Download plants - Dr Magrann

PLANTS
1. THE BASIC PLANT ORGANS
Plants draw resources from two very different environments: below-ground and aboveground. Plants must absorb water and minerals from below the ground and carbon
dioxide in light from above the ground. Therefore, they have three 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.
A. 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
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.
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 type 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.
The entire root system helps anchor the 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.
B. 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.
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.
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
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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.
C. 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.
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.
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.
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.
A. 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.
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B. 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.
C. 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.
3. 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 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 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 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.
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.
A. 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. 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.
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 dome-shaped 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.
B. 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.
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.
4. FOOD, WATER, AND AIR
A. 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 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.
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
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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.
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. 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.
B. 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.
C. 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.
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.
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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.
D. 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.
E. 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 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.
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.
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. 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
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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 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. 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.
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 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. 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. 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.
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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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.
F. 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 ammonium (NH4)
or nitrate (NO3). These 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.
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 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.
5. 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.
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. 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.
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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. 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.
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.
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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.
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. 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.
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 Human Biology by Sylvia Mader (p. 31)
There’s nothing quite like the taste of a juicy, vine-ripened 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. 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.
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
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enzyme. As a result, the bioengineered tomatoes could be allowed to ripen on the vine
before being picked, package, and shipped to market.
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.
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.
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.
1. 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?
2. 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?
3. Is the fear of increased allergic reactions to genetically modified foods justified?
4. 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?
5. What steps could corporations take to increase public acceptance of genetically
modified foods?
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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.
According to the endosymbiotic theory of the origin of chloroplasts, photosynthetic
prokaryotic cells were ingested by larger cells in plants. 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.
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. 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.
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.
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 gametophyte. Pollen grains develop in the anther (male reproductive
segment) and are transferred to the stigma (female reproductive segment).
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PLANTS
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.
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. 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 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.
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PLANTS
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. 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.
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.
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.
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.
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.
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