants split open to
release seeds with structural adaptations for
dispersal by wind or by clinging to animal fur.
Seed Dispersal
4. Cling onto animals fur
5. May float away if found
near the water