Transcript Plant Biology

Plant Biology
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 in light from
above the ground. This led to the development of
free basic organs: roots, stems, and leaves. 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.
ROOTS

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 at the
plant consumes wearing 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.
ROOTS
 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. Roots
that arise from this damn are said to be
adventitious. A fibrous root system is
usually shallower than a taproot system
this system makes grassroots particularly
useful because they hold the top soil in
place, preventing erosion.
ROOTS
 The
entire route system helps anchor of
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.
STEMS

A stem is an organ system consisting of nodes
(the points at which leaves are attached),
and internodes (the stem segments between
nodes). 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. 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.
STEMS

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.
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. 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. 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. A bulb is a vertical,
underground shoot consisting mostly of the enlarged bases
of leaves that store food. An example is an onion.
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. Plants differ in the
arrangement of veins, which are the
vascular tissue of leaves.
LEAVES
 Most
monocot leaves (like grass) have
parallel major veins that run the length of
the leaf blade. In contrast, eudicot
leaves (like trees and most other plants)
generally have a multi-branched network
of major veins. Plants are sometimes
classified according to the shape of the
leaves and the pattern of the veins.
LEAVES

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 are modified leaves which allow a pea plant
to cling for support. The spines of a cactus are
modified leaves which serve as protection.
Succulent plants, such as the ice plant, have storage
leaves for storing water. The red parts of a poinsettia
plant are often mistaken for petals but are actually
modified leaves called bracts that attract pollinators.
Some leaves are modified for reproduction, such as
those which produce tiny plantlets, which fall off the
leaf and take root in the soil.
PLANT TISSUES
 2.
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.
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. In woody plants, protective tissues known as
periderm replace the epidermis in older regions of the
stems and roots. 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.
VASCULAR TISSUE SYSTEM

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.
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.
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
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
generally live two years, often
including a cold period (winter) between
vegetative growth (first spring/summer)
and flowering (second sprain/summer).
Beets and carrots are biennials but are
rarely left in the ground long enough to
flower.
Perennials
 Perennials
live many years and include
trees, shrubs, and some grasses. 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.
Meristems

Plants have embryonic tissues called meristems
that allow the plant to grow indefinitely. Apical
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. 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. The vascular cambium adds
layers of secondary xylem (wood) and secondary
phloem. The cork cambium replaces the
epidermis with periderm which is thicker and
tougher.
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.
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.
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.
PRIMARY GROWTH OF ROOTS

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.
PRIMARY GROWTH OF SHOOTS
 The
apical meristem of a shoot is a domeshaped mass of dividing cells at the tip of
the terminal bud. 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.
SECONDARY GROWTH

Secondary growth adds girth 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 and apical meristem elongates a stem
or root, secondary growth commences where
a primary growth has stopped.
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.
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. Plants lose an astonishing amount
of water by transpiration, the loss of water vapor
from Leeds. 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.
XYLEM

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.
XYLEM

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 transpirationcohesion-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.
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.
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. 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


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.
TRANSPIRATION

A leaf may transpire are 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. 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.
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.
NUTRIENTS

Watch a large plant grow from a tiny seed, and
you cannot help wondering where all the mass
comes from. 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. 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.
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. Texture refers to
the relative amounts of various sizes of soil
particles. 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.
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.
The eventual result of all this activity is topsoil,
a mixture of rock particles, living organisms,
and humus, the remains of partially decayed
organic material.
Texture of topsoil

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 (mediumsize 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.
Soil composition

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

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 air ration of roots. It is also a
reservoir of mineral nutrients that are returned
gradually to the soil as microorganisms
decomposed 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.
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.
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.
Soil conservation

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.
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-PK ratio, indicating the content of these minerals. A
fertilizer marked as 15-10-5 indicates the
percentage of each mineral. 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.
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.
Fertilizers

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.
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.
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.
NITROGEN

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 to
absorb mobile 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.
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. 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.
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.
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.
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.
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.
Reducing World Hunger and
Malnutrition

800 million people on Earth suffer from nutritional
deficiencies. 40,000 people die each day of malnutrition,
half of them children. 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. 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.
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.
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.
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.
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.
The Debate over Plant
Biotechnology

Many ecologists are concerned that the growing of GM
crops might have unforeseen effects on nontarget
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

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.
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.
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.
Genetically Modified Foods

There’s nothing quite like the taste of a juicy, vineripened tomato fresh from the garden in the
summer time. Tomatoes have become a staple of
our Western diets, and demand for them has
never been greater. However, bringing them to
market has never been easy. If allowed to ripen
naturally, tomatoes become mushy and mealy,
and often do not survive shipment. Thus, they are
picked while still green, shipped to market, and
ripened artificially using ethylene gas. While this
causes the tomato to appear ripened on the
surface, it remains mostly unripe.
Genetically Modified Foods
 As
anyone who has eaten a store-bought
tomato can confirm, the flavor and
texture are usually not as appealing as
that of vine ripened tomatoes. New
biotechnology techniques are being
utilized to address this problem and many
others, attracting both praise and scorn
alike, and igniting a national discussion on
the future of genetically altered food.
Genetically Modified Foods

Tomatoes become mushy and mealy mostly
after pectin, a complex carbohydrate that
gives tomatoes their firmness, breaks down.
When tomatoes ripen, they make an enzyme
that degrades pectin in the tomato, causing
the tomato to become soft and mushy. To
solve these problems, scientists produced a
genetically altered tomato lacking the
enzyme. As a result, the bioengineered
tomatoes could be allowed to ripen on the
vine before being picked, package, and
shipped to market.
Genetically Modified Foods

In 1994, the genetically altered tomato
received FDA approval. Many scientists and
consumers alike praised the tomato for its
quality and hardiness, and embraced the
technology behind it. The tomatoes initially
sold well in the marketplace, indicating
acceptance by the general population.
Although the flavor was not quite as good as
that of tomatoes fresh from the garden, it was
close.
Genetically Modified Foods

However, not everyone has embraced genetically
modified foods. Consumer advocacy groups and
environmental groups have questioned the safety of such
foods. In particular, questions remain regarding the stability
of the genetically modified crops, the possible
accumulation of toxins in the modified tomatoes, and the
potential of foreign proteins in these crops to induce
allergies in some individuals. Many also questioned
whether environmental damage would result from the
accidental transfer of genetic alterations to native plants
and animals, primarily because some plants are both pest
and herbicide resistant. Critics derided the tomato and
other genetically modeled crops as dangerous to our
health.
Genetically Modified Foods

The genetically altered tomato was eventually
pulled from the supermarket shelves because of a
disagreement with tomato growers. Tepid sales
were also blamed, having fallen off after the initial
consumer exuberance. Despite the failure of the
bioengineered tomato, the technology used to
produce it has led to the development of many
other genetically modified crops that have
weathered the marketplace and have found their
way to our dinner tables. However, many
consumer advocates, government entities, and
scientists remain wary of the long-term effects of
these modifications on our health and on the
environment.
Genetically Modified Foods





Despite the promises of higher crop yields on the tastier
foods, and improved nutritional value, much fear and
skepticism remains. Do you think that this fear is justified?
Do you believe that it is possible that the changes in
genetically altered crops may be transferred to other
organisms? How do you think this might occur?
Is the fear of increased allergic reactions to genetically
modified foods justified?
How should genetically modified foods be labeled in
supermarkets? Should producers be required to disclose
the presence of genetically modified food ingredients on
food labels?
What steps could corporations take to increase public
acceptance of genetically modified foods?
PLANT EVOLUTION AND
DIVERSITY

Plants evolved from green algae from shallow
water habitats which were subject to
occasional drying. Natural selection would
have favored algae that could survive
periodic droughts. Once plants were able to
make a transition to land, they would have
thrived from the limitless bright sunlight, the
abundance of carbon dioxide in the air, and
the relatively few pathogens and plant eating
animals.
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.
PLANT EVOLUTION AND
DIVERSITY

According to the endosymbiotic theory of the
origin of chloroplasts, photosynthetic
prokaryotic cells were incorporated by larger
cells. 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.
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). 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. Is in all plants, but 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.
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 wind-dispersed
pollen. 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.
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.
PARTS OF A FLOWER
 The
anther is the male organ in which
pollen grains develop. A pollen grain is
called a male sporangium. Pollen grains
develop in the (male) anther and are
trapped by the stigma (female).
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. 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.
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
at the base. The ovary is a protective chamber where the
eggs develop. 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.
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. 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.
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.
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. 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
 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.
Angiosperms
 Many
flowers pollinated by birds are red
or pink, colors to which bird eyes are
especially sensitive. 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.
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. 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.
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.
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.