Plant Biology
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PLANT ORGANS
 1.
THE BASIC PLANT ORGANS
 Plants draw resources from two very
different environments: below-ground
and above-ground. Plants must absorb
water and minerals from below the
ground and carbon dioxide and light
from above the ground.
 Therefore, they have three basic organs:
roots, stems, and leaves.
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 Roots
are not photosynthetic and would
starve without the organic nutrients
imported from the stems and leaves.
 Conversely, the stems and leaves depend
on the water and minerals that roots
absorb from the soil.
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ROOTS

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
The root is an organ that anchors a vascular plant,
usually to the soil. It absorbs minerals and water,
and often stores organic nutrients.
A taproot system consists of one main vertical root
which gives rise to lateral roots. The taproot often
stores organic nutrients that the plant consumes
during flowering and fruit production. For this
reason, root crops such as carrots, turnips, and
sugar beets are harvested before they flower.
Taproot systems generally penetrate deeply into
the ground.
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Taproots
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ROOTS

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In seedless vascular plants and grasses, many
small roots grow from the stem in what is
called a fibrous root system. No roots stand
out as the main one.
These roots are said to be adventitious.
A fibrous root system is usually shallower than
a taproot system.
Grass roots are fibrous root systems that hold
the top soil in place, preventing erosion.
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Fibrous Roots
vs.
Tap Root
ROOTS


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The entire root system helps
anchor a plant, but the
absorption of water and
minerals occurs primarily near
the root tips, where vast
numbers of tiny root hairs
increase the surface area of the
root enormously.
A root hair is an extension of a
root at the dermal cell.
Absorption is often enhanced
by symbiotic relationships
between plant roots and fungi
and bacteria.
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Root Hairs from Dermal Cells
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STEMS
A
stem is an organ
system consisting
of nodes (the
points at which
leaves are
attached), and
internodes (the
stem segments
between nodes).
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STEMS

In the angle
formed by each
leaf and the
stem is an axillary
bud, a structure
that has the
potential to form
a lateral shoot,
commonly
called a branch.
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STEMS
 Most
axillary buds of a young shoot are
dormant.
 Thus, elongation of a young shoot is
usually concentrated near the shoot apex
(tip), which consists of a terminal bud with
developing leaves.
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STEMS

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The resources of a plant are concentrated at the
apex for elongation growth to increase the plant's
exposure to light. But what if an animal eats the end
of the shoot? Or what if light is obstructed there?
Under such conditions, axillary buds began growing.
A growing axillary bud gives rise to a lateral shoot
with its own terminal bud, leaves, and axillary buds.
Removing the terminal bud usually stimulates the
growth of axillary buds resulting in more lateral
shoots.
That is why pruning trees and shrubs and pinching
back houseplants will make them bushier.
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 Removal
of
the growing
tip will
produce a
short and
bushy plant.
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STEMS
 Modified
stems with different functions
have evolved in many plants as an
adaptation to the environment.
 These modified stems, which include
stolons, rhizomes, tubers, and bulbs, are
often mistaken for roots.
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A
stolon is a horizontal stem that grows
along the surface of the soil. These
runners enable a plant to reproduce
asexually, as plantlets form at nodes
along each runner. An example is found
in the strawberry plant.

A rhizome is a horizontal
stem that grows just below
the surface of the soil. An
example is the edible base
of a ginger plant.
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
A tuber is an enlarged
end of a rhizome that has
become specialized for
storing food. An example
is a potato. The eyes of a
potato are clusters of
axillary buds that mark
nodes.
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
A bulb is a vertical,
underground shoot
consisting mostly of the
enlarged bases of leaves
that store food. An
example is an onion.
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Garlic Bulb
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LEAVES
 The
leaf is the main photosynthetic organ of most
plants, although green stems also perform
photosynthesis.
 Leaves generally consist of a flattened blade and
a stalk (the petiole), which joins the leaf to a node
of the stem.
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 Plants
differ in the arrangement of veins,
which are the vascular tissue of leaves.
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LEAVES
 Most
monocot leaves (like grass) have
parallel major veins that run the length of
the leaf blade.
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LEAVES
 In
contrast, eudicot leaves (like trees and
most other plants) generally have a multibranched network of major veins.
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 Plants
are sometimes classified according
to the shape of the leaves and the
pattern of the veins.
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 Plants
are sometimes classified according
to the shape of the leaves and the
pattern of the veins.
LEAVES
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
Most leaves are specialized for
photosynthesis. However, some plant species
have leaves that have become adapted for
other functions, such as support, protection,
storage, or reproduction.

Tendrils
Spines
Storage leaves
Bracts
Reproduction

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LEAVES
 Tendrils
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are modified leaves which allow a
pea plant to cling for support.
LEAVES
 The
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spines of a cactus are modified
leaves which serve as protection.
LEAVES
 Succulent
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plants, such as the ice plant,
have storage leaves for storing water.
LEAVES
 The
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red parts of a poinsettia plant are often
mistaken for petals but are actually modified
leaves called bracts that attract pollinators.
LEAVES
 Some
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leaves are modified for
reproduction, such as those which
produce tiny plantlets, which fall off the
leaf and take root in the soil.
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PLANT TISSUES
 Each
plant organ (root, stem, or leaf) has
dermal, vascular, and ground tissues.
 A tissue system consists of one or more tissues
organized into a functional unit connecting the
organs of a plant.
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DERMAL TISSUE SYSTEM



The dermal tissue system is the outer protective covering of a
plant.
Like our skin, it forms the first line of defense against physical
damage and pathogenic (disease causing) organisms.
In non-woody plants, the dermal tissue usually consists of a
single layer of tightly packed cells called the epidermis.
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In woody plants,
protective tissues
known as
periderm replace
the epidermis in
older regions of
the stems and
roots.
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DERMAL TISSUE SYSTEM



In addition to protecting the plant from water loss
and disease, the epidermis has special
characteristics in each organ.
For example, at the tip of roots, the epidermis has
extensions called root hairs which absorb water and
minerals.
In the epidermis of leaves and most stems, a waxy
coating called the cuticle prevents water loss.
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Cuticle
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VASCULAR TISSUE SYSTEM

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The vascular tissue system carries out long
distance transport of materials between roots and
shoots.
The two vascular tissues are xylem and phloem.
Xylem conveys water and dissolved minerals
upward from roots in to be shoots.
Phloem transports nutrients such as sugars from
where they are made (usually the leaves) to
where they are needed (usually the roots,
developing leaves, and fruits).
The vascular tissue of a root or stem is collectively
called the stele.
Stele
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GROUND TISSUE SYSTEM



Tissues that are neither dermal nor vascular are part of the
ground tissues system.
Ground tissue that is internal to the vascular tissue is called pith,
and ground tissue that is external to the vascular tissue is called
cortex.
The ground tissues system includes various cells specialized for
functions such as storage, photosynthesis, and support.
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TYPES OF GROWTH
 Unlike
most animals, plant growth occurs
throughout the life of the plant.
 Except for periods of dormancy, most
plants grow continuously.
 Eventually of course, plants die.
 Based on the length of their lifecycle,
flowering plants can be categorized as
annuals, biennials, or perennials.
Annuals
 Annuals
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complete their lifecycle (from
germination to flowering to seed production to
death) in a single year or less.
 Many wildflowers are annuals, as are the most
important food crops, including the cereal
grains and legumes.
Biennials
 Biennials
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generally live two years, often including
a cold period (winter) between vegetative
growth (first spring/summer) and flowering
(second spring/summer).
 Beets and carrots are biennials but are rarely left
in the ground long enough to flower.
Perennials
 Perennials
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live many years and include trees,
shrubs, and some grasses.
Perennials

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Some buffalo grass of the North American plains is believed
to have been growing for 10,000 years from seeds that
sprouted at the close of the last ice age.
When a
perennial dies, it
is usually not
from old age,
but from an
infection or
some
environmental
trauma, such as
fire or severe
drought.
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Meristems
 Plants
have embryonic tissues called meristems
that allow the plant to grow indefinitely.
 Apical meristems
 Lateral meristems
 Vascular cambium
 Cork cambium
Meristems
 Apical
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meristems, located at the tips of roots
and in the buds of shoots, enable a plant to
grow in length, a process known as primary
growth.
Meristems
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 Lateral
meristems allow for growth in thickness,
known as secondary growth. In woody plants,
the lateral meristems are called the vascular
cambium and the cork cambium.
Lateral
meristem
Meristems
 The
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vascular cambium adds layers of
secondary xylem (wood) and secondary
phloem.
Meristems
 The
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cork cambium replaces the epidermis with
periderm which is thicker and tougher.
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PRIMARY GROWTH
 Primary
growth lengthens roots and
shoots. The new growth produced by
apical meristems affects the entire plant if
it is herbaceous.
 In woody plants, it only affects the
youngest parts which have not yet
become woody.
 Although apical meristems lengthen both
roots and shoots, there are differences in
the primary growth of these two systems.
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PRIMARY GROWTH OF ROOTS
 The
root tip is
covered by a root
cap, which
protects the
delicate apical
meristem as the
root pushes
through the
abrasive soil during
primary growth.
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PRIMARY GROWTH OF ROOTS
 Growth
occurs just
behind the root tip, in
three zones of cells at
successive stages of
primary growth.
 Moving away from
the root tip, they are
the zones of cell
division, elongation,
and maturation.
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PRIMARY GROWTH OF ROOTS

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The primary growth of roots produces the
epidermis, ground tissue, and vascular tissue.
Water and minerals absorb from the soil must enter
through the epidermis.
Root hairs enhance this process by greatly
increasing the surface area of epidermal cells. In
most roots, the stele is a vascular cylinder, a solid
core of xylem and phloem.
However, in many roots, the vascular tissue consists
of a central core of parenchyma cells surrounded
by alternating rings of xylem and phloem.
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PRIMARY GROWTH OF SHOOTS
Leaves arise as leaf
primordia, which
are finger-like
projections along
both sides of the
apical meristem.
Axillary buds can
form lateral shoots
as well.
Within a bud, leaf
primordia grow in
length due to both
cell division and
cell elongation.
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SECONDARY GROWTH
 Secondary
growth adds girth (width) to stems
and roots in woody plants.
 Secondary growth is produced by lateral
meristems.
 The vascular cambium adds secondary xylem
and secondary phloem.
 Cork cambium produces a tough, thick covering
consisting mainly of cork cells.
 Primary and secondary growth occurs
simultaneously like in different regions.
 While an apical meristem elongates a stem or
root, secondary growth commences where a
primary growth has stopped.
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SECONDARY GROWTH
 The
vascular cambium is a cylinder of
meristematic cells one layer thick.
 It increases in circumference and also lays down
successive layers of secondary xylem to its interior
and secondary phloem to its exterior.
 In this way, it is primarily responsible for the
thickening of a root or stem.
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XYLEM
 In
plants, vascular tissue made of dead cells that
transport water and minerals from the roots is
called xylem.
 Water and minerals ascend from roots to shoots
through the xylem.
 The xylem sap flows upward from the roots
throughout the shoot system to veins that branch
throughout each leaf.
 Leaves depend on this delivery method for their
supply of water.
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XYLEM
 Plants
lose an astonishing amount of water by
transpiration, the loss of water vapor from leaves.
 A single plant can lose 125 L of water during a
growing season.
 Unless the water is replaced, the leaves will wilt in
the plant will eventually die.
 The upward flow of xylem sap also brings mineral
nutrients to the shoots.
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XYLEM

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
Xylem sap needs to rise more than 100 m in the
tallest trees.
To get to this height, it is either pushed up from the
roots or pulled upward by the leaves.
Root pressure pushes the xylem sap upward,
especially at night.
The root pressure at night sometimes causes more
water to enter the leaves then is transpired, resulting
in exudation of water droplets that can be seen in
the morning on tips of grass blades or the margins of
leaves.
This is not the same thing as dew, which is
condensed moisture produced during transpiration.
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Transpiration vs. Dew
 Transpiration
 Dew
XYLEM



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Root pressure can only force water upward a few meters,
and it cannot keep pace with transpiration after sunrise.
For the most part, xylem sap is pulled upward by the
leaves themselves.
This is accomplished by the transpiration-cohesion-tension
mechanism, like sucking liquid through a straw.
As moisture escapes the leaves by
transpiration, one water molecule
sticks to the other water molecules
by cohesion, and the entire column
of water rises.
This transpiration pull can extend
down to the roots only if the chain of
water molecules is unbroken.
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XYLEM




If an air pocket forms, such as when xylem sap freezes in the winter,
the resulting air bubbles will break the chain.
Air bubbles can also occur if there is an excess rate of evaporation of
water from the leaves.
This is common when the leaves are exposed to windy conditions,
such as when plants are transported in the back of a truck.
A plant can be killed in as little as 20 minutes of exposure to these
conditions if the soil is not thoroughly watered before the trip.
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PHLOEM
 In
plants, vascular tissue that consists of living
cells that distribute sugars throughout the plant is
called phloem.
 Organic nutrients (the products of
photosynthesis) are translocated through the
phloem.
 Phloem is arranged in sieve tubes that are
positioned end to end.
 Between the cells are sieve plates, structures that
allow the flow of sap along the sieve tubes.
PHLOEM
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PHLOEM
 The
main component of phloem sap is sugar
(sucrose). This gives the sap a syrupy thickness.
 A sugar source is a plant organ that produces
sugar by photosynthesis. Mature leaves are the
primary sugar sources.
 A sugar sink is an organ that is a consumer or
storage site of sugar. Growing roots, buds, stems,
and fruits are sugar sinks. A storage organ, such
as a tuber or a bulb, may be a source or a sink,
depending on the season.
TRANSPIRATION

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Gas exchange (transpiration) in plants occurs through
structures called stomata.
The rate of transpiration is regulated by stomata, which are
pores in the leaves.
Carbon dioxide enters through the stomata into airspaces
formed by the spongy parenchyma cells.
This increases the internal surface area of the leaf by up to
30 times greater than what it appears when we look at the
leaves.
This increase in surface area improves the rate of
photosynthesis however it also increases water loss through
the stomata.
Therefore, a plant requires a tremendous amount of water
to make food by photosynthesis.
By opening and closing the stomata, guard cells balance
water conservation during photosynthesis.
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Stomata and Guard Cells
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Transpiration
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TRANSPIRATION
A
leaf may transpire more than its
weight in water every day and water
may move through the xylem at a rate
which is about equal to the speed of
the tip of a second hand sweeping
around a clock.
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TRANSPIRATION


If transpiration continues to pull sufficient water upward to the
leaves, they will not wilt.
But the rate of transpiration is greatest on a day that is sunny,
warm, dry, and windy because of the increase in evaporation.
Plants adjust to these
conditions by regulating
the size of the stomatal
openings, but some
evaporation still occurs
when the stomata are
closed.
As cells lose water
pressure, leaves begin to
wilt.
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TRANSPIRATION
 Transpiration
also results in evaporation cooling.
 This prevents the leaf from reaching
temperatures that could damage enzymes
involved in photosynthesis.
Cactus plants have
low rates of
transpiration, but
have evolved to
tolerate high leaf
temperatures.
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NUTRIENTS
 Watch
a large plant grow from a tiny seed, and
you cannot help wondering where all the mass
comes from.
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NUTRIENTS
 About
90% of a plant is water which has
accumulated within their cells.
 However, soil, water, and air all contribute to
plant growth.
 Plants extract essential mineral nutrients from the
soil, especially phosphorus and nitrogen.
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NUTRIENTS
 They
also require other minerals as well. The
symptoms of a mineral deficiency depend partly
on the nutrient’s function.
 For example, a deficiency of magnesium, a
component of chlorophyll, causes yellowing of
the leaves, known as chlorosis.
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SOIL QUALITY
 Along
with climate, the major factors
determining whether a particular plant can
grow well in a certain location are the
texture and composition of the soil.
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Soil Texture
Texture refers to
the relative
amounts of various
sizes of soil
particles.
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Soil Composition
Composition
refers to the
organic and
inorganic
chemical
components
of the soil.
In turn, plants
affect the soil,
taking part in a
chemical cycle
that sustains the
balance of
terrestrial
ecosystems.
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SOIL QUALITY
 Soil
originally comes from the weathering
of solid rock.
 Rocks break apart over time from several
mechanisms.
 Water can seep into crevices, freeze, and
the expansion can fracture rocks.
 Acids dissolved in the water can also
break down rocks chemically.
 Roots that grow in fissures can also cause
fracturing.
SOIL QUALITY
The eventual
result of all this
activity is topsoil
(O), a mixture of
rock particles,
living organisms,
and humus (A),
the remains of
partially decayed
organic material.
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Texture of topsoil

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The texture of topsoil depends on the size of its particles,
which range from coarse sand to microscopic clay.
The most fertile soils are loams, made up of equal
amounts of sand, silt (medium-size particles), and clay.
The fine particles provide a large surface area for
retaining minerals and water.
Coarse particles provide airspaces containing oxygen
that can be used by roots for cellular respiration.
If soil does not drain adequately, roots suffocate because
the air spaces are replaced by water; the roots may also
be attacked by molds that favor wet soil.
These are common hazards for houseplants that are
overwatered in pots with poor drainage.
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Soil composition

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Soil composition includes organic components as well as
minerals.
Topsoil has an astonishing number and variety of
organisms.
A teaspoon of topsoil has about 5 billion bacteria along
with various fungi, algae, insects, and worms.
The activities of all these organisms affect the soils
properties.
Earthworms aerate the soil by their burrowing and add
mucus that holds find soil particles together.
The metabolism of bacteria changes the mineral
composition of the soil.
Plant roots can release organic acids, changing the soil
pH.
Plant roots also reinforce the soil against erosion.
Soil composition

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Humus consists of decomposing organic material
formed by the action of bacteria and fungi on
dead organisms, feces, fallen leaves, etc.
Humus prevents clay from packing together and
builds a crumbly soil that retains water but is still
porous enough for adequate aeration of roots.
It is also a reservoir of mineral nutrients that are
returned gradually to the soil as microorganisms
decompose the organic matter.
During heavy rain or irrigation, nitrogen and
phosphate is leached away from the soil and
drained into the groundwater deeper down,
making them less available for uptake by roots.
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Soil conservation



Soil conservation is essential.
It may take centuries for a soil to become
fertile through the breakdown of rock and the
accumulation of organic material, but human
management can destroy that fertility within
a few years.
Before the arrival of farmers, the Great Plains
of the United States was covered by hardy
grasses that held the soil in place despite of
the long recurrent droughts and torrential
rains characteristic of that region.
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Great Plains
became dustbowl
due to farming the
same crops each
year.
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Soil conservation




In the late 1800s, many homesteaders settled in
the region, planting wheat and raising cattle.
These land uses left the topsoil exposed to erosion
by winds that often swept over the area.
During drought seasons, much of the topsoil was
blown away rendering millions of acres of
farmland into what was called the Dust Bowl.
This forced hundreds of thousands of people to
abandon their homes and land, as found in the
story, The Grapes of Wrath.
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Soil conservation

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
In healthy ecosystems, mineral nutrients must be
recycled by the decomposition of dead organic
material in the soil.
When farmers harvest of crop, essential elements are
removed.
To grow 1000 kg of wheat, the soil gives up 20 kg of
nitrogen, 4 kg of phosphorus, and 4 kg of potassium.
Each year, soil fertility diminishes unless fertilizers replace
these lost minerals.
Additional irrigation is also necessary.
More than 30% of the world's farmland suffers from low
productivity stemming from poor soil conditions.
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Fertilizers



Fertilizers are essential. Commercially produced
fertilizers are enriched with nitrogen (N), phosphorus
(P), and potassium (K).
They are labeled with a three-number code called
the N-P-K ratio, indicating the content of these
minerals.
A fertilizer marked as 15-10-5 indicates the
percentage of each mineral.
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Fertilizers
 Manure,
fish meal, and compost are called
organic fertilizers because they are of biological
origin and contain decomposing organic
material.
Before plants can use
organic material,
however, it must be
decomposed into the
inorganic nutrients that
roots can absorb.
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Fertilizers



Whether from organic fertilizer or a chemical
factory, the minerals a plant extracts are in
the same form, but organic fertilizers release
minerals gradually, whereas commercial
fertilizers are immediately available but may
not be retained by the soil for long.
Excess minerals not absorbed by the roots are
usually wasted because they are leached
from the soil by irrigation.
To make matters worse, mineral runoff may
pollute groundwater, streams, and lakes.
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Fertilizers

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

Agricultural researchers are developing ways
to maintain crop yields while reducing fertilizer
use.
One approach is to genetically engineer
“smart” plants that inform the grower when a
nutrient deficiency is imminent, before
damage has occurred.
One type of smart plant will produce a blue
pigment in the leaves when phosphate is
being depleted in the soil.
Therefore, the farmer can add phosphate
without needing to add other minerals that
would be wasted.
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Smart Plants
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Soil erosion
 Soil
erosion is another main concern.
 Thousands of acres of topsoil is lost to
water and wind erosion each year in the
United States alone.
 Certain precautions, such as planting
rows of trees as windbreaks, terracing
hillside crops, and cultivating in a contour
pattern, can prevent loss of topsoil.
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Windbreaks
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Terracing hillside crops
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Cultivating in a contour pattern
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Alfalfa vs.
Corn Rows
Crops such as alfalfa and wheat
provide good ground cover and
protect the soil better then corn and
other crops that are usually planted in
more widely spaced rows.
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NITROGEN

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
Nitrogen is often the mineral that has the greatest
effect on plant growth and crop yields.
It is ironic that plants can suffer from nitrogen
deficiency because the atmosphere is nearly 80%
nitrogen.
However atmospheric nitrogen is in a gas form
(N2) that plants cannot use.
For plants to absorb nitrogen, it must first be
converted to of ammonium (NH4) or nitrate (NO3).
These absorbable forms of nitrogen do not come
from the breakdown of rock.
They are generated by the decomposition of
dead vegetation by certain kinds of bacteria,
called nitrogen-fixing bacteria.
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NITROGEN



All life on Earth depends on these special
bacteria that can perform nitrogen fixation.
Several species of these bacteria live freely in
the soil, while others live in plant roots in
symbiotic relationships.
One of the most important crops that has this
symbiotic relationship is the legume family,
including peas, beans, soybeans, peanuts,
alfalfa, and clover.
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NITROGEN


Nitrogen-fixing bacteria
live in the nodules of
these plants and
generate more useful
nitrogen for themselves
and the soil than all
industrial fertilizers.
When farmers plant the
right amounts of these
legumes at the right time,
the soil becomes
enriched at virtually no
cost to the farmer.
112
Crop rotation
 Crop
rotation improves the quality of the
soil.
 In this practice, a non-legume such as
corn is planted one year, and the
following year alfalfa or some other
legume is planted to restore the
concentration of nitrogen in the soil.
113
PLANT BIOTECHNOLOGY
 Plant
biotechnology refers to innovations
in the use of plants or substances
obtained from plants to make products
that are useful to humans.
 Genetic engineering is a form of
biotechnology that refers to the use of
genetically modified organisms to
produce beneficial results.
114
PLANT BIOTECHNOLOGY



Corn is a staple crop in many developing countries, but the most
common varieties are poor sources of protein, requiring that
diets be supplemented with other protein sources, such as
beans.
The proteins in the most popular variety of corn are very low in
several essential amino acids that humans require in the diet.
Forty years ago, researchers discovered a new mutant species
of corn that has much higher levels of these essential amino
acids; this variety of corn is more nutritious.
Swine who are fed this variety of
corn gained weight three times
faster than those fed with normal
corn. However, the kernels are soft
and are more vulnerable to attack
by pests.
115
PLANT BIOTECHNOLOGY



Using conventional methods, plant breeders crossbred the
soft kernel species with a more desirable type; this transition
took hundreds of scientists nearly 20 years to accomplish.
With modern methods of genetic engineering, one
laboratory can accomplish this sort of thing in only a few
years.
Unlike traditional cross-breeding techniques, modern plant
biotechnologists are not limited to transferring genes
between closely related species of plants.
For instance, traditional breeding techniques could not be
used to insert a desired gene from a daffodil plant into a
rice plant. However, modern genetic engineering makes
this possible.
116
Reducing World Hunger and
Malnutrition

There is much disagreement about the causes of such hunger.
Some argue that there is a food shortage because the world is
overpopulated. Others say that there is enough food available,
but poor people cannot afford it. Whatever the cause, increasing
food production is a humane objective.
800 million people
on Earth suffer
from nutritional
deficiencies.
40,000 people die
each day of
malnutrition, half
of them children.
117
Reducing World Hunger and
Malnutrition



Because land and water are the most limiting
resources for food production, the best option
will be to increase yields on the available
land.
Based on estimates of population growth, the
world's farmers will have to produce 40% more
grain per acre to feed the human population
in the year 2020.
Plant biotechnology can help make these
crop yields possible.
118
Transgenic crops






Transgenic crops are those which contain genes from
particular bacteria that produce a protein that repels insect
pests.
When the gene from the bacteria is inserted into the plant,
the plant is now able to repel insects by itself, without the
use of insecticide.
Examples of transgenic crops include cotton, corn, and
potatoes.
This natural insecticide is completely harmless to humans
and all other invertebrates because it is only activated by a
substance found in the intestines of insects.
Researchers are also engineering plants with enhanced
resistance to disease.
In one case, a transgenic papaya resistant to a ring spot
virus was introduced into Hawaii, thereby saving its papaya
industry.
119
120
The Debate over Plant
Biotechnology




One concern about plant genetic engineering is
that certain molecules within a plant cause
allergies in humans.
Some people are concerned that these allergy
molecules will be transferred to a plant used for
food.
However, biotechnologists remove the genes that
encode for the allergenic proteins from soybeans
and other crops.
So far, there is no evidence that genetically
modified plants designed for human consumption
have adverse effects on human health.
121
The Debate over Plant
Biotechnology




In fact, some genetically modified foods are
potentially a healthier alternative.
For example, a particular species of corn contains
a cancer-causing toxin that has been found in
high concentrations in some batches of processed
corn products ranging from corn flakes to beer.
This toxin is produced by a fungus that can infect
corn which has been damaged by an insect.
Genetically modified corn contains 90% less of this
toxin.
122
The Debate over Plant
Biotechnology
 Nevertheless,
because of health concerns,
opponents lobby for the clear labeling of all
foods containing products of genetically
modified organisms (GMO).
Some people also argue
for strict regulations
against the mixing of GM
foods with non-GM foods
during transportation,
storage, and processing.
123
The Debate over Plant
Biotechnology





Many ecologists are concerned that the growing of GM crops
might have unforeseen effects on non-target organisms.
One study indicated that the caterpillars of Monarch butterflies
died following consumption of milkweed leaves (their preferred
food) which had been heavily dusted with pollen from genetically
modified corn. This study has since been discredited.
As it turns out, when the original researchers showered the corn
pollen onto the milkweed leaves in the laboratory experiment, other
floral parts also rained onto the leaves.
Subsequent research found that it was these other floral parts, not
the pollen, which contained a toxin that killed the butterflies.
Unlike pollen, these floral parts would not be carried by the wind to
neighboring milkweed plants under natural field conditions.
The Debate over Plant
Biotechnology
124




Perhaps the most serious concern is the possibility of the introduced
genes escaping from a transgenic crop into related weeds by
natural cross-pollination.
The fear is that the undesirable weeds will become resistant to
insects, creating a “superweed” that would be difficult to control in
the field.
Because of this concern, efforts are underway to breed male sterility
into transgenic crops.
These plants will still produce seeds and fruit if pollinated, but they
will produce no pollen.
125
The Debate over Plant
Biotechnology



One way to accomplish this is “Terminator
Technology” which uses “suicide genes” that
disrupt critical developmental sequences,
which prevent pollen development.
Plants that are genetically modified to
undergo the Terminator process grow
normally until the last stages of pollen
maturation.
At this point, a gene expressing a particular
protein becomes active and stops the pollen
from forming.
126
The Debate over Plant
Biotechnology
 On
a case-by-case basis, scientists and
the public must assess possible benefits of
transgenic products versus the risks society
is willing to take.
 The best scenario is for these discussions
and decisions to be based on sound
scientific information and testing rather
than on reflexive fear or blind optimism.
127
Genetically Modified Foods:
Discussion
128
PLANT EVOLUTION AND
DIVERSITY
 Plants
are multicellular eukaryotes that
make organic molecules by
photosynthesis.
 Unlike algae, plants have growth regions
called apical meristems as well as male
and female gametangia (pollen and
ovum) and multi-cellular, dependent
embryos.
129
PLANT EVOLUTION AND
DIVERSITY
 According
to the endosymbiotic theory of
the origin of chloroplasts, photosynthetic
prokaryotic cells (bacteria)were ingested
by larger cells in plants.
130
131
PLANT EVOLUTION AND
DIVERSITY
 Plants
have always had chloroplasts, even
before they went from living in the oceans
to living on land.
 However, the key adaptations plants had
to make before they could live on land
are: flowers, dependent embryos,
gametangia, organized vascular tissues,
and seeds.
132
PLANT EVOLUTION AND
DIVERSITY




Reproduction on land presents challenges.
For algae, the surrounding water insures that
gametes and offspring stay moist and
provides the means for their dispersal.
Plants, however, must keep their gametes
and developing embryos from drying out in
the air.
Land plants produce gametes in male and
female gametangia (protective jackets
around the gametes).
133
134
PLANT EVOLUTION AND
DIVERSITY



The egg remains in the female gametangia and is
fertilized there.
Pollen containing sperm are carried by the wind or by
animals toward the egg.
The fertilized egg (zygote) develops into an embryo while
attached to and nourished by parent plant.
This is called a
dependent
embryo,
which
distinguishes
plants from
algae.
135
PLANT EVOLUTION AND
DIVERSITY
 Plants
that produce seeds rely upon wind
or animals to disperse their offspring.
As a matter of fact, the
key step in the
adaptation of SEED
PLANTS to dry land was
the evolution of winddispersed pollen.
136
PLANT EVOLUTION AND
DIVERSITY



Plant reproduction may also include the
production of spores which are encased in a
protective jacket called a sporangium.
A spore is a cell that can develop into a new
organism without fusing with another cell.
Plants that do not produce seeds (such as
ferns) often rely on these tough-walled,
resistant spores for dispersal.
137
Fern
Sporangium
138
PLANT EVOLUTION AND
DIVERSITY




Among the earliest seed plants were the
gymnosperms, which are “naked seeds”
because they are not enclosed in any
chamber.
The largest group of gymnosperms is the
conifers, consisting mainly of cone bearing
trees such as pine, spruce, and fir.
Later on, flowering plants evolved, known as
angiosperms.
The dominant types of seed plants today are
the conifers and angiosperms.
139
140
PARTS OF A FLOWER
 The
anther is the male organ in which pollen
grains develop.
 A pollen grain is called a male gametophyte.
 Pollen grains develop in the anther (male
reproductive segment) and the pollen is
transferred to the stigma (female reproductive
segment).
141
PARTS OF A FLOWER
 Sepals
are green leaves which enclose the
flower before the flower opens.
 Petals are usually the most striking part of a
flower, and they function to attract
hummingbirds and insects.
142
PARTS OF A FLOWER
 Plants
dependent on nocturnal pollinators
typically have flowers that are highly scented.
When the insect
comes to collect the
nectar, it picks up
some pollen grains
and carries them to
the stigma of another
flower.
143
PARTS OF A FLOWER
 Fertilization
in angiosperms usually occurs
immediately after pollination.
 The carpel consists of a stalk with the
stigma at the top (which catches the
pollen) and an ovary (or ovule) at the
base.
 The ovary is a protective chamber where
the eggs develop.
144
145
PARTS OF A FLOWER
 The
ripened ovary of a flower, which is adapted
to disperse seeds, is called a fruit.
 Fruits protect and help disperse seeds.
 Seeds develop within fruits, and the fruits develop
at the base of flowers.
146
PARTS OF A FLOWER
 The
structure of a fruit reflects its function in seed
dispersal.
 Some angiosperms depend on wind for seed
dispersal.
For example, the fruit
of a maple tree acts
like a propeller,
spinning a seed away
from the parent tree
on wind currents.
147
PARTS OF A FLOWER
 Some
fruits hitch a ride on animals.
 The barbs of cockleburs hook to the fur of animals.
 These fruits may be carried for miles before they
open and release their seeds.
Angiosperms




Many angiosperms produce
fleshy, edible fruits that are
attractive to animals as food.
When a mouse eats a berry, it
digests the fleshy part of the
fruit, but most of the tough
seeds pass unharmed
through its digestive tract.
The mouse may then deposit
the seeds, along with a
supply of natural fertilizer,
some distance away from
where it ate the fruit.
The dispersal of seeds in fruits
is one of the main reasons
angiosperms are so
widespread and successful.
148
149
Angiosperms
 Angiosperms
often have mutually
dependent relationships with animals.
 They disperse their seeds by producing
fleshy, edible fruits that are consumed by
animals which defecate the seeds; seeds
sometimes attach to animals, or the seeds
may catch the wind.
150
Angiosperms
 Most
angiosperms depend on insects, birds, or
mammals for pollination and seed dispersal and
most land animals depend on angiosperms for
food.
 These mutual dependencies tend to improve the
reproductive success of both the plants and
animals.
151
Angiosperms


Many
angiosperms
produce flowers
that attract
pollinators that rely
entirely on the
flower’s nectar
and pollen for
food.
Nectar is a high
energy fluid that is
of use to the plant
only for attracting
pollinators.
Angiosperms




152
The color and fragrance of a flower are usually keyed
to a particular type of animal or insect.
Many flowers also have markings that attract
pollinators, leading them past pollen bearing organs
on the way to the nectar.
For example, flowers that are pollinated by bees
often have markings that reflect ultraviolet light.
Such markings are invisible to us, but vivid to bees.
153
Angiosperms
 Many
flowers pollinated by birds are red or
pink, colors to which bird eyes are especially
sensitive.
154
Angiosperms
 The shape of the flower may also be important.
 Flowers that depend largely on hummingbirds, for
example, typically have their nectar located
deep in a floral tube, where only the long, thin
beak and tongue of a hummingbird are likely to
reach.
155
Angiosperms
 Insects
and birds are active mainly during the
day.
 Some flowering plants, however, depend on
nocturnal pollinators, such as bats.
 These plants typically have large, light colored,
highly scented flowers that can easily be found
at night.
156
Angiosperms
 An
example of this is a cactus.
 As the bats eat part of the flower, its body
becomes dusted with pollen which it passes on
to other flowers.
157
Angiosperms





Human agriculture is based almost entirely on
angiosperms.
Whereas gymnosperms supply most of our lumber
and paper, flowering plants provide nearly all our
food.
Corn, rice, wheat, and other grains are dried fruits,
the main food source for most of the world’s
population and their domestic animals.
Many food crops are fleshy fruits, such as
strawberries, apples, cherries, oranges, tomatoes,
squash, and cucumbers.
Others are modified roots, such as carrots and
sweet potatoes, or modified stems, such as onions
and potatoes.
158
Angiosperms
 We
also grow angiosperms for spices,
fiber, medications, perfumes, and
decoration.
 Hardwoods, such as oak, cherry, and
walnut, are flowering plants.
 Two of the world's most popular
beverages come from coffee beans and
tea leaves, and cocoa and chocolate
also come from angiosperms.
159
Videos
 Non-vascular
plants 3 mins
 Seed plants 4 mins
 Parasitic Plants 3 mins
 Prehistoric Plants 2 mins
 Tour of a Plant Cell 2 mins
160